Simple LINQ to SQL in C#

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Introduction

This article provides an introduction to employing LINQ to SQL within a Windows Forms application. The article will address the incorporation of LINQ to SQL into a WinForms project, how to use LINQ to SQL to select, insert, update, and delete data, and how to use LINQ to SQL to execute stored procedures. Select query examples will demonstrate ordering, filtering, aggregation, returning typed lists, returning single objects and values, and how to query across entity sets (which are essentially related tables associated by foreign keys).

The demonstration project included with the article is a simple WinForms application; this example contains a DataGridView control and a menu; the menu is used to execute each example query contained in the demonstration.

The application provides the following functionality:

• Return Full Tables
• Return Typed Lists
• Return Single Typed Values
• Insert Data
• Update Data
• Delete Data
• Execute Stored Procedures
• Select Filtered Lists
• Select Ordered Lists
• Perform Aggregate Functions

There is a great deal more that one can do with LINQ to SQL that is not contained in this demonstration; however, the demonstration was geared towards the mechanics of performing the most typical types of queries that might be required within a data driven application.

LINQ to SQL Statements

This section will discuss some of the common techniques used in LINQ to SQL statement construction. In a nutshell, LINQ to SQL provides the developer with the means to conduct queries against a relational database through a LINQ to SQL database model and related data context.

Data Context

The data context provides the mapping of all entities (essentially tables) to the database. It is through the data context that the application can query the database, and it is through the data context that changes to the database can be executed.

Anatomy of LINQ to SQL Statements

Example 1 – A Simple Select

This is an example of a very simple LINQ to SQL statement:

public void SimpleQuery()
{
    DataClasses1DataContext dc = new DataClasses1DataContext();

    var q =
        from a in dc.GetTable<Order>()
        select a;

    dataGridView1.DataSource = q;
}

In the example, an instance of the data context is created, and then a query is formed to get all of the values in the table; once the query runs, the result is used as the data source of a DataGridView control, and the results are displayed in the grid:

var q =
    from a in dc.GetTable<Order>()
    select a;

Since the GetTable function in the data context returns the entire table, this query is pretty useless, but it does work, and it is representative of a simple select query. You could accomplish the same task using this code:

public void SimpleQuery2()
{
    DataClasses1DataContext dc = new DataClasses1DataContext();
    dataGridView1.DataSource = dc.GetTable<Order>();
}

If you were to create a project, add either bit of code to a method and run it, the results would look like this:

Example 2 – Select with a Where Clause

The next example shows a LINQ to SQL query that incorporates a where clause. In this example, we get a data context to work with first, and then query the Orders table to find a customer with the customer ID starting with the letter “A”; the results are then bound to a DataGridView control.

public void SimpleQuery3()
{
    DataClasses1DataContext dc = new DataClasses1DataContext();

    var q =
        from a in dc.GetTable<Order>()
        where a.CustomerID.StartsWith("A")
        select a;

    dataGridView1.DataSource = q;
}

If you were to run the query, the results would appear as follows:

Example 3 – Select with a Where Clause

In a slight variation to the previous query, this example looks for an exact match in its where clause:

public void SimpleQuery3()
{
    DataClasses1DataContext dc = new DataClasses1DataContext();

    var q =
        from a in dc.GetTable<Order>()
        where a.CustomerID == "VINET"
        select a;

    dataGridView1.DataSource = q;
}

Running this code will display this result:

Example 4 – Generating an Ordered List

In this query, the list of orders is ordered (using “orderby a.OrderDate ascending”):

public void SimpleQuery5()
{
    DataClasses1DataContext dc = new DataClasses1DataContext();

    var q =
        from a in dc.GetTable<Order>()
        where a.CustomerID.StartsWith("A")
        orderby a.OrderDate ascending
        select a;

    dataGridView1.DataSource = q;
}

Example 5 – Working with a Custom Type

In this example, a query is built to return a list of a custom type (CustomerOrderResult).

public void GetCustomerOrder()
{
    DataClasses1DataContext dc = new DataClasses1DataContext();

    var q= (from orders in dc.GetTable<Order>()
            from orderDetails in dc.GetTable<Order_Detail>()
            from prods in dc.GetTable<Product>()
            where ((orderDetails.OrderID == orders.OrderID) &&
                 (prods.ProductID == orderDetails.ProductID) &&
                 (orders.EmployeeID == 1))
            orderby orders.ShipCountry
            select new CustomerOrderResult
            {
                CustomerID = orders.CustomerID,
                CustomerContactName = orders.Customer.ContactName,
                CustomerCountry = orders.Customer.Country,
                OrderDate = orders.OrderDate,
                EmployeeID = orders.Employee.EmployeeID,
                EmployeeFirstName = orders.Employee.FirstName,
                EmployeeLastName = orders.Employee.LastName,
                ProductName = prods.ProductName
            }).ToList<CustomerOrderResult>();

    dataGridView1.DataSource = q;
}

The “select new” in the query defines the result type, and then sets each of the properties in the type to a value returned by the query. At the end of the query, the output is converted to a List of the CustomerOrderResult type.

The displayed results of running the query are:

The CustomerOrderResult class used as the type behind the list is as follows:

public class CustomerOrderResult
{
    public System.String CustomerID
        {get;set;}
    public System.String CustomerContactName
        {get;set;}
    public System.String CustomerCountry
        {get;set;}
    public System.Nullable<System.DateTime> OrderDate
        {get;set;}
    public System.Int32 EmployeeID
        {get;set;}
    public System.String EmployeeFirstName
        {get;set;}
    public System.String EmployeeLastName
        {get;set;}
    public System.String ProductName
        {get;set;}
}

Example 6 – Searching an Existing List<T> Using LINQ to Objects

In this example, a typed list is created (as in the previous example) using LINQ to SQL, populated, and then the returned typed list is queried using LINQ to Objects. In this case, the query includes a where clause that only returns matches were the customer ID begins with “RICAR”:

public void GetCustomerOrder2()
{
    DataClasses1DataContext dc = new DataClasses1DataContext();

    var query = (from orders in dc.GetTable<Order>()
             from orderDetails in dc.GetTable<Order_Detail>()
             from prods in dc.GetTable<Product>()
             where ((orderDetails.OrderID == orders.OrderID)
                  && (prods.ProductID == orderDetails.ProductID)
                  && (orders.EmployeeID == 1))
             orderby orders.ShipCountry
             select new CustomerOrderResult
             {
                 CustomerID = orders.CustomerID,
                 CustomerContactName = orders.Customer.ContactName,
                 CustomerCountry = orders.Customer.Country,
                 OrderDate = orders.OrderDate,
                 EmployeeID = orders.Employee.EmployeeID,
                 EmployeeFirstName = orders.Employee.FirstName,
                 EmployeeLastName = orders.Employee.LastName,
                 ProductName = prods.ProductName
             }).ToList<CustomerOrderResult>();

    var matches = (from c in query
                  where c.CustomerID == "RICAR"
                  select c).ToList<CustomerOrderResult>();

    dataGridView1.DataSource = matches;
}

Example 7 – Searching an Existing List<T> Using LINQ to Objects and Returning a Single Result

In this example, a typed list is created (as in the previous example), populated, and then queried using LINQ to Objects. In this case, a single result of type “Parts” is returned:

public void GetEmployeeLastName()
{
    DataClasses1DataContext dc = new DataClasses1DataContext();

    var query = (from orders in dc.GetTable<Order>()
                 select orders);

    var matches = (from c in query
                   where c.OrderID == 10248
                   select 
                   c.Employee.LastName).SingleOrDefault<System.String>();

    MessageBox.Show(matches);

}

The results are displayed as:

The preceding examples were intended to provide a simple overview as to how to conduct some basic queries against collections using LINQ to SQL and LINQ to Objects; there are certainly a great number of more complex operations that can be executed using similar procedures (groups and aggregation, joins, etc.), however, the examples provided are representative of some of the more common types of queries.

Getting Started

There is a single solution included with this download. The solution contains a WinForms project called “LINQSQLCS”; this project contains a form - the main form used to display the results of the demonstration queries (frmMain), a LINQ to SQL database model (NorthWindDataClasses.dbml) along with the models designer code and the layout file, and a class, “Accessor”, which contains the code used to perform the LINQ to SQL queries used in the demonstration.

If you open the attached project into Visual Studio 2008, you should see the following in the Solution Explorer:

The demonstration relies upon an instance of the Northwind database running in SQL Server 2005. The database can be downloaded from here; the database was created for SQL Server 2000, but you can install the database and attach to it using a local instance of SQL Server 2005.

Once the database is installed, you will want to update the connection string found in the project settings. Open the settings and click on the button (showing an ellipsis) to set the connection string.

When starting from scratch, in order to add LINQ to SQL to a project, open the “Add New Item” dialog, and select the LINQ to SQL Classes item (Figure 11); name the data classes and then select the “Add” button to add the item to the project. Once set, set the connection string for the data classes, and then open the Server Explorer to drag tables and stored procedures onto the designer (dragging the tables into the left hand side of the workspace and stored procedures into the right hand side of the workspace (Figure 12)). Once that is done, build the project to generate the LINQ to SQL code.

This project is intended for Visual Studio 2008 with the .NET framework version 3.5.

Code: Accessor.cs

The Accessor class is used to the store all of the functions used to execute LINQ to SQL queries against the database. The functions contained in this class are all static, and include a mixed bag of selects, inserts, updates, deletes, and stored procedure evocations. You may wish to implement some business logic in the extensibility methods defined in the auto-generated code contained in the designer file, but I chose not to in this example.

The class begins with the normal and default imports:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Data.Linq;
using System.Text;

The next section contains the namespace and class declarations.

namespace LINQSQLCS
{

    /// <summary>
    /// This class defines functions used to
    /// select, insert, update, and delete data
    /// using LINQ to SQL and the defined
    /// data context
    /// </summary>
    public class Accessor
    {

Next up is a region containing all of the functions used to return full tables from the database through the data context. All of the functions work essentially the same way; the data context includes a function called GetTable<T>, which is used to return a table of the indicated type. Each example gets a data context, and then evokes the GetTable function to return the full table of the indicated type.

#region  Full Table

    // This section contains examples of
    // pulling back entire tables from
    // the database

    /// <summary>
    /// Displays the full Employee table
    /// </summary>
    /// <returns></returns>
    public static System.Data.Linq.Table<Employee> GetEmployeeTable()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();
        return dc.GetTable<Employee>();
    }


    /// <summary>
    /// Displays the full Shipper table
    /// </summary>
    /// <returns></returns>
    public static System.Data.Linq.Table<Shipper> GetShipperTable()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();
        return dc.GetTable<Shipper>();
    }


    /// <summary>
    /// Displays the full Order table
    /// </summary>
    /// <returns></returns>
    public static System.Data.Linq.Table<Order> GetOrderTable()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();
        return dc.GetTable<Order>();
    }


    /// <summary>
    /// Displays the full EmployeeTerritory table
    /// </summary>
    /// <returns></returns>
    public static System.Data.Linq.Table<EmployeeTerritory> 
    GetEmployeeTerritoryTable()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();
        return dc.GetTable<EmployeeTerritory>();
    }


    /// <summary>
    /// Displays Territory Table
    /// </summary>
    /// <returns></returns>
    public static System.Data.Linq.Table<Territory> GetTerritoryTable()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();
        return dc.GetTable<Territory>();
    }

    /// <summary>
    /// Displays the full Region table
    /// </summary>
    /// <returns></returns>
    public static System.Data.Linq.Table<Region> GetRegionTable()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();
        return dc.GetTable<Region>();
    }

    /// <summary>
    /// Displays the full Customer table
    /// </summary>
    /// <returns></returns>
    public static System.Data.Linq.Table<Customer> GetCustomerTable()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();
        return dc.GetTable<Customer>();
    }


    /// <summary>
    /// Displays the full CustomerCustomerDemo table
    /// </summary>
    /// <returns></returns>
    public static System.Data.Linq.Table<CustomerCustomerDemo> 
    GetCustomerDemoTable()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();
        return dc.GetTable<CustomerCustomerDemo>();
    }


    /// <summary>
    /// Displays the full CustomerDemographic table
    /// </summary>
    /// <returns></returns>
    public static System.Data.Linq.Table<CustomerDemographic> 
    GetCustomerDemographicTable()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();
        return dc.GetTable<CustomerDemographic>();
    }


    /// <summary>
    /// Displays the full Order_Detail table
    /// </summary>
    /// <returns></returns>
    public static System.Data.Linq.Table<Order_Detail> GetOrderDetailsTable()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();
        return dc.GetTable<Order_Detail>();
    }


    /// <summary>
    /// Displays the full Product table
    /// </summary>
    /// <returns></returns>
    public static System.Data.Linq.Table<Product> GetProductTable()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();
        return dc.GetTable<Product>();
    }


    /// <summary>
    /// Displays the full Supplier table
    /// </summary>
    /// <returns></returns>
    public static System.Data.Linq.Table<Supplier> GetSupplierTable()
    {
        NorthWindDataClassesDataContext dc = new             
        NorthWindDataClassesDataContext();
        return dc.GetTable<Supplier>();
    }


    /// <summary>
    /// Displays the full Category table
    /// </summary>
    /// <returns></returns>
    public static System.Data.Linq.Table<Category> GetCategoryTable()
    {
        NorthWindDataClassesDataContext dc = new  
        NorthWindDataClassesDataContext();
        return dc.GetTable<Category>();
    }

#endregion

That next region contained in the Accessor class is the Queries region; this region contains examples of different types of select queries that may be performed using LINQ to SQL. Each query is described in the annotation:

#region Queries

    // This region contains examples of some
    // of the sorts of queries that can be
    // executed using LINQ to SQL

    /// <summary>
    /// Example:  Where Clause
    /// Returns an employee where the
    /// employee ID matches the value
    /// passed in as empID
    /// </summary>
    /// <param name="empId"></param>
    /// <returns>The single matching or default value</returns>
    public static Employee GetEmployeeById(int empId)
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();

        return (from e in dc.GetTable<Employee>()
                where (e.EmployeeID == empId)
                select e).SingleOrDefault<Employee>();
    }

    /// <summary>
    /// Example:  Select to a single returned object
    /// using a Where Clause
    /// 
    /// Returns the first matching order
    /// </summary>
    /// <param name="orderId"></param>
    /// <returns>The single matching or default value</returns>
    public static Order GetOrderById(int orderId)
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();

        return (from ord in dc.GetTable<Order>()
                where (ord.OrderID == orderId)
                select ord).SingleOrDefault<Order>();
    }

    /// <summary>
    /// Example:  Select to a typed List
    /// using a Where Clause
    /// </summary>
    /// <param name="orderId"></param>
    /// <returns></returns>
    public static List<Order> GetOrdersById(int orderId)
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();

        return (from ord in dc.GetTable<Order>()
                where (ord.OrderID == orderId)
                select ord).ToList<Order>();
    }

    /// <summary>
    /// Example:  Return an ordered list
    /// 
    /// Converts the returned value to a List
    /// of type Employee; the list is ordered
    /// by hire date
    /// </summary>
    /// <returns></returns>
    public static List<Employee> GetEmployeesByHireDate()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();

        return (from emp in dc.GetTable<Employee>()
                orderby emp.HireDate ascending
                select emp).ToList<Employee>();
    }

    /// <summary>
    /// This class is used to define the return type
    /// for the next function - OrdersAndDetails
    /// 
    /// When results are extracted from multiple tables
    /// you can either return the results as anonymous
    /// or as a type; this class defines the return
    /// type used by OrdersAndDetails
    /// </summary>
    public class OrdersAndDetailsResult
    {
        public System.String CustomerID
        { get; set; }
        public System.Nullable<System.DateTime> OrderDate
        { get; set; }
        public System.Nullable<System.DateTime> RequiredDate
        { get; set; }
        public System.String ShipAddress
        { get; set; }
        public System.String ShipCity
        { get; set; }
        public System.String ShipCountry
        { get; set; }
        public System.String ShipZip
        { get; set; }
        public System.String ShippedTo
        { get; set; }
        public System.Int32 OrderID
        { get; set; }
        public System.String NameOfProduct
        { get; set; }
        public System.String QtyPerUnit
        { get; set; }
        public System.Nullable<System.Decimal> Price
        { get; set; }
        public System.Int16 QtyOrdered
        { get; set; }
        public System.Single Discount
        { get; set; }
    }

    /// <summary>
    /// Example:  Joins
    /// Joining using the join keyword
    /// 
    /// The values are set to each of the
    /// properties contained in the 
    /// OrdersAndDetailsResult class
    /// 
    /// The value returned is converted
    /// to a list of the specified type
    /// </summary>
    /// <returns></returns>
    public static List<OrdersAndDetailsResult> OrdersAndDetails()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();

        return (from ords in dc.GetTable<Order>()
                join dets in dc.GetTable<Order_Detail>()
                    on ords.OrderID equals dets.OrderID
                orderby ords.CustomerID ascending
                    select new OrdersAndDetailsResult
                    {
                        CustomerID = ords.CustomerID,
                        OrderDate = ords.OrderDate,
                        RequiredDate = ords.RequiredDate,
                        ShipAddress = ords.ShipAddress,
                        ShipCity = ords.ShipCity,
                        ShipCountry = ords.ShipCountry,
                        ShipZip = ords.ShipPostalCode,
                        ShippedTo = ords.ShipName,
                        OrderID = ords.OrderID,
                        NameOfProduct = dets.Product.ProductName,
                        QtyPerUnit = dets.Product.QuantityPerUnit,
                        Price = dets.Product.UnitPrice,
                        QtyOrdered = dets.Quantity,
                        Discount = dets.Discount
                    }
                ).ToList <OrdersAndDetailsResult>();
    }


    /// <summary>
    /// Defined to support following function:
    /// GetOrderAndPricingInformation - this class
    /// supplies the return type for that function
    /// </summary>
    public class OrderandPricingResult
    {
        public System.Int32 OrderID
        { get; set; }
        public System.String Company
        { get; set; }
        public System.String OrderCountry
        { get; set; }
        public System.String ProductName
        { get; set; }
        public System.Nullable<System.Decimal> UnitPrice
        { get; set; }
        public System.Int16 UnitsOrder
        { get; set; }
        public System.String ShipperName
        { get; set; }
        public System.String SalesFirstName
        { get; set; }
        public System.String SalesLastName
        { get; set; }
        public System.String SalesTitle
        { get; set; }
    }

    /// <summary>
    /// Example:  Query across an entity ref
    /// This example collections information from the orders table
    /// and the order_details table through the orders table
    /// entity association to the orders_details table. 
    /// 
    /// An entity is a representation in the model of a table
    /// in the database, foreign key relationships are maintained
    /// as entity references to the related tables in the model.
    /// It is possible to query across tables through this
    /// relationship in LINQ to SQL
    /// </summary>
    /// <returns></returns>
    public static List<OrderandPricingResult> GetOrderAndPricingInformation()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();

        return (from ords in dc.Orders // orders table
                from dets in ords.Order_Details     // entity set in orders table
                select new OrderandPricingResult
                { 
                    OrderID = ords.OrderID, 
                    Company =ords.Customer.CompanyName,   
                    OrderCountry = ords.Customer.Country,
                    ProductName = dets.Product.ProductName,
                    UnitPrice = dets.Product.UnitPrice,
                    UnitsOrder = dets.Quantity,
                    ShipperName = ords.Shipper.CompanyName,
                    SalesFirstName = ords.Employee.FirstName,
                    SalesLastName = ords.Employee.LastName,
                    SalesTitle = ords.Employee.Title
                }).ToList < OrderandPricingResult>();
    }

    /// <summary>
    /// Example:  Query across entity ref with Where class
    /// Same as previous function with added where clause
    /// 
    /// An entity is a representation in the model of a table
    /// in the database, foreign key relationships are maintained
    /// as entity references to the related tables in the model.
    /// It is possible to query across tables through this
    /// relationship in LINQ to SQL
    /// </summary>
    /// <param name="orderId"></param>
    /// <returns></returns>
    public static List<OrderandPricingResult> 
    GetOrderAndPricingInformationByOrderId(int orderId)
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();

        return (from ords in dc.Orders  // orders table
                from dets in ords.Order_Details // entity set in orders table
                where ords.OrderID == orderId
                select new OrderandPricingResult
                {
                    OrderID = ords.OrderID,
                    Company = ords.Customer.CompanyName,
                    OrderCountry = ords.Customer.Country,
                    ProductName = dets.Product.ProductName,
                    UnitPrice = dets.Product.UnitPrice,
                    UnitsOrder = dets.Quantity,
                    ShipperName = ords.Shipper.CompanyName,
                    SalesFirstName = ords.Employee.FirstName,
                    SalesLastName = ords.Employee.LastName,
                    SalesTitle = ords.Employee.Title
                }).ToList<OrderandPricingResult>();
    }

    /// <summary>
    /// Example:  Aggregation
    /// 
    /// Returns the total sum of the order 
    /// selected by order ID by selecting
    /// unit price multiplied by quantity
    /// ordered and then calling sum for 
    /// the total
    /// </summary>
    /// <param name="orderID"></param>
    /// <returns></returns>
    public static decimal? GetOrderValueByOrderId(int orderID)
    {

        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();

        var matches = 
            (from od in dc.GetTable<Order_Detail>()
            where od.OrderID == orderID
            select od.Product.UnitPrice * od.Quantity).Sum();

        return matches;

    }

    /// <summary>
    /// Example:  Using Take to get a limited
    /// number of returned values for display and 
    /// using Skip to sequence to a different 
    /// starting point within the returned values -
    /// can be used to navigate through a large
    /// list
    /// </summary>
    /// <param name="SkipNumber"></param>
    /// <returns></returns>
    public static List<Order> GetTopFiveOrdersById(int SkipNumber)
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();

        return (from ord in dc.GetTable<Order>()
                orderby ord.OrderID ascending
                select ord).Skip(SkipNumber).Take(5).ToList<Order>();
    }

#endregion

The next region is “Inserting, Updating, and Deleting Data”; it contains examples of how to insert or update data, and an example showing how to delete data from the database. Each function is described in its annotation:

#region Inserting, Updating, Deleting Data

    /// <summary>
    /// Insert or Update a Customer Record
    /// 
    /// If the customer ID exists, the existing
    /// customer record is updated.
    /// 
    /// If the customer ID does not exist, the
    /// new customer record is inserted into
    /// the database
    /// </summary>
    /// <param name="customerId"></param>
    /// <param name="companyName"></param>
    /// <param name="contactName"></param>
    /// <param name="contactTitle"></param>
    /// <param name="address"></param>
    /// <param name="city"></param>
    /// <param name="region"></param>
    /// <param name="postalCode"></param>
    /// <param name="country"></param>
    /// <param name="phone"></param>
    /// <param name="fax"></param>
    public static void InsertOrUpdateCustomer(string customerId, string 
        companyName, string contactName, string contactTitle, string address, 
        string city, string region, string postalCode, string country, string 
        phone, string 
        fax)
    {

        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();

        var matchedCustomer = (from c in dc.GetTable<Customer>()
                               where c.CustomerID == customerId
                               select c).SingleOrDefault();

        if(matchedCustomer == null)
        {
            try
            {
                // create new customer record since customer ID
                // does not exist
                Table<Customer> customers = Accessor.GetCustomerTable();
                Customer cust = new Customer();

                cust.CustomerID = customerId;
                cust.CompanyName = companyName;
                cust.ContactName = contactName;
                cust.ContactTitle = contactTitle;
                cust.Address = address;
                cust.City = city;
                cust.Region = region;
                cust.PostalCode = postalCode;
                cust.Country = country;
                cust.Phone = phone;
                cust.Fax = fax;

                customers.InsertOnSubmit(cust);
                customers.Context.SubmitChanges();
            }
            catch (Exception ex)
            {
                throw ex;
            }  
        }
        else
        {
            try
            {
                matchedCustomer.CompanyName = companyName;
                matchedCustomer.ContactName = contactName;
                matchedCustomer.ContactTitle = contactTitle;
                matchedCustomer.Address = address;
                matchedCustomer.City = city;
                matchedCustomer.Region = region;
                matchedCustomer.PostalCode = postalCode;
                matchedCustomer.Country = country;
                matchedCustomer.Phone = phone;
                matchedCustomer.Fax = fax;

                dc.SubmitChanges();
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }

    }


    /// <summary>
    /// Delete a customer by customer ID
    /// </summary>
    /// <param name="customerID"></param>
    public static void DeleteCustomer(string customerID)
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();

        var matchedCustomer = (from c in dc.GetTable<Customer>()
                               where c.CustomerID == customerID
                               select c).SingleOrDefault();

        try
        {
            dc.Customers.DeleteOnSubmit(matchedCustomer);
            dc.SubmitChanges();
        }
        catch (Exception ex)
        {
            throw ex;
        }
    }

#endregion

The last region of the class contains the code used to execute stored procedures. The stored procedures, once added to the project, may be immediately accessed through the data context; to access a stored procedure, just get an instance of the data context and call the stored procedure, passing along any required parameters as arguments to the function call:

#region Stored Procedures

    /// <summary>
    /// Stored Procedure:  Sales By Year
    /// </summary>
    /// <param name="beginningYear"></param>
    /// <param name="endingYear"></param>
    /// <returns></returns>
    public static List<Sales_by_YearResult> SalesByYear(DateTime? beginningYear, 
    DateTime? endingYear)
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();
        
        return dc.Sales_by_Year(beginningYear, 
        endingYear).ToList<Sales_by_YearResult>();
    }

    /// <summary>
    /// Stored Procedure:  Ten Most Expenisve Products
    /// </summary>
    /// <returns></returns>
    public static List<Ten_Most_Expensive_ProductsResult> 
    TenMostExpensiveProducts()
    {
        NorthWindDataClassesDataContext dc = new 
        NorthWindDataClassesDataContext();

        return          
        dc.Ten_Most_Expensive_Products().ToList<Ten_Most_Expensive_ProductsResult>();

    }

#endregion

That concludes the description of the “Accessor” class.

Code: Main Application Form (frmMain.cs)

This is the main form of the application; this form is used to provide a test harness for testing each of the functions defined in the Accessor class. All functions defined in the Accessor class have a corresponding menu item, and the Click event handler for each menu item executes an Accessor class function, supplying any arguments necessary as canned values.

The structure for the main form’s menu is as follows:

• Menu
• File
• Exit
• Read
• Tables
• Queries
• Stored Procedures
• Insert/Update/Delete

The class begins with the normal and default imports:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

The next section contains the namespace and class declarations.

namespace LINQSQLCS
{
    /// <summary>
    /// This class is used to demonstrate each of the
    /// queries defined in the accessor class
    /// </summary>
    public partial class frmMain : Form
    {

Next is the definition of a private variable used to maintain the position within the orders table; it is used in an example showing how to make use of the Skip and Take functions.

// used to support take/skip example
private int OrderPosition;

The next region of code in the class contains the constructor. The constructor sets the OrderPosition integer value to zero.

#region Constructor

public frmMain()
{
    InitializeComponent();

    // set order position to zero
    OrderPosition = 0;
}

#endregion

The next code region is called ‘Full Table Requests’. Each of the functions operates in a similar manner in that a function creates a list of the type returned by the Accessor class function called, evokes the Accessor function, and then assigns the returned list to the data source property of the DataGridView control contained in the main form.

#region  Full Table Requests

    private void employeesToolStripMenuItem_Click(object sender, EventArgs e)
    {
        System.Data.Linq.Table<Employee> emp = Accessor.GetEmployeeTable();
        dataGridView1.DataSource = emp;
    }


    private void shippersToolStripMenuItem_Click(object sender, EventArgs e)
    {
        System.Data.Linq.Table<Shipper> ship = Accessor.GetShipperTable();
        dataGridView1.DataSource = ship;
    }


    private void ordersToolStripMenuItem_Click(object sender, EventArgs e)
    {
        System.Data.Linq.Table<Order> orders = Accessor.GetOrderTable();
        dataGridView1.DataSource = orders;
    }

    private void employeeTerritoryToolStripMenuItem_Click(object sender, 
    EventArgs e)
    {
        System.Data.Linq.Table<EmployeeTerritory> empTerrs = 
        Accessor.GetEmployeeTerritoryTable();

        dataGridView1.DataSource = empTerrs;
    }


    private void territoryToolStripMenuItem_Click(object sender, EventArgs e)
    {
        System.Data.Linq.Table<Territory> terrs = Accessor.GetTerritoryTable();
        dataGridView1.DataSource = terrs;
    }


    private void regionToolStripMenuItem_Click(object sender, EventArgs e)
    {
        System.Data.Linq.Table<Region> regs = Accessor.GetRegionTable();
        dataGridView1.DataSource = regs;
    }


    private void customerToolStripMenuItem_Click(object sender, EventArgs e)
    {
        System.Data.Linq.Table<Customer> cust = Accessor.GetCustomerTable();
        dataGridView1.DataSource = cust;
    }


    private void customerDemoToolStripMenuItem_Click(object sender, EventArgs e)
    {
        System.Data.Linq.Table<CustomerCustomerDemo> custdemo = 
        Accessor.GetCustomerDemoTable();

        dataGridView1.DataSource = custdemo;
    }


    private void customerDemographicToolStripMenuItem_Click(object sender, 
    EventArgs e)
    {
        System.Data.Linq.Table<CustomerDemographic> custdemograph = 
        Accessor.GetCustomerDemographicTable();

        dataGridView1.DataSource = custdemograph;
    }


    private void orderDetailsToolStripMenuItem_Click(object sender, EventArgs e)
    {
        System.Data.Linq.Table<Order_Detail> ordDetails = 
        Accessor.GetOrderDetailsTable();
        
        dataGridView1.DataSource = ordDetails;
    }


    private void productToolStripMenuItem_Click(object sender, EventArgs e)
    {
        System.Data.Linq.Table<Product> prods = Accessor.GetProductTable();
        dataGridView1.DataSource = prods;
    }

    private void supplierProductToolStripMenuItem_Click(object sender, EventArgs e)
    {
        System.Data.Linq.Table<Supplier> prods = Accessor.GetSupplierTable();
        dataGridView1.DataSource = prods;
    }


    private void categoToolStripMenuItem_Click(object sender, EventArgs e)
    {
        System.Data.Linq.Table<Category> cats = Accessor.GetCategoryTable();
        dataGridView1.DataSource = cats;
    }

#endregion

The next region contains the menu item Click event handlers used to execute each of the queries described in the queries region of the Accessor class. Each function is annotated to describe what it does and what it is intended to demonstrate.

#region Queries

    /// <summary>
    /// Find and display an employee by 
    /// the employee's ID
    /// </summary>
    /// <param name="sender"></param>
    /// <param name="e"></param>
    private void employeeByIDToolStripMenuItem_Click(object sender, EventArgs e)
    {

        Employee emp = Accessor.GetEmployeeById(1);

        StringBuilder sb = new StringBuilder();
        sb.Append("Employee 1: " + Environment.NewLine);
        sb.Append("Name: " + emp.FirstName + " " + emp.LastName + 
        Environment.NewLine);

        sb.Append("Hire Date: " + emp.HireDate + Environment.NewLine);
        sb.Append("Home Phone: " + emp.HomePhone + Environment.NewLine);

        MessageBox.Show(sb.ToString(),