Populate Data from Database in a ComboBox

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Introduction

This article is the second way (version) of "How to Populate Data in a ComboBox" - program. For doing that, I make use of 'TableMapping' and 'DataViewManager' that I'll explain later in the section: "Code and How It Works".

The application allows you to select a StudentID in a ComboBox and displays the columns StudentID, Student Subject and Student Name from the table in the 3 TextBoxes.

It also shows how to get connected to a MS-Access database which you can also find in the project included (sudentDB.mdb). I chose MS-Access database because not many people have SQL-Server running.

Form:

Code and How It Works

Before starting, a short explanation of what TableMapping and DataViewManager means.

TableMapping is the process that controls how data adapters copy tables and columns of data from a physical data source to ADO.NET in-memory objects. When a data adapter reads data from a data source, it determines where to put the data in the corresponding DataSet table (or tables), using a table mapping. If you create a mapping in a DataAdapter, it allows you to establish a correspondence between columns in the data source and columns in a DataSet table.

A DataAdapter contains a collection of DataTableMapping objects in its TableMappings property. You can pass the DataTableMapping name in place of the DataTable name to the Fill method of the DataAdapter.

The following example creates a DataTableMapping named "MyStudentMappings" for the table "studentTable".

dAdapter.TableMappings.Add("MyStudentMappings", "studentTable");

When you call the Fill method of the DataAdapter and do not specify a TableName or DataTableMapping name, the DataAdapter looks for a DataTableMapping called "Table". If you leave out that DataTableMapping, TableName of the DataTable will be "Table". This is the default DataTableMapping.

For example:

dAdapter.TableMappings.Add("Table", "studentTable");

If the SELECT command creates a result set with a default name of Table, then its contents go into a new or existing DataTable object called "studentTable". The DataTableMapping object describes a mapped relationship between a SQL-result set and a DataTable object in a DataSet.

The DataViewManager represents a view onto an entire DataSet and represents a databindable, customized view of a DataTable for sorting, filtering, searching, editing, and navigation, whereas the DataView class acts as a view onto a single DataTable. A DataViewManager is an object that contains a collection of DataViews. The DataViewManager returned by the DefaultViewManager property allows you to create custom settings for each DataTable in the DataSet.

When you want to bind a control to more than one table of a DataSet, binding to a DataViewManager is the ideal choice. When there is only a single DataTable then use DataView. When binding a DataSet, .NET automatically uses the corresponding DataViewManager provided through the DataSet.DefaultViewManager property:

For example:

this.dviewmanager=dset.DefaultViewManager;

Here is the code for the method fnGetConnectedToDatabase():

private void fnGetConnectedToDatabase() 
{
    //Connection string 
    string conStr="Provider=Microsoft.Jet.OLEDB.4.0;" + 
                  " Data Source=..\\..\\studentDB.mdb"; 
    try 
     {
        //Instantiate OleDbConnection and open 
        //the connection to the database 
        myConn = new OleDbConnection(conStr); 
        myConn.Open(); 
    }
    catch(OleDbException ex)    {
        //get the error message if connection failed 
        MessageBox.Show("Error in connection ..."+ex.Message); 
    } 
    string sqlStr ="SELECT * FROM studentTable;"; 
    //Instantiate a DataAdapter by passing the sqlStr and myConn. 
    //now data is in raw form 
    dAdapter = new OleDbDataAdapter(sqlStr,myConn); 
    //Instantiate a DataSet 
    dset = new DataSet(); 
    //Gets a collection that provides the master mapping 
    // between a source table and a DataTable 
    dAdapter.TableMappings.Add("Table", "studentTable"); 
    //A data adapter object utilizes the Fill method 
    //to populate a DataSet or a DataTable object with
    //data retrieved by a SELECT command. 
    dAdapter.Fill(dset); 
    //When binding a DataSet, .NET automatically uses the corresponding 
    //DataViewManager provided through the DataSet.DefaultViewManager property 
    this.dviewmanager=dset.DefaultViewManager; 
    this.comboBox1.DataSource=this.dviewmanager; 
    //display "studentTable.StudentID" in the ComboBox  
    this.comboBox1.DisplayMember="studentTable.StudentID"; 
    //DataBinding for the TextBox controls 
    this.textBox1.DataBindings.Add("Text", 
              this.dviewmanager,"studentTable.StudentID"); 
    this.textBox2.DataBindings.Add("Text", 
              this.dviewmanager, "studentTable.StudentSubject"); 
    this.textBox3.DataBindings.Add("Text", 
              this.dviewmanager, "studentTable.StudentName"); 
    // Close the connection to the database. 
    this.myConn.Close(); 
}

As soon as you select a StudentID from the ComboBox, the columns StudentID, StudentSubject and StudentName in the table "studentTable" are displayed in the three TextBoxes.

Conclusion

There are, of course, many ways to get data into a ComboBox control. I just tried to show a different way.

Good coding!

History

• 17^th August, 2004: Initial version

License

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