expression tree problem

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Hello
I made one page with 2 text boxes and 1 button and 1 grid view. Then I used the below code for button click of the page. Now when I enter some value in text boxes for example "France" in textbox1 and "Paris" in textbox2 nothing will be happen and I have no result.
Also there is error for this line:

C#

var query = Queryable.Where(db.Customers, (Func<Customer, Boolean>)predicate.Compile());

Thanks in advanced.
Code:

C#

namespace WebApplication1
{
    public partial class WebForm2 : System.Web.UI.Page
    {
       

        protected void Button1_Click(object sender, EventArgs e)
        {
            DataClasses1DataContext db = new DataClasses1DataContext();
            db.Log = Console.Out;
            var customer = Expression.Parameter(typeof(Customer), "customer");
            var countryExpression = Expression.Equal(
Expression.Property(customer,"country"),
Expression.Constant(TextBox1.Text));
            var cityExpression = Expression.Equal(
Expression.Property(customer, "City"),
Expression.Constant(TextBox2.Text));
            var andExpression = Expression.And(countryExpression,
cityExpression);
            var predicate = Expression.Lambda(andExpression, customer);
            var query = Queryable.Where(db.Customers, (Func<Customer, Boolean>)predicate.Compile());

            GridView1.DataSource = query;
            GridView1.DataBind();

        }
    }
}

Posted 23-Oct-11 11:18am

alimahdipour

Updated 23-Oct-11 17:00pm

Sergey Alexandrovich Kryukov

v2

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Sergey Alexandrovich Kryukov · Answer 1 · 2011-10-23T16:58:00

Solution 1

This is not nice to say "there is error for this line" without telling the error message. Why, to make answering harder? Why, again.

Nevertheless, in this particular case it's apparent, that you're trying to case (Func<customer,>)predicate.Compile(), that is, a function with not arguments to a predicate of one argument of Customer, so how can you possible expect it to work? The declaration of predicate is not shown.

As to the rest of it, the idea is just not clear, and the shown code is incomplete. "I have no results" is not enough if you don't explain what results did you expect and why did you need such "results".

—SA

Posted 23-Oct-11 16:58pm

Sergey Alexandrovich Kryukov

Comments

alimahdipour 24-Oct-11 4:16am

Excuse me SAKryukov , my error is like below:

Error 1 The type arguments for method 'System.Linq.Queryable.Where<tsource>(System.Linq.IQueryable<tsource>, System.Linq.Expressions.Expression<system.func<tsource,int,bool>>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

Actually in the book, I am studying; there is one code for linq to objects like below that is working, just in my previous code i liked to change the code in the book:
1- Linq to sql (northwind) instead of linq to objects.
2- Using text boxes instead of constant value.
3- Using Queryable instead of Enumerable.

Please help what changes I have to do in expression tree.
Thanks a lot.

The code in the book:

var book = Expression.Parameter(typeof(Book), "book");
var titleExpression = Expression.NotEqual(
Expression.Property(book, "Title"),
Expression.Constant("Funny Stories"));
var pageCountExpression = Expression.GreaterThan(
Expression.Property(book, "PageCount"),
Expression.Constant(100));
var andExpression = Expression.And(titleExpression,
pageCountExpression);
var predicate = Expression.Lambda(andExpression, book);
var query = Enumerable.Where(SampleData.Books,
(Func<book, boolean="">)predicate.Compile());

Sergey Alexandrovich Kryukov 24-Oct-11 14:20pm

OK, this is different story. Shouldn't it be Funk<book>, not Funk<book,>? Still, there is no definition of Compile.
--SA

Espen Harlinn 24-Oct-11 14:15pm

Good guess :)

Sergey Alexandrovich Kryukov 24-Oct-11 14:16pm

Thank you, Espen.
--SA

alimahdipour 25-Oct-11 3:26am

my book is "LINQ in Action" and i do not think this is funk, maybe my question is not clear and understandable because i am new to linq.
thanks.

Amir Mahfoozi · Answer 2 · 2011-10-23T20:07:00

If you had already checked the Pluralize or singularize generated object names you have access to the customers list by Customers object. So now if you do something like this you will probably get to what you want:

C#

DataClasses1DataContext db = new DataClasses1DataContext();

GridView1.DataSource = db.Customers.Where(i=> i.country == textbox1.Text && i.City == textbox2.Text);
GridView1.DataBind();