Question

I have a combo box in Silverlight. It has a collection of values built out of the properties of one of my LINQ-to-SQL objects (ie Name, Address, Age, etc…). I would like to filter my results based off the value selected in a combo box.

Example: Say I want everyone with a last name ‘Smith’. I’d select ‘Last Name’ from the drop down list and enter smith into a textbox control. Normally I would write a LINQ query similar to…

var query = from p in collection
where p.LastName == textbox.Text
select p;

Is it possible to decide the property dynamically, maybe using Reflection? Something like

var query = from p in collection
where p.(DropDownValue) == textbox.Text
select p;

Answer 1

Assuming:

public class Person {     public string LastName { get; set; } }  IQueryable<Person> collection; 

your query:

var query =     from p in collection     where p.LastName == textBox.Text     select p; 

means the same as:

var query = collection.Where(p => p.LastName == textBox.Text); 

which the compiler translates from an extension method to:

var query = Queryable.Where(collection, p => p.LastName == textBox.Text); 

The second parameter of Queryable.Where is an Expression<Func<Person, bool>>. The compiler understands the Expression<> type and generates code to build an expression tree representing the lambda:

using System.Linq.Expressions;  var query = Queryable.Where(     collection,     Expression.Lambda<Func<Person, bool>>(         Expression.Equal(             Expression.MakeMemberAccess(                 Expression.Parameter(typeof(Person), 'p'),                 typeof(Person).GetProperty('LastName')),             Expression.MakeMemberAccess(                 Expression.Constant(textBox),                 typeof(TextBox).GetProperty('Text'))),         Expression.Parameter(typeof(Person), 'p')); 

That is what the query syntax means.

You are free to call these methods yourself. To change the compared property, replace this:

typeof(Person).GetProperty('LastName') 

with:

typeof(Person).GetProperty(dropDown.SelectedValue);