Building Dynamic LINQ Queries based on Combobox Value

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);

Answer 2

Scott Guthrie has a short series on dyamically built LINQ to SQL queries:

http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx

That's the easy way...then there's another way that's a bit more involved:

http://www.albahari.com/nutshell/predicatebuilder.aspx

Answer 3

You can also use the library I created: http://tomasp.net/blog/dynamic-linq-queries.aspx. You would store the properties in ComboBox as lambda expressions and then just write:

var f = (Expression<Func<Product, string>>)comboBox.SelectedValue;
var query =
    from p in collection
    where f.Expand(textBox.Text)
    select p;