Expression.Lambda and query generation at runtime, simplest "Where" example

Question

I was trying to generate a simple Lambda Expression at runtime with no luck... something like this:

var result = queryableData.Where(item => item.Name == "Soap") 

Here is my example class and a fixture queryable:

public class Item {     public int Id { get; set; }     public string Name { get; set; } }  IQueryable<Item> queryableData = ...; 

Then I generate a lambda expression at runtime correct code follows:

//"item" in "item =>..." var item = Expression     .Parameter(typeof(Item), "item");  //property of my item, this is "item.Name" var prop = Expression     .Property(item, "Name");  //then "Soap" in '... => item.Name=="Soap"' var value = Expression.Constant("Soap");  //equality expression "==" in my primer var equals = Expression.Equal(prop, value);  //then lambda var lambda = Expression.Lambda<Func<Item, bool>>(equals, item);  //and here are the results     var results = queryableData.Where(lambda); 

Big thanks to dtb for advice!

Answer 1

In the following query

var result = query.Where(item => item.Name == "Soap") 

the lambda expression is

item => item.Name == "Soap" 

You only need to construct this part, not the Where call which accepts an expression tree.

The expression tree for the lambda expression looks like this:

                     Lambda                       /  \                    Equal  Parameter                    /   \    item               Property  \                "Name"   Constant                  |       "Soap"              Parameter                         item 

In code:

var item = Expression.Parameter(typeof(Item), "item");  var prop = Expression.Property(item, "Name");  var soap = Expression.Constant("Soap");  var equal = Expression.Equal(prop, soap);  var lambda = Expression.Lambda<Func<Item, bool>>(equal, item);  var result = queryableData.Where(lambda);