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c#.netlambdaexpression-treesdynamic-language-runtime

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


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!


Solution

  • 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);