Question

I am attempting to build a simple Where clause.

This is the code that does not work:

EDIT this code works fine now (thanks to answers below).

public class Item
{
    public int Value { get; set; }
    public string Name { get; set; }
}

var _List = new List<Item>
{
    new Item{ Name = "Smith", Value = 1},
    new Item{ Name = "Smith", Value = 2},
    new Item{ Name = "Wesson", Value = 3},
    new Item{ Name = "Wesson", Value = 4},
};

// Where(x => x.Value == 1)
var _Type = typeof(Item);
var _Prop = _Type.GetProperty("Value");
var _Param = Expression.Parameter(_Type, _Prop.Name);
var _Left = Expression.PropertyOrField(_Param, _Prop.Name);
var _Right = Expression.Constant(1, _Prop.PropertyType); 
var _Body = Expression.Equal(_Left, _Right);
var _Where = Expression.Lambda<Func<Item, bool>>(_Body, _Param); 
var _Result = _List.AsQueryable().Where(_Where);

Thank you.

Answer 1

There are several problems with your code:

1. You need to pass 1 and not "1" for the integer constant 1.

   var _Right = Expression.Constant(1, _Prop.PropertyType);
   

2. Expression.Equals if two expression trees are equal. It returns a bool.

   Expression.Equal returns an expression tree that represents an equality check.

   var _Body = Expression.Equal(_Left, _Right);
   

3. The parameter is of type Item and not int.

   var _Where = Expression.Lambda<Func<Item, bool>>(_Body, _Param);
   

4. The List<T> implements IEnumerable<T>, but not IQueryable<T>.

   IEnumerable<T> works with delegates, while IQueryable<T> works with expression trees.

   So you need to either compile your expression tree to a delegate

   var _Where = Expression.Lambda<Func<Item, bool>>(_Body, _Param).Compile();
   var _Result = _List.Where(_Where);
   

   or convert the list to IQueryable<T>.

   var _Where = Expression.Lambda<Func<Item, bool>>(_Body, _Param);
   var _Result = _List.AsQueryable().Where(_Where);
   

Working code:

// Where(x => x.Value == 1)
var _Param = Expression.Parameter(typeof(Item), "x");
var _Left = Expression.PropertyOrField(_Param, "Value");
var _Right = Expression.Constant(1);
var _Body = Expression.Equal(_Left, _Right);
var _Where = Expression.Lambda<Func<Item, bool>>(_Body, _Param).Compile();
var _Result = _List.Where(_Where);