Cannot convert expression type 'lambda expression' to return type 'System.Linq.Expressions.Expression<System.Func<IProduct,string,bool>>'

Question

Ok, I'm lost. Why is the 1st function WRONG (squiglies in the lambda expression), but the 2nd one is RIGHT (meaning it compiles)?

    public static Expression<Func<IProduct, string, bool>> IsValidExpression(string val)
    {
        return (h => h.product_name == val);

    }

    public static Expression<Func<IProduct, bool>> IsValidExpression2()
    {
        return (m => m.product_name == "ACE");

    }

Answer 1

Your first function is going to need two arguments. Func<x,y,z> defines two parameters and the return value. Since you have both an IProduct and a string as parameters, you'll need two arguments in your lambda.

  public static Expression<Func<IProduct, string, bool>> IsValidExpression(string val)
  {
        return ((h, i) => h.product_name == val);
  }

Your second function is only Func<x,y> , so that means that the function signature has but one parameter, and thus your lambda statement compiles.

Answer 2

What is the middle string intended to do? You can make it compile by:

public static Expression<Func<IProduct, string, bool>> IsValidExpression(string val)
{
    return (h,something) => h.product_name == val;
}

Or maybe you mean:

public static Expression<Func<IProduct, string, bool>> IsValidExpression()
{
    return (h,val) => h.product_name == val;
}

?

Answer 3

Func<IProduct, string, bool> is a delegate to a method with the following signature:

bool methodName(IProduct product, string arg2)
{
  //method body
  return true || false;
}

so

public static Expression<Func<IProduct, string, bool>> IsValidExpression(string val)
{
    return (h => h.product_name == val);
}

has a differance between the return type and the returned value. you are trying to return an object of the type Expression<Func<IProduct, bool>> .

the val argument is not an argument to the method you're delegating to but will be hoisted (made part of a class implementing the resulting function) and since it's not an argument to the resulting method it should not be part of the Func type delclaration

Answer 4

Before trying to fix the lambda expression be sure that the following references were added to the related cs file:

using System.Linq;
using System.Linq.Expressions;

The lack of these references may cause the same error as well ( "Cannot convert lambda expression to type 'System.Linq.Expressions.Lambda Expression' because it is not a delegate type" ).