Question

Let’s say i had two parameters:

parameter 1 an expression that takes a T and fetches the associated product’s ID from it

Expression<Func<T,int>> prodIdProperty = x=>x.Product.Id

parameter 2 a productid to compare with , let’s say

int productid = 5;

Is there any way I could runtime transform this (in code) into a new expression like this:

Expression<Func<T,bool>> prodIdProperty = x=>x.Product.Id == 5

I basically need something like this:

Expression<Func<T,bool>> TransformToPredicate(Expression<Func<T,int>> prodIdProperty,int productid){
//insert expression transform magic I don't seem to grasp...
}

I need this because I want to pass it as a predicate to my EF Where clause, that means I cannot included Compiled versions of my prodIdProperty as It requires calling Invoke() wich is not supported

(I already tried formulating my problem another way in this question, but I think I overcomplicated it there)

Answer 1

It sounds like you want something like:

Expression<Func<T, bool>> TransformToPredicate(
    Expression<Func<T, int>> prodIdProperty,
    int productId)
{
    Expression equals = Expression.Equal(prodIdProperty.Body,
                                         Expression.Constant(productId));
    return Expression.Lambda<Func<T, bool>>(equals, prodIdProperty.Parameters);
}

Sample code showing it in action:

using System;
using System.Linq.Expressions;

class Test
{    
    static Expression<Func<T, bool>> TransformToPredicate<T>(
        Expression<Func<T, int>> prodIdProperty,
        int productId)
    {
        Expression equals = Expression.Equal(prodIdProperty.Body,
                                             Expression.Constant(productId));
        return Expression.Lambda<Func<T, bool>>(equals, prodIdProperty.Parameters);
    }

    static void Main()
    {
        Expression<Func<string, int>> length = x => x.Length;

        var predicate = TransformToPredicate(length, 5);
        var compiled = predicate.Compile();

        Console.WriteLine(compiled("Hello")); // True
        Console.WriteLine(compiled("Foo")); // False
    }
}