Kanekotic/C# - Extension methods overlap with linq

Created Thu, 28 Jan 2016 09:56:49 +0200 Modified Sun, 05 Dec 2021 10:26:06 +0000

I have hit a corner case of extension methods and LINQ. Today I was declaring some extension methods to make my code more readable.So I created an extension method that gets an object and performs a direct cast:

public static class GeneralExtensions
{
    public static T Cast<T>(this object o)
    {
    	return (T)o;
    }
}

The intention was to be able to call my direct castings by something like this:

MyObject.CastTo<MyInterface>();

It happens that in the same namespace I have an extension method that has a LINQ expression

using System;
using System.Collections.Generic;
using System.Linq;

public static class EnumExtenstions
{
	public static IEnumerable<string> UseLinq(this IEnumerable<object> collection)
	{
		return (from object value in collection select value.ToString() ).ToList();
	}
}

Adding this first extension method to my code base cause the next error

Error	CS1936	Could not find an implementation of the query pattern for source type 'object'.  'Select' not found.

Having both extension methods in different namespaces (and not referred), or rename Cast<T> to something different solves the issue. This is caused for an overlap of the extension methods where the nearest one to the code is the one called.

##How bad Extension Methods over object could go?

This is an extract from the answer of Eric Lippert, regarding the code:

public static class GeneralExtensions
{
    public static T Cast<T>(this object o)
    {
    	return (T)o;
    }
}

Side Effects of the cast<T>:

  • Cast<int>(123) unnecessarily boxes the int, (int)123 does not.
  • Cast< short >(123) fails but (short)123 succeeds. There is no conversion from a boxed int to a short.
  • Suppose you have a user-defined conversion from Animal to Shape. Cast<Shape>(new Tiger()) fails but (Shape) new Tiger() succeeds.
  • Suppose q is a nullable int that happens to be null. Cast<string>(q) succeeds! But (string)q would fail at compile time
  • Etc

Cast method has some overlap with the real cast operator but is not a substitute for it. To capture the semantics of the cast operator there is a need to use dynamic, which starts the compiler at runtime and does the compile time analysis on runtime types.