Impossible to use ref and out for first ("this") parameter in Extension methods?

c# .net extension-methods
Hun1Ahpu picture Hun1Ahpu · Apr 11, 2010 · Viewed 12k times · Source

Why is it forbidden to call Extension Method with ref modifier?

This one is possible:

public static void Change(ref TestClass testClass, TestClass testClass2)
{
    testClass = testClass2;
}

And this one not:

public static void ChangeWithExtensionMethod(this ref TestClass testClass, TestClass testClass2)
{
    testClass = testClass2;
}

But why?

Answer

Jon Skeet picture Jon Skeet · Apr 11, 2010

You have to specify ref and out explicitly. How would you do this with an extension method? Moreover, would you really want to?

TestClass x = new TestClass();
(ref x).ChangeWithExtensionMethod(otherTestClass);
// And now x has changed?

Or would you want to not have to specify the ref part, just for the first parameter in extension methods?

It just sounds weird to me, to be honest, and a recipe for unreadable (or at least hard-to-predict) code.

Related questions

Extension methods must be defined in a non-generic static class

I'm getting the error: Extension methods must be defined in a non-generic static class On the line: public class LinqHelper Here is the helper class, based on Mark Gavells code. I'm really confused as to what this error means as …

Read More
Convert string[] to int[] in one line of code using LINQ

I have an array of integers in string form: var arr = new string[] { "1", "2", "3", "4" }; I need to an array of 'real' integers to push it further: void Foo(int[] arr) { .. } I tried to cast int and it of course failed: Foo(…

Read More
Why there is no ForEach extension method on IEnumerable?

Inspired by another question asking about the missing Zip function: Why is there no ForEach extension method in the Enumerable class? Or anywhere? The only class that gets a ForEach method is List<>. Is there a reason why …

Read More