How do I write an extension method for a generic type with constraints on type parameters?

Question

I'm working with a task specific .NET plattform, which is precompiled and not OpenSource. For some tasks I need to extend this class, but not by inheriting from it. I simply want to add a method.

At first I want to show you a dummycode existing class:

public class Matrix<T> where T : new() {
    ...
    public T values[,];
    ...
}

I want to extend this class in the following way:

public static class MatrixExtension {
    public static T getCalcResult<T>(this Matrix<T> mat) {
        T result = 0;
        ...
        return result;
    }
}

I've got this syntax from many google links so no idea whether it is correct. The compiler tells me no error, but in the end it doesn't work. In the end I want to call this function in the following way:

Matrix<int> m = new Matrix<int>();
...
int aNumber = m.getCalcResult();

So anyone got an idea? Thank you for your help!

Regards Nem

Answer 1

You need to add the same type parameter constraints on the extension method.

This is my attempt at the closest reconstruction of your example that compiles and runs, without any error:

public class Matrix<T>  where T : new() {
     public T[,] values;
 }


 public static class MatrixExtension {
     public static T getCalcResult<T>(this Matrix<T> mat)  where T : new() {
         T result = new T();
         return result;
     }
 }

 class Program {
     static void Main(string[] args)  {
        Matrix<int> m = new Matrix<int>();
        int aNumber = m.getCalcResult();
        Console.WriteLine(aNumber); //outputs "0"
 }