Question

I am trying to write a generic extension method for adding a fixed matrix to an “elastic” matrix. The extension method compiles and (I assume) its code works fine in a regular method. Knowing I’ll be using this function a lot for various types, I would much prefer to figure this problem out instead of limping along with a band-aid:

    public void AddMatrix<T>(this List<T[]> MyList, T[,] Matrix)
    {
        if (MyList == null) throw new ArgumentNullException("MyList");
        if (Matrix == null) throw new ArgumentNullException("Matrix");

        for (int i = 0; i < Matrix.GetLength(0); i++)
        {
            T[] aLine = new T[Matrix.GetLength(1)];
            for (int j = 0; j < Matrix.GetLength(1); j++)
                aLine[j] = Matrix[i, j];
            MyList.Add(aLine);
        }
    }
    public void testAddMatrix()
    {
        List<string[]> aMyBigMatrix = new List<string[]>();
        string[,] aSmallerMatrix = 
        {
        {
            "foo",
            "bar", 
            "what"
        }
        };

        aMyBigMatrix.AddMatrix(aSmallerMatrix);               // .AddMatrix is not showing up here in Intellisense?
    }

Answer 1

From MSDN:

To defining and call the extension method

1. Define a static class to contain the extension method. The class must be visible to client code.

2. Implement the extension method as a static method with at least the same visibility as the containing class.

3. The first parameter of the method specifies the type that the method operates on; it must be preceded with the this modifier.

Your method is not static (2.).