c# help..

[yigster]

namespace week4Matrix
{
    class Program
    {
        int[,] a;
        int[,] b;
        int[,] c;

        static void Main(string[] args)
        {
        }

       
        public void TransposeMatrix()
        {
            for (int i = 0; i < a.GetLength(0); i++)
            {
                for (int j = 0; j < a.GetLength(1); j++)
                {
                    c[j, i] = a[i, j];
                }
            }
        }

        public void AddMatrix()
        {
            int i, j;
            if (a.GetLength(0) == b.GetLength(0) && a.GetLength(1) == b.GetLength(1))
            {
                c = new int[a.GetLength(0), b.GetLength(1)];
                for (i = 0; i < a.GetLength(0); i++)
                {
                    for (j = 0; j < a.GetLength(0); j++)
                    {
                        c[i, j] = 0;
                        for (int k = 0; k < a.GetLength(0); k++)
                        {
                            c[i, j] = a[i, j] + b[i, j];
                        }
                    }
                }
            }
        }
        public void MultiplyMatrix()
        {
            if (a.GetLength(1) == b.GetLength(0))
            {
                c = new int[a.GetLength(0), b.GetLength(1)];
                for (int i = 0; i < c.GetLength(0); i++)
                {
                    for (int j = 0; j < c.GetLength(1); j++)
                    {
                        c[i, j] = 0;
                        for (int k = 0; k < a.GetLength(1); k++) // OR k<b.GetLength(0)
                            c[i, j] = c[i, j] + a[i, k] * b[k, j];
                    }
                }
            }
            else
            {
                Console.WriteLine("\n Number of columns in Matrix1 is not equal to Number of rows in Matrix2.");
                Console.WriteLine("\n Therefore Multiplication of Matrix1 with Matrix2 is not possible");
                Environment.Exit(-1);
            }
        }
    }
}

ok so this is what I have so far.. I want the program to start off asking the user what he/she wants to do;
1. Addition
2. Multiplication
3. Transpose
4. Determinant

and then according to the choice call one of the methods.

Also I couldn't find a way to find the determinant except for this;

http://msdn.microsoft.com/en-us/library/system.windows.media.matrix.determinant.aspx

the problem with this is that I dont know how to use it..

Thank you in advance.