static void Main(string[] args) { int[,] matrix = new int[2, 2]; FillMatrix(matrix); // matrix is now full of -1 } public static void FillMatrix(int[,] matrix) { for (int i = 0; i < matrix.GetLength(0); i++) { for (int j = 0; j < matrix.GetLength(1); j++) { matrix[i, j] = -1; } } }