How can I multiply two matrices in C#?

Multiplication of 2 matrixes:

    public double[,] MultiplyMatrix(double[,] A, double[,] B)
    {
        int rA = A.GetLength(0);
        int cA = A.GetLength(1);
        int rB = B.GetLength(0);
        int cB = B.GetLength(1);

        if (cA != rB)
        {
            Console.WriteLine("Matrixes can't be multiplied!!");
        }
        else
        {
            double temp = 0;
            double[,] kHasil = new double[rA, cB];

            for (int i = 0; i < rA; i++)
            {
                for (int j = 0; j < cB; j++)
                {
                    temp = 0;
                    for (int k = 0; k < cA; k++)
                    {
                        temp += A[i, k] * B[k, j];
                    }
                    kHasil[i, j] = temp;
                }
            }

            return kHasil;
        }
    }

Although you can multiply matrices by an iterative approach (for loops), performing the calculations with linear algebra will clean up your code and will give you performance gains that are several times faster!

There is a free library available in nuget - MathNet.Numerics. It makes it extremely easy to multiply matrices:

Matrix<double> a = DenseMatrix.OfArray(new double[,] { { 1, 2, 3 }, { 4, 5, 6 }, { 7, 8, 9 } });
Matrix<double> b = DenseMatrix.OfArray(new double[,] { { 1 }, { 2 }, { 3 } });
Matrix<double> result = a * b;

It has no dependencies and can be used in .net core 2.0, making it an excellent choice to avoid iterative matrix multiplication techniques and take advantage of linear algebra.

There is nothing built into .NET. You will have to write the multiplication yourself or use some third party library. I've blogged about one way to achieve this comparing two different implementations : a standard naive algorithm and one using unsafe code.

Whilst there's no built in Maths framework to do this in .NET (could use XNA's Maths library), there is a Matrix in the System.Windows.Media namespace. The Matrix structure has a Multiply method which takes in another Matrix and outputs a Matrix.

Matrix matrix1 = new Matrix(5, 10, 15, 20, 25, 30);
Matrix matrix2 = new Matrix(2, 4, 6, 8, 10, 12);

// matrixResult is equal to (70,100,150,220,240,352) 
Matrix matrixResult = Matrix.Multiply(matrix1, matrix2);

// matrixResult2 is also
// equal to (70,100,150,220,240,352) 
Matrix matrixResult2 = matrix1 * matrix2;

This is mainly used for 2D transformation:

Represents a 3x3 affine transformation matrix used for transformations in 2-D space.

but if it suits your needs, then there's no need for any third party libraries.