C#: Rotation Matrix

Rotation matrices are used in 3D graphics to rotate vectors.
I use homogeneous coordinates, so the matrices are 4x4. If you want 3x3, just remove the last column and last row.
I wrote several functions:

• Rotation around X axis
• Rotation around Y axis
• Rotation around Z axis
• Rotation around all axes
• Rotation around any given axis
• Rotation from normal vector to normal vector

Apparently the 5th function is enough, because for example "Rotation around X axis" can be replace by rotation around (1,0,0), and "Rotation around all axes" is merely the product of 3 matrices. BUT, one should use the specific function he or she needs, because it is more efficient.
For matrix operations you can use this post.

Code:

public static Matrix4 GetRotationMatrixX(double angle)
{
    if (angle == 0.0)
    {
        return Matrix4.I;
    }
    float sin = (float)Math.Sin(angle);
    float cos = (float)Math.Cos(angle);
    return new Matrix4(new float[4, 4] {
        { 1.0f, 0.0f, 0.0f, 0.0f }, 
        { 0.0f, cos, -sin, 0.0f }, 
        { 0.0f, sin, cos, 0.0f }, 
        { 0.0f, 0.0f, 0.0f, 1.0f } });
}

public static Matrix4 GetRotationMatrixY(double angle)
{
    if (angle == 0.0)
    {
        return Matrix4.I;
    }
    float sin = (float)Math.Sin(angle);
    float cos = (float)Math.Cos(angle);
    return new Matrix4(new float[4, 4] {
        { cos, 0.0f, sin, 0.0f }, 
        { 0.0f, 1.0f, 0.0f, 0.0f }, 
        { -sin, 0.0f, cos, 0.0f }, 
        { 0.0f, 0.0f, 0.0f, 1.0f } });
}

public static Matrix4 GetRotationMatrixZ(double angle)
{
    if (angle == 0.0)
    {
        return Matrix4.I;
    }
    float sin = (float)Math.Sin(angle);
    float cos = (float)Math.Cos(angle);
    return new Matrix4(new float[4, 4] {
        { cos, -sin, 0.0f, 0.0f }, 
        { sin, cos, 0.0f, 0.0f }, 
        { 0.0f, 0.0f, 1.0f, 0.0f }, 
        { 0.0f, 0.0f, 0.0f, 1.0f } });
}

public static Matrix4 GetRotationMatrix(double ax, double ay, double az)
{
    Matrix4 my = null;
    Matrix4 mz = null;
    Matrix4 result = null;
    if (ax != 0.0)
    {
        result = GetRotationMatrixX(ax);
    }
    if (ay != 0.0)
    {
        my = GetRotationMatrixY(ay);
    }
    if (az != 0.0)
    {
        mz = GetRotationMatrixZ(az);
    }
    if (my != null)
    {
        if (result != null)
        {
            result *= my;
        }
        else
        {
            result = my;
        }
    }
    if (mz != null)
    {
        if (result != null)
        {
            result *= mz;
        }
        else
        {
            result = mz;
        }
    }
    if (result != null)
    {
        return result;
    }
    else
    {
        return Matrix4.I;
    }
}

public static Matrix4 GetRotationMatrix(Vector3 axis, double angle)
{
    if (angle == 0.0)
    {
        return Matrix4.I;
    }

    float x = axis.x;
    float y = axis.y;
    float z = axis.z;
    float sin = (float)Math.Sin(angle);
    float cos = (float)Math.Cos(angle);
    float xx = x * x;
    float yy = y * y;
    float zz = z * z;
    float xy = x * y;
    float xz = x * z;
    float yz = y * z;

    float[,] matrix = new float[4, 4];

    matrix[0, 0] = xx + (1 - xx) * cos;
    matrix[1, 0] = xy * (1 - cos) + z * sin;
    matrix[2, 0] = xz * (1 - cos) - y * sin;
    matrix[3, 0] = 0.0f;

    matrix[0, 1] = xy * (1 - cos) - z * sin;
    matrix[1, 1] = yy + (1 - yy) * cos;
    matrix[2, 1] = yz * (1 - cos) + x * sin;
    matrix[3, 1] = 0.0f;

    matrix[0, 2] = xz * (1 - cos) + y * sin;
    matrix[1, 2] = yz * (1 - cos) - x * sin;
    matrix[2, 2] = zz + (1 - zz) * cos;
    matrix[3, 2] = 0.0f;

    matrix[3, 0] = 0.0f;
    matrix[3, 1] = 0.0f;
    matrix[3, 2] = 0.0f;
    matrix[3, 3] = 1.0f;

    return new Matrix4(matrix);
}

/// <param name="source">Should be normalized</param>
/// <param name="destination">Should be normalized</param>
public static Matrix4 GetRotationMatrix(Vector3 source, Vector3 destination)
{
    Vector3 rotaxis = Vector3.CrossProduct(source, destination);
    if (rotaxis != Vector3.Zero)
    {
        rotaxis.Normalize();
        float cos = source.DotProduct(destination);
        double angle = Math.Acos(cos);
        return GetRotationMatrix(rotaxis, angle);
    }
    else
    {
        return Matrix4.I;
    }
}