simple C# Matrix manipulation for Silverlight transforms

MatrixUtils.cs

using System;
using System.Windows.Media;

namespace Stamen
{
    /*
     * Extension methods for Matrix manipulation in Silverlight.
     * 
     * Note that by default C# passes structs by value (and 
     * extension functions can't pass by reference) so the matrix 
     * won't be modified unless you reassign the result.
     * 
     * As a side-effect, since these methods return Matrix they 
     * can be chained neatly. 
     * 
     * e.g. to scale by 5x around point (1,2)
     * 
     * Matrix m = new Matrix();
     * m = m.Translate(-1,-2).Scale(5).Translate(1,2);
     * 
     * e.g. to rotate by pi radians around point (1,2)
     * 
     * Matrix m = new Matrix();
     * m = m.Translate(-1,-2).Rotate(Math.PI).Translate(1,2);
     * 
     * This is still less verbose than without the functions, 
     * especially for rotations.
     * 
     */
    public static class MatrixUtils
    {
        public static Matrix Scale(this Matrix m, double sc)
        {
            m.M11 *= sc;
            m.M22 *= sc;
            m.OffsetX *= sc;
            m.OffsetY *= sc;
            return m;
        }

        public static Matrix Translate(this Matrix m, double tx, double ty)
        {
            m.OffsetX += tx;
            m.OffsetY += ty;
            return m;
        }

        public static Matrix Rotate(this Matrix m, double radians)
        {
            double sin = Math.Sin(radians);
            double cos = Math.Cos(radians);
            double m11 = m.M11;
            double m12 = m.M12;
            double m21 = m.M21;
            double m22 = m.M22;
            double offsetX = m.OffsetX;
            double offsetY = m.OffsetY;
            m.M11 = m11 * cos - m12 * sin;
            m.M12 = m11 * sin + m12 * cos;
            m.M21 = m21 * cos - m22 * sin;
            m.M22 = m21 * sin + m22 * cos;
            m.OffsetX = offsetX * cos - offsetY * sin;
            m.OffsetY = offsetX * sin + offsetY * cos;
            return m;
        }
    }
}