static class ExtensionMethod
{
    public static void ScaleAt(
        this System.Drawing.Drawing2D.Matrix matrix, 
        float scaleX, 
        float scaleY, 
        System.Drawing.PointF point
        )
    {
        // 原点へ移動
        matrix.Translate(-point.X, -point.Y, System.Drawing.Drawing2D.MatrixOrder.Append);
        // 倍率変換
        matrix.Scale(scaleX, scaleY, System.Drawing.Drawing2D.MatrixOrder.Append);
        // 元の位置へ戻す
        matrix.Translate(point.X, point.Y, System.Drawing.Drawing2D.MatrixOrder.Append);
    }
}

var mat = new System.Drawing.Drawing2D.Matrix();

// （20,30）を中心に３０°回転（通常のメソッド）
mat.RotateAt(30, new PointF(20, 30), System.Drawing.Drawing2D.MatrixOrder.Append);

// (100, 150)を中心にX方向に２倍、Ｙ方向に３倍（拡張メソッド）
mat.ScaleAt(2, 3, new PointF(100, 150));

public static double AddScale(double a, double b, 
                              double scale = 1.0, double offset = 0.0)
{
    return (a + b) * scale + offset;
}

double ans;
ans = AddScale(10, 5);            // ans = 15.0
ans = AddScale(10, 5, 1.0);       // ans = 15.0
ans = AddScale(10, 5, 1.0, 0.0);  // ans = 15.0

var ans = AddScale(10, 5, 2.0, 1.0); // ans = 31.0

public static double AddScale(double a, double b, 
                              double scale = 1.0, double offset)  // これはエラー
{
    return (a + b) * scale + offset;
}

public static double AddScale(double a, double b, 
                              double scale = 1.0, double offset = 0.0)
{
    return (a + b) * scale + offset;
}

var ans = AddScale(a: 10, b: 5, scale: 1.0, offset: 0.0); // ans = 15.0

var ans = AddScale(scale: 2.0, offset: 5.0, b: 5, a: 10); // ans = 35.0

var ans = AddScale(a: 10, b: 5, offset: 2.0); // ans = 17.0

var ans = AddScale(10, 5, offset: 3.0); // ans = 18.0

string text = "Sample";

var text = "Sample";

System.Diagnostics.Stopwatch sw = new System.Diagnostics.Stopwatch();

var sw = new System.Diagnostics.Stopwatch();

List<string> a = new List<string>();

var a = new List<string>();

var a; // これはエラーになる

class Test
{
    var a = "Sample";   // エラー

    public Test()
    {
        var b = "Sample";   // OK
    }
}