Both these lines:
Bitmap bmp = new Bitmap(this.ClientRectangle.Width, this.ClientRectangle.Height);
this.DrawToBitmap(bmp, this.ClientRectangle);
allow you to specify which part of the client area you want to draw to your bitmap. Here it does exactly want you asked for: it draws the entire client area. If you want only a portion of it, you have to define a rectangle delimiting this portion, and use this rectangle instead of
ClientRectangle
.