Read HitTest to determine the position of the mouse when it is over the grid or when the grid has captured the mouse. HitTest may represent a position that is not within the client area of the grid control if the user is moving or resizing the rows or columns, or if the user is selecting a range of cells using the mouse. HitTest is the position of the mouse cursor in the coordinate system where (0,0) is the upper left corner of the grid control's client area. 

ClientOrigin 

MouseCoord