pinvoke.net: Edit: user32.InvalidateRect

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Summary: The InvalidateRect API. From MSDN: The InvalidateRect function adds a rectangle to the specified window's update region. The update region represents the portion of the window's client area that must be redrawn.

!!!!C# Signature:
 [DllImport("user32.dll")]
 static extern bool InvalidateRect(IntPtr hWnd, IntPtr lpRect, bool bErase);

!!!!VB Signature:
 Public Overloads Declare Function InvalidateRect Lib "User32" Alias "InvalidateRect" (ByVal hWnd As Int32, ByRef lpRect As RECT, ByVal bErase As Boolean) As Boolean
 Public Overloads Declare Function InvalidateRect Lib "User32" Alias "InvalidateRect" (ByVal hWnd As Int32, ByVal lpRect As IntPtr, ByVal bErase As Boolean) As Boolean
 Public Overloads Shared Function InvalidateRect(ByVal hWnd As Int32, ByVal bErase As Boolean) As Boolean
     Return InvalidateRect(hWnd, IntPtr.Zero, bErase)
 End Function
 Public Overloads Shared Function InvalidateRect(ByVal hWnd As Int32, ByVal lpRect As System.Drawing.Rectangle, ByVal bErase As Boolean) As Boolean
     Return InvalidateRect(hWnd, RECT.FromRectangle(lpRect), bErase)
 End Function

!!!!User-Defined Types:
[RECT]

!!!!Notes:
Useful to force a redraw when you don't have control of the area, when using WindowFromPoint() for example. If you do have control of the object, you can called the managed Control.Invalidate()

!!!!Tips & Tricks:
Setting the second param (lpRect) to IntPtr.Zero will redraw the whole client area.

!!!!Sample Code:

Context: [POINT] and [RECT] structures are defined elsewhere. you've gotten the rectangle (RECT) you wish to highlight

POINT pt;
 RECT rct;
 /** converted coords **/
 Rectangle R;

/** get current current cursor location **/
 GetCursorPos(out pt)

/** get the window handle **/
 IntPtr hWnd = WindowFromPoint(pt);
 GetWindowRect(hWnd, out rct);

/** draw a red border around your rectangle **/
 ControlPaint.DrawBorder(System.Drawing.Graphics.FromHwnd(hWnd), R, Color.Red, ButtonBorderStyle.Solid);

/** when you want to clear this rectangle, use this **/
 InvalidateRect(hWnd, IntPtr.Zero, true);

!!!!Alternative Managed API:
 Invalidate method
 Invalidate() - invalidates the entire surface causing the control to be repainted.
 Invalidate(bool invalidateChilden) - Optionally, invalidates the child controls assigned to the control.
 Invalidate(Rectangle rect) - Adds the specified rectangle to the update region of the control
                            and causes a paint message to be sent to the control.
 Invalidate(Rectangle rect, bool invalidateChildern) - combination of the previous two.
 Invalidate(Region rgn) - Adds the specified graphics region to the update region of the control
                        and causes a paint message to be sent to the control.
 Invalidate(Region rgn, bool invalidateChildren)
 
 Control.Invalidate(), Form1.Invalidate(), this.Invalidate(), Me.Invalidate()
 
Documentation: InvalidateRect@msdn on MSDN