The .NET Framework Class Library does not provide a method to determine which control has keyboard focus, but you can invoke a Win32 API function to do this.

Here's a custom form class that adds a GetFocusControl method to retrieve the .NET control that has the keyboard focus. If the window that has the focus is not a .NET control, the method returns null (Nothing in VB).

[C#]

using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;

public class FormWithGetFocusControl : Form
{
  // Import GetFocus() from user32.dll
  [ DllImport( "user32.dll", CharSet=CharSet.Auto,
      CallingConvention=CallingConvention.Winapi ) ] 
  internal static extern IntPtr GetFocus(); 

  protected Control GetFocusControl() 
  { 
    Control focusControl = null; 
    IntPtr focusHandle = GetFocus(); 
    if ( focusHandle != IntPtr.Zero ) 
      // returns null if handle is not to a .NET control
      focusControl = Control.FromHandle( focusHandle );
    return focusControl; 
  } 
}

[Visual Basic]

Public Class FormWithGetFocusControl
  Inherits Form

  ' Import GetFocus() from user32.dll
  Public Shared Function GetFocus() As IntPtr
  End Function

  Protected Function GetFocusControl() As Control
    Dim focusControl As Control = Nothing
    ' To get hold of the focused control: 
    Dim focusHandle As IntPtr = GetFocus()
    If IntPtr.Zero.Equals(focusHandle) Then
      ' returns Nothing if handle is not to a .NET control
      focusControl = Control.FromHandle(focusHandle)
    End If
    Return focusControl
  End Function

End Class

Contributed from George Shepherd's Windows Forms FAQ