[StructLayout(LayoutKind.Sequential)]
public struct FLASHWINFO
{
    public UInt32 cbSize;
    public IntPtr hwnd;
    public UInt32 dwFlags;
    public UInt32 uCount;
    public UInt32 dwTimeout;
}

public enum FlashWindow : uint
    {
    /// <summary>
    /// Stop flashing. The system restores the window to its original state.
    /// </summary>
    FLASHW_STOP = 0,

Notes:

//Stop flashing. The system restores the window to its original state.
public const UInt32 FLASHW_STOP = 0;
//Flash the window caption.
public const UInt32 FLASHW_CAPTION = 1;
//Flash the taskbar button.
public const UInt32 FLASHW_TRAY = 2;
//Flash both the window caption and taskbar button.
//This is equivalent to setting the FLASHW_CAPTION | FLASHW_TRAY flags.
public const UInt32 FLASHW_ALL = 3;
//Flash continuously, until the FLASHW_STOP flag is set.
public const UInt32 FLASHW_TIMER = 4;
//Flash continuously until the window comes to the foreground.
public const UInt32 FLASHW_TIMERNOFG = 12;

    /// <summary>
    /// Flash the window caption
    /// </summary>
    FLASHW_CAPTION = 1,

    /// <summary>
    /// Flash the taskbar button.
    /// </summary>
    FLASHW_TRAY = 2,

Tips & Tricks:

Please add some!

    /// <summary>
    /// Flash both the window caption and taskbar button.
    /// This is equivalent to setting the FLASHW_CAPTION | FLASHW_TRAY flags.
    /// </summary>
    FLASHW_ALL = 3,

    /// <summary>
    /// Flash continuously, until the FLASHW_STOP flag is set.
    /// </summary>
    FLASHW_TIMER = 4,

    /// <summary>
    /// Flash continuously until the window comes to the foreground.
    /// </summary>
    FLASHW_TIMERNOFG = 12
    }

Notes:

Tips & Tricks:

Please add some!

Sample Code:

/// <summary>
/// Flashes a window
/// </summary>
/// <param name="hWnd">The handle to the window to flash</param>
/// <returns>whether or not the window needed flashing</returns>
public static bool FlashWindowEx(IntPtr hWnd)
{
    FLASHWINFO fInfo = new FLASHWINFO();

    fInfo.cbSize = Convert.ToUInt32(Marshal.SizeOf(fInfo));
    fInfo.hwnd = hWnd;
    fInfo.dwFlags = FLASHW_ALL;
    fInfo.uCount = UInt32.MaxValue;
    fInfo.dwTimeout = 0;

    return FlashWindowEx(ref fInfo);
}

...

/// Minor adjust to the code above
/// <summary>
/// Flashes a window until the window comes to the foreground
/// Receives the form that will flash
/// </summary>
/// <param name="hWnd">The handle to the window to flash</param>
/// <returns>whether or not the window needed flashing</returns>
public static bool FlashWindowEx(Form frm)
{
        IntPtr hWnd = frm.Handle;
        FLASHWINFO fInfo = new FLASHWINFO();

        fInfo.cbSize = Convert.ToUInt32(Marshal.SizeOf(fInfo));
        fInfo.hwnd = hWnd;
        fInfo.dwFlags = FLASHW_ALL | FLASHW_TIMERNOFG;
        fInfo.uCount = UInt32.MaxValue;
        fInfo.dwTimeout = 0;

        return FlashWindowEx(ref fInfo);
}

Example Usage/Link:

Flash Window in Taskbar via Win32 FlashWindowEx:

http://pietschsoft.com/post/2009/01/26/CSharp-Flash-Window-in-Taskbar-via-Win32-FlashWindowEx.aspx

Alternative Managed API:

Do you know one? Please contribute it!

Documentation

FlashWindowEx on MSDN

VB Code:

Private Declare Function FlashWindowEx Lib "User32" (ByVal fwInfo As FLASHWINFO) As Boolean

#Region " Flash Window Routines"

    ' As defined by: http://msdn.microsoft.com/en-us/library/ms679347(v=vs.85).aspx
    Public Enum FlashWindowFlags As UInteger
    ' Stop flashing. The system restores the window to its original state.
    FLASHW_STOP = 0
    ' Flash the window caption.
    FLASHW_CAPTION = 1
    ' Flash the taskbar button.
    FLASHW_TRAY = 2
    ' Flash both the window caption and taskbar button.
    ' This is equivalent to setting the FLASHW_CAPTION | FLASHW_TRAY flags.
    FLASHW_ALL = 3
    ' Flash continuously, until the FLASHW_STOP flag is set.
    FLASHW_TIMER = 4
    ' Flash continuously until the window comes to the foreground.
    FLASHW_TIMERNOFG = 12
    End Enum
  Private Declare Function FlashWindowEx Lib "User32" (ByRef FWInfo As FLASHWINFO) As Int32

    Public Structure FLASHWINFO
    Public cbSize As Long
    Public hwnd As Long
    Public dwFlags As FlashWindowFlags
    Public uCount As Long
    Public dwTimeout As Long
    End Structure
  Private Const FLASHW_STOP As Int32 = 0
  Private Const FLASHW_CAPTION As Int32 = &H1&
  Private Const FLASHW_TRAY As Int32 = &H2&
  Private Const FLASHW_ALL As Int32 = FLASHW_CAPTION Or FLASHW_TRAY
  Private Const FLASHW_TIMER As Int32 = &H4&
  Private Const FLASHW_TIMERNOFG As Int32 = &HC&

    Public Function FlashWindow(ByRef frm As Windows.Forms.Form) As Boolean
    Return FlashWindow(frm, True, True, 5)
    End Function
    Public Function FlashWindow(ByRef frm As Windows.Forms.Form, ByVal FlashTitleBar As Boolean, ByVal FlashTray As Boolean) As Boolean
    Return FlashWindow(frm, FlashTitleBar, FlashTray, 5)
    End Function
    Public Function FlashWindow(ByRef frm As Windows.Forms.Form, ByVal FlashCount As Integer) As Boolean
    Return FlashWindow(frm, True, True, FlashCount)
    End Function
    Public Function FlashWindow(ByRef frm As Windows.Forms.Form, ByVal FlashTitleBar As Boolean, ByVal FlashTray As Boolean, ByVal FlashCount As Integer) As Boolean
    If frm Is Nothing Then Return False
    If frm.IsDisposed Then Return False
    If frm.Handle = 0 Then Return False
  Private Structure FLASHWINFO
    Dim cbSize As Int32
    Dim hwnd As IntPtr
    Dim dwFlags As Int32
    Dim uCount As Int32
    Dim dwTimeout As Int32
  End Structure

    Try
        Dim fwi As New FLASHWINFO
        With fwi
        .hwnd = frm.Handle
        If FlashTitleBar Then .dwFlags = .dwFlags Or FlashWindowFlags.FLASHW_CAPTION
        If FlashTray Then .dwFlags = .dwFlags Or FlashWindowFlags.FLASHW_TRAY
        .uCount = FlashCount
        If FlashCount = 0 Then .dwFlags = .dwFlags Or FlashWindowFlags.FLASHW_TIMERNOFG
        .dwTimeout = 0 ' Use the default cursor blink rate.
        .cbSize = System.Runtime.InteropServices.Marshal.SizeOf(fwi)
        End With
  Public Shared Function FlashWindow(ByRef frm As Form) As Int32
    Return FlashWindow(frm, True, True, 5)
  End Function
  Public Shared Function FlashWindow(ByRef frm As Form, ByVal FlashTitleBar As Boolean, ByVal FlashTray As Boolean) As Int32
    Return FlashWindow(frm, FlashTitleBar, FlashTray, 5)
  End Function
  Public Shared Function FlashWindow(ByRef frm As Form, ByVal NumTimes2Flash As Integer) As Int32
    Return FlashWindow(frm, True, True, NumTimes2Flash)
  End Function
  Public Shared Function FlashWindow(ByRef frm As Form, ByVal FlashTitleBar As Boolean, ByVal FlashTray As Boolean, ByVal NumTimes2Flash As Integer) As Int32
    Try
      Dim fwi As New FLASHWINFO
      With fwi
    .cbSize = System.Runtime.InteropServices.Marshal.SizeOf(fwi)
    .hwnd = IIf(frm Is Nothing, 0, frm.Handle)
    If FlashTitleBar Then .dwFlags = .dwFlags Or FLASHW_CAPTION
    If FlashTray Then .dwFlags = .dwFlags Or FLASHW_TRAY
    .uCount = NumTimes2Flash
    .dwTimeout = 250
      End With

        Return FlashWindowEx(fwi)
    Catch
        Return False
    End Try
    End Function
      Return FlashWindowEx(fwi)
    Catch
      Return -1
    End Try
  End Function

#End Region

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