C# Signature:

[DllImport("user32.dll")]
static extern void FlashWindowEx(ref FLASHWINFO pwfi);

User-Defined Types:

[StructLayout(LayoutKind.Sequential)]

public struct FLASHWINFO

{

    public UInt16 cbSize;
    public IntPtr hwnd;
    public UInt32 dwFlags;
    public UInt16 uCount;
    public UInt32 dwTimeout;

}

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;

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 = (ushort)Marshal.SizeOf(fInfo);
    fInfo.hwnd = hWnd;
    fInfo.dwFlags = FLASHW_ALL;
    fInfo.uCount = UInt16.MaxValue;
    fInfo.dwTimeout = 0;

    return (FlashWindowEx(ref fInfo) == 0);

}

...

if (!FlashWindowEx(someForm.Handle))

{

    //The window was already active

}

Alternative Managed API:

Do you know one? Please contribute it!