C# Signature:

[DllImport("user32.dll")]
static extern Int16 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:

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