/// <summary>
/// Sets a form's opacity.
/// </summary>
/// <param name="form"></param>
/// <param name="opacity"></param>
/// <remarks>
/// Note to implementors: This may be implemented as just "form.Opacity = opacity".
/// This method works around some visual clumsiness in .NET 2.0 related to
/// transitioning between opacity == 1.0 and opacity != 1.0.</remarks>
public static void SetFormOpacity(Form form, double opacity)
{
if (opacity < 0.0 || opacity > 1.0)
{
throw new ArgumentOutOfRangeException("opacity", "must be in the range [0, 1]");
}
uint exStyle = SafeNativeMethods.GetWindowLongW(form.Handle, NativeConstants.GWL_EXSTYLE);
byte bOldAlpha = 255;
if ((exStyle & NativeConstants.GWL_EXSTYLE) != 0)
{
uint dwOldKey;
uint dwOldFlags;
bool result = SafeNativeMethods.GetLayeredWindowAttributes(form.Handle, out dwOldKey, out bOldAlpha, out dwOldFlags);
}
byte bNewAlpha = (byte)(opacity * 255.0);
uint newExStyle = exStyle;
if (bNewAlpha != 255)
{
newExStyle |= NativeConstants.WS_EX_LAYERED;
}
if (newExStyle != exStyle || (newExStyle & NativeConstants.WS_EX_LAYERED) != 0)
{
if (newExStyle != exStyle)
{
SafeNativeMethods.SetWindowLongW(form.Handle, NativeConstants.GWL_EXSTYLE, newExStyle);
}
if ((newExStyle & NativeConstants.WS_EX_LAYERED) != 0)
{
SafeNativeMethods.SetLayeredWindowAttributes(form.Handle, 0, bNewAlpha, NativeConstants.LWA_ALPHA);
}
}
GC.KeepAlive(form);
}
