C# Signature:

[DllImport("user32.dll")]
static extern bool SetLayeredWindowAttributes(IntPtr hwnd, uint crKey,
   byte bAlpha, uint dwFlags);

VB Signature:

Public Declare Auto Function SetLayeredWindowAttributes Lib "User32.Dll" _
    (ByVal hWnd As IntPtr, ByVal crKey As Integer, ByVal Alpha As Byte, ByVal dwFlags As Integer) As Boolean

User-Defined Types:

None.

Notes:

This method may only be called on Windows 2000 and above.

Tips & Tricks:

Sample Code:

    public const int GWL_EXSTYLE = -20;
    public const int WS_EX_LAYERED = 0x80000;
    public const int LWA_ALPHA = 0x2;
    public const int LWA_COLORKEY = 0x1;

    //set the window style to alpha appearance
    private void button4_Click(object sender, EventArgs e)
    {
        SetWindowLong(Handle, GWL_EXSTYLE, GetWindowLong(Handle, GWL_EXSTYLE) ^ WS_EX_LAYERED);
        SetLayeredWindowAttributes(Handle, 0, 128, LWA_ALPHA);
    }

Alternative Managed API:

When setting Layered Window Attributes for window transparency, you can set the .Opacity property of your form instead.

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