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setlayeredwindowattributes (user32)
 
.
Summary

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

PB Signature:

Function Long SetLayeredWindowAttributes(long hWnd, Long crKey , char /*Byte*/ bAlpha , Long dwFlags) &
    Library "user32" alias for "SetLayeredWindowAttributes;Ansi"

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);
    }

Sample PB Code:

  // Tip submitted by Olivier Citeau (ociteau@yahoo.fr)
  // Combined it with the numeric module from Simon Harris (simon@addease.com.au)
  // As found on the PBDR.com website

  //Open event of your window
  Constant long LWA_COLORKEY = 1, LWA_ALPHA = 2
  Constant long GWL_EXSTYLE = - 20
  Constant long WS_EX_LAYERED = 524288 //2^19
  Long ll_Ret, ll_handle

  n_cst_numerical lnc_N

  // Set the window style to 'Layered'
  ll_handle = Handle (this)
  ll_Ret = GetWindowLong(ll_handle, GWL_EXSTYLE)

  //ll_Ret = ll_Ret Or WS_EX_LAYERED
  ll_Ret = lnc_N.of_bitwiseor( ll_Ret, WS_EX_LAYERED)

  SetWindowLong (ll_handle, GWL_EXSTYLE, ll_Ret)

  // Set the opacity of the layered window to 128
  SetLayeredWindowAttributes (ll_handle, 0, char(128),LWA_ALPHA)

  //Note that if you set opacity to 0, your window will be
  //invisible. With 255, it will be a "normal" window.

Alternative Managed API:

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

Documentation

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