C# Signature:

[DllImport("gdi32.dll")]
static extern IntPtr CreatePolygonRgn(POINT [] lppt, int cPoints,
   int fnPolyFillMode);

User-Defined Types:

POINT

Notes:

Values for fnPolyFillMode can be 1=Alternate or 2=Winding

Tips & Tricks:

System.Drawing.Point can be used instead POINT.

[DllImport("gdi32.dll")]
static extern IntPtr CreatePolygonRgn(Point[] lppt, int cPoints, int fnPolyFillMode);

VBNET Signatute :

   <DllImport("gdi32.dll")>
    Private Shared Function CreatePolygonRgn(ByVal lppt As Point(), ByVal NumberOfPoints As Integer, ByVal fnPolyFillMode As Integer) As IntPtr

    End Function

Sample Code:

/// <summary>
/// Create a border for custom label control
/// </summary>
/// <returns>
/// if int is returned. Function was successful.
/// if string is returned. Function was failed.
/// </returns>
/// <remarks>Created by Jim Vazquez Castan. jim.vazquez@gmail.com</remarks>
private string SetBorder()
{
    try
    {
    // First we create polygon
    IntPtr forma = CreatePolygonRgn(edges, edges.Length, 1);
    // then we create area
    int iRes = SetWindowRgn(this.Handle, forma, true);

    return iRes.ToString();
    }
    catch (Exception ex) { return ex.Message; }
}

Alternative Managed API:

Do you know one? Please contribute it!