[StructLayout(LayoutKind.Sequential)]
public struct RECT
{
    public int left;
    public int top;
    public int right;
    public int bottom;

    public RECT(int left, int top, int right, int bottom)
    {
        this.left = left;
        this.top = top;
        this.right = right;
        this.bottom = bottom;
    }

    public Rectangle ToRectangle
    {
        get
        {
            return new Rectangle(this.left, this.top, this.right-this.left, this.bottom-this.top);
        }
    }
}

VB .NET Signature:

<StructLayout(LayoutKind.Sequential)> _
Public Structure RECT()
    Left As Integer
    Right As Integer
    Top As Integer
    Bottom As Integer
End Structure

Public Function ToRectangle() As Rectangle
    Return New Rectangle(RECT.Left, RECT.Top, RECT.Right-RECT.Left, RECT.Bottom-RECT.Top)
End Function