C# Signature:

  [Serializable, 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 int Height { get { return Bottom - Top; } }
    public int Width { get { return Right - Left; } }
    public Size Size { get { return new Size(Width, Height); } }

    public Point Location { get { return new Point(Left, Top); } }

    // Handy method for converting to a System.Drawing.Rectangle
    public Rectangle ToRectangle()
      { return Rectangle.FromLTRB(Left, Top, Right, Bottom); }

    public static RECT FromRectangle(Rectangle rectangle) {
      return new RECT(rectangle.Left, rectangle.Top, rectangle.Right, rectangle.Bottom);
    }
  }

VB Definition:

     <Serializable, StructLayout(LayoutKind.Sequential)> _
    Public Structure Rect
    Public Left As Integer
    Public Top As Integer
    Public Right As Integer
    Public Bottom As Integer

    Public Overrides Function ToString() As String
        ToString = String.Format("({0},{1})-({2},{3})", Left, Top, Right, Bottom)
    End Function

    'Conversion to rectangle
    Public ReadOnly Property Rectangle() As Rectangle
        Get
        Return Rectangle.FromLTRB(Left, Top, Right, Bottom)
        End Get
    End Property
    End Structure

Notes:

This can be used when a PInvoke signature requires a "Win32 Rectangle" otherwise you should be using System.Drawing.Rectangle. Note that a Win32 rectangle is specified by two opposite corners (LTRB), while a .NET rectangle is specified by one corner and width/height (LTWH).