C# Signature:

[Serializable, StructLayout(LayoutKind.Sequential)]
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).