C# Signature:

[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;
   }

   // Handy method for converting to a System.Drawing.Rectangle
   public Rectangle Rectangle
   {
     get { return new Rectangle(this.left, this.top, this.right - this.left, this.bottom - this.top); }
   }
}

VB Definition:

     <System.Runtime.InteropServices.StructLayout(Runtime.InteropServices.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 New Rectangle(Left, Top, Right - Left, Bottom - Top)
        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.