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.bottom); }
   }
}

VB Definition:

Structure RECT
    Public Left, Top, Right, Bottom As Int32
End Structure

Notes:

This can be used when a PInvoke signature requires a "Win32 rectangle," otherwise you should be using System.Drawing.Rectangle.