/// <summary>Rectangle Structure</summary>
[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;
    }

    #region Operator overloads: Cast To And From System.Drawing.Rectangle

    /// <summary>Implicitly casts a RECT to a <see cref="Rectangle"/>.</summary>
    /// <param name="r">The <c>RECT</c> instance to cast to a <c>Rectangle</c>
    /// instance.</param>
    /// <returns>A new <c>Rectangle</c> structure.</returns>
    public static implicit operator Rectangle( RECT r )
    {
        return new Rectangle(r.left, r.top, r.right-r.left, r.bottom-r.top);
    }

    /// <summary>Implicitly casts a <see cref="Rectangle"/> to a <c>RECT</c>.</summary>
    /// <param name="r">The <c>Rectangle</c> instance to cast to a <c>RECT</c>
    /// instance.</param>
    /// <returns>A new <c>RECT</c> structure.</returns>
    public static implicit operator RECT( Rectangle r )
    {
        return new RECT(r.Left, r.Top, r.Right, r.Bottom);
    }

    #endregion
}

VB .NET Signature:

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

Public Shared Function ToRectangle(ByVal sourceRect As RECT) As Rectangle
   Return New Rectangle(sourceRect.Left, sourceRect.Top, sourceRect.Right - sourceRect.Left + 1, sourceRect.Bottom - sourceRect.Top + 1)
End Function