/// <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.bottom);
    }

    /// <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()
    Left As Integer
    Right As Integer
    Top As Integer
    Bottom As Integer
End Structure

Public Function ToRectangle() As Rectangle
    Return New Rectangle(RECT.Left, RECT.Top, RECT.Right, RECT.Bottom)
End Function