为什么我的形式越来越小，后响应到WM_NCCALCSIZE

我想自己画的标题栏，但我的标题栏的高度是不系统的，增加的标题栏的高度，然后，我响应WM_NCCALCSIZE
但如果我不断地最大化和恢复，形式会越来越小
出了什么事？
希望有人能回答，谢谢！

using System;

using System.Runtime.InteropServices;

using System.Windows.Forms;

 

namespace WindowsFormsApplication1

{

    public partial class Form1 : Form

    {

        public Form1()

        {

            InitializeComponent();

        }

 

        [DllImport("user32")]

        public static extern int GetSystemMetrics(int nIndex);

        private const int SM_CYCAPTION = 4;

        private const int SM_CXFRAME = 32;

        private const int SM_CYFRAME = 33;

        private const int WM_NCCALCSIZE = 0x83;

 

        [StructLayout(LayoutKind.Sequential)]

        public struct RECT

        {

            public int Left;

            public int Top;

            public int Right;

            public int Bottom;

        }

 

        [StructLayout(LayoutKind.Sequential)]

        public struct PWINDOWPOS

        {

            public IntPtr hwnd;

            public IntPtr hwndInsertAfter;

            public int x;

            public int y;

            public int cx;

            public int cy;

            public uint flags;

        }

 

        [StructLayout(LayoutKind.Sequential)]

        public struct NCCALCSIZE_PARAMS

        {

            [MarshalAs(UnmanagedType.ByValArray, SizeConst = 3)]

            public RECT[] rgrc;

            public PWINDOWPOS lppos;

        }

 

        protected override void WndProc(ref Message m)

        {

            if (m.Msg == WM_NCCALCSIZE && m.WParam != IntPtr.Zero)

            {

                NCCALCSIZE_PARAMS Params;

                Params = (NCCALCSIZE_PARAMS)m.GetLParam(typeof(NCCALCSIZE_PARAMS));

                Params.rgrc[0].Top += 40 - (GetSystemMetrics(SM_CYCAPTION) + GetSystemMetrics(SM_CYFRAME)); //caption height

                Marshal.StructureToPtr(Params, m.LParam, false);

            }

            base.WndProc(ref m);

        }

    }

}