ひらがなをカタカナに自動的に変換する。

using System;
using System.Text;
using System.Windows.Forms;
using System.Runtime.InteropServices;


namespace MyTextBox
{
    public class TextBoxEx :TextBox
    {
        const int WM_IME_COMPOSITION = 0x010F;
        const int GCS_RESULTREADSTR = 0x0200;

        [DllImport("Imm32.dll")]
        public static extern int ImmGetContext(IntPtr hWnd);
        [DllImport("Imm32.dll")]
        public static extern int ImmGetCompositionString(
            int hIMC,
            int dwIndex,
            StringBuilder lpBuf,
            int dwBufLen 
            );
        [DllImport("Imm32.dll")]
        public static extern bool ImmReleaseContext(IntPtr hWnd,int hIMC);

        [Serializable]
        public class CompositionEventArgs:EventArgs
        {
            public string ImeStr;
        }
        public delegate void CompositionEventHandler(object sender,CompositionEventArgs e);
        public event CompositionEventHandler CompositionEvent;

        protected override void WndProc(ref Message m)
        {
            if (CompositionEvent != null && m.Msg == WM_IME_COMPOSITION)
            {
                if (((int)m.LParam & GCS_RESULTREADSTR)>0)
                {
                    int Imc = ImmGetContext(this.Handle);
                    int sz = ImmGetCompositionString(Imc,GCS_RESULTREADSTR,null,0);
                    StringBuilder str = new StringBuilder(sz);
                    ImmGetCompositionString(Imc,GCS_RESULTREADSTR,str,str.Capacity);
                    ImmReleaseContext(this.Handle,Imc);
                    CompositionEventArgs args = new CompositionEventArgs();
                    args.ImeStr = str.ToString();
                    CompositionEvent(this,args);
                }
            }
            base.WndProc(ref m);
        }
    }
}

        private void CompoEvent(object sender, TextBoxEx.CompositionEventArgs e)
        {
            label1.Text += e.ImeStr;
        }

        private void Form1_Load(object sender, System.EventArgs e)
        {
            TextBoxEx tb = new TextBoxEx();
            tb.Name = "textBoxEx1";
            tb.Text = "textBoxEx1";
            tb.CompositionEvent += 
                         new TextBoxEx.CompositionEventHandler(this.CompoEvent);
            this.Controls.Add(tb);

        }