#region 修正输入法全角/半角的问题
//声明一些API函数 
[DllImport("imm32.dll")]
public static extern IntPtr ImmGetContext(IntPtr hwnd);
[DllImport("imm32.dll")]
public static extern bool ImmGetOpenStatus(IntPtr himc);
[DllImport("imm32.dll")]
public static extern bool ImmSetOpenStatus(IntPtr himc, bool b);
[DllImport("imm32.dll")]
public static extern bool ImmGetConversionStatus(IntPtr himc, ref int lpdw, ref  int lpdw2);
[DllImport("imm32.dll")]
public static extern int ImmSimulateHotKey(IntPtr hwnd, int lngHotkey);
private const int IME_CMODE_FULLSHAPE = 0x8;
private const int IME_CHOTKEY_SHAPE_TOGGLE = 0x11;
protected override void OnActivated(EventArgs e)
{
  base.OnActivated(e);
  IntPtr HIme = ImmGetContext(this.Handle);  
  //如果输入法处于打开状态 
  if (ImmGetOpenStatus(HIme))
  {
    int iMode = 0;
    int iSentence = 0;
    //检索输入法信息 
    bool bSuccess = ImmGetConversionStatus(HIme, ref iMode, ref iSentence);  
    if (bSuccess)
    {
      //如果是全角,转换成半角 
      if ((iMode & IME_CMODE_FULLSHAPE) > 0)    
        ImmSimulateHotKey(this.Handle, IME_CHOTKEY_SHAPE_TOGGLE);    
    }
  }
}
#endregion 修正输入法全角/半角的问题

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
public class Form1 : System.Windows.Forms.Form
{
  RichTextBox rtb = new RichTextBox();
  public Form1()
  {
    this.Controls.Add(rtb);
    rtb.Dock = DockStyle.Fill;
    this.InputLanguageChanged += new InputLanguageChangedEventHandler(languageChange);
  }
  private void languageChange(Object sender, InputLanguageChangedEventArgs e)
  {
    // If the input language is Japanese, set the initial IMEMode to Katakana.
    if (e.InputLanguage.Culture.TwoLetterISOLanguageName.Equals("ja"))
    {
      rtb.ImeMode = System.Windows.Forms.ImeMode.Katakana;
    }
  }
  public static void Main(string[] args)
  {
    Application.Run(new Form1());
  }
}