C# Code:

public class frmTest : Form
{
   #region 自动转换为半角
   
   //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;
   
   //重载Form的OnActivated
   protected override void OnActivated(EventArgs e)
   {
      base.OnActivated(e);
      IntPtr HIme = ImmGetContext(this.Handle);
      if (ImmGetOpenStatus(HIme)) //如果输入法处于打开状态
      {
         int iMode = 0;
         int iSentence = 0;
         bool bSuccess = ImmGetConversionStatus(HIme, ref iMode, ref iSentence); //检索输入法资讯
         if (bSuccess)
         {
            if ((iMode & IME_CMODE_FULLSHAPE) > 0) //如果是全形
            ImmSimulateHotKey(this.Handle, IME_CHOTKEY_SHAPE_TOGGLE); //转换成半形
         }
      }
   }
   
   #endregion
}

//来源:C/S框架网(www.csframework.com) QQ:1980854898