using System.Runtime.InteropServices;

[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 ImmGetConversionStatus(IntPtr hIMC, 
    ref int conversion, ref int sentence);

[DllImport("imm32.dll")]
public static extern bool ImmReleaseContext(IntPtr hWnd, IntPtr hIMC);

[DllImport("imm32.dll")]
public static extern bool ImmSetConversionStatus(IntPtr hIMC, 
    int conversion, int sentence);

const int IME_CMODE_FULLSHAPE = 0x0008;

public static void SetHalfShape(IntPtr AHandle)
{
    IntPtr vIme = ImmGetContext(AHandle);
    if (ImmGetOpenStatus(vIme)) // 输入法是打开的  
    {
        int vMode = 0, vSentence = 0;
        if (ImmGetConversionStatus(vIme, ref vMode, ref vSentence)) // 获取输入法状态
        {
            if ((vMode & IME_CMODE_FULLSHAPE) > 0) // 是全角  
            {
                vMode &= (~IME_CMODE_FULLSHAPE);
                ImmSetConversionStatus(vIme, vMode, vSentence);
            }
        }
    }
    ImmReleaseContext(AHandle, vIme);
}

private void textBox1_Enter(object sender, EventArgs e) // 设置焦点的时候
{
    SetHalfShape(((TextBox)sender).Handle);
}

------解决方案--------------------
安装.NET 2.0的一个补丁

------解决方案--------------------
你可以把一楼的方法做成一个控件,在父窗体上添加这个控件,在子窗体上作一下设置就可以了.