WPF中 键盘输入事件OnPreviewKeyDown中没有e.KeyChar()的解决方法

不知道为啥C#有这个方法而WPF没有，在需要获取输入字符的情景下，虽然可以用e.key.toString()去获得字母，但是想获取小键盘的数字或者字符的时候就不会返回正确的结果。

贴上一个别人写的方法，本质上是调用的winform的dll。新建cs复制粘贴即可使用。

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

namespace ClassicPassWd
{
    public class GetCharFromKey
    {
        public enum MapType : uint
        {
            MAPVK_VK_TO_VSC = 0x0,
            MAPVK_VSC_TO_VK = 0x1,
            MAPVK_VK_TO_CHAR = 0x2,
            MAPVK_VSC_TO_VK_EX = 0x3,
        }

        [DllImport("user32.dll")]
        public static extern int ToUnicode(
            uint wVirtKey,
            uint wScanCode,
            byte[] lpKeyState,
            [Out, MarshalAs(UnmanagedType.LPWStr, SizeParamIndex = 4)]
            StringBuilder pwszBuff,
            int cchBuff,
            uint wFlags);

        [DllImport("user32.dll")]
        public static extern bool GetKeyboardState(byte[] lpKeyState);

        [DllImport("user32.dll")]
        public static extern uint MapVirtualKey(uint uCode, MapType uMapType);

        public static char KeyChars(Key key)
        {
            char ch = ' ';

            int virtualKey = KeyInterop.VirtualKeyFromKey(key);
            byte[] keyboardState = new byte[256];
            GetKeyboardState(keyboardState);

            uint scanCode = MapVirtualKey((uint)virtualKey, MapType.MAPVK_VK_TO_VSC);
            StringBuilder stringBuilder = new StringBuilder(2);

            int result = ToUnicode((uint)virtualKey, scanCode, keyboardState, stringBuilder, stringBuilder.Capacity, 0);
            switch (result)
            {
                case -1:
                    break;
                case 0:
                    break;
                case 1:
                {
                    ch = stringBuilder[0];
                    break;
                }
                default:
                {
                    ch = stringBuilder[0];
                    break;
                }
            }
            return ch;
        }
    }
}