C＃预期为常数

Question

所以我有我的case函数，并且我一直都收到此错误。我究竟做错了什么？ 我想念什么？

每个case语句之后，错误始终发生。

private string GetData(Type DotNetType) {
        switch (true) {
            case DotNetType.Equals(typeof(TimeSpan)): //here
                return "timespan";
                break;
            case (DotNetType == typeof(DateTime)): //here
                return "datetime";
                break;
            case (DotNetType == typeof(Decimal)): //here
                return "number";
                break;
            case (DotNetType == typeof(int)): //here
                return "number";
                break;
            case (DotNetType == typeof(double)): //here
                return "number";
                break;
            case (DotNetType == typeof(float)): //here
                return "number";
                break;
            case (DotNetType == typeof(System.Drawing.Bitmap)): //here
                return "image";
                break;
            case (DotNetType == typeof(byte[])): //here
                return "binary";
                break;
            case (DotNetType == typeof(bool)): //here
                return "flag";
                break;
            default:
                return "text";
                break;
        }
    }