Java翻译成C#

public class SignUtil {

        public static String sign(){

        Map<String, Object> params = new HashMap<>();

        params.put("key", key);

        params.put("secret", secret);

        params.put("userMobile", userMobile);

        params.put("refereeMobile", refereeMobile);

        StringBuilder builder = new StringBuilder();

        Set<String> keysSet = params.keySet();

        Object[] keys = keysSet.toArray();

        Arrays.sort(keys);

        boolean first = true;

        for (Object key : keys) {

            if (first) {

                first = false;

            } else {

                builder.append("&");

            }

            builder.append(key).append("=");

            Object value = params.get(key);

            String str = "";

            if (value != null) {

                str = String.valueOf(value);

            }

            builder.append(str);

        }

        return DigestUtil.md5Hex(builder.toString()).toUpperCase();

    }

}