那么，第一个问题可以通过使用来解决这个代替：

Well, the first problem can be solved using this instead:

var PrintURL = currurl + (String.IsNullOrEmpty(querystring) ? 
   HttpContext.Current.Request.QueryString.Add("print", "y") : string.Empty);

所有这一切从原来的code改变时只需将关闭括号从（String.IsNullOrEmpty（查询字符串））（它是不必要的），以结束的：条款。这使得它明确清楚你想要做的事。
否则，编译器试图串联的 String.IsNullOrEmpty（查询字符串）（这是一个布尔）以 currUrl - 不正确的，你首先希望不要什么。

All that's changed from your original code is simply moving the closing paren from (String.IsNullOrEmpty(querystring)) (where it was unnecessary) to the end of the ?: clause. This makes it explicitly clear what you're trying to do.
Otherwise, the compiler tries to concatenate the result of String.IsNullOrEmpty(querystring) (which is a bool) to currUrl -- incorrect, and not what you intended in the first place.

不过，你有一个第二个问题 HttpContext.Current.Request.QueryString.Add（打印，Y）语句。这将返回无效，不是字符串。你需要修改这部分的三元EX pression，使其返回一个字符串 - 什么是你想怎么办

However, you've got a second problem with the HttpContext.Current.Request.QueryString.Add("print", "y") statement. This returns void, not a string. You'll need to modify this part of your ternary expression so that it returns a string -- what are you trying to do?