三元运算符; 这个语法有什么问题？

Question

I'm trying to create a MailMessage, and I'm getting the following error...

 Cannot implicitly convert type 'string' to 'bool' 

This is my init statement:

MailMessage msg = new MailMessage("DoNotReply@optoma.com",
                      myTbl.Rows[i]["Requester"].ToString().Trim(),
                      subject, 
                      "Dear " + myTbl.Rows[i]["Ship_Attention"].ToString() + ",<br/><br/>" +
                      body + "<br/>Your ISO ID is " + myTbl.Rows[i]["ISO_ID"].ToString() + 
                      (Convert.ToInt32(myTbl.Rows[i]["EmailType"]) == 1) ? 
                          ("<br/>Tracking Number: " + myTbl.Rows[i]["Tracking_No"].ToString()) : 
                          ("") + "<br/><br/>Please examine the loaned items for this transaction:<br/><br/>" +
                      sw.ToString());

I'm trying to add to the string at runtime based on a boolean expression. Why can't I do this? Am I not doing it right?

string + (true | false) ? "somestring" : "anotherstring" + string

Answer 1

the ? : operator has very low precedence. Put it in parenthesis and I think you'll resolve your issue.

((true|false)?"somestring":"anotherstring")

Answer 2

When you have string + (bool)?"somestring":"anotherstring" + string the + gets evaluated before the ? , so you need parentheses:

string + ((bool)?"somestring":"anotherstring") + string

Answer 3

just cleaning it up a wee bit.... and you won't run into operator precedence problems so much

  void SendMessage(DataRow  row, string subject, string body, string sw)
    {
        var to = row["Requester"].ToString().Trim();
        var isoId = row["ISO_ID"].ToString();
        var attention = row["Ship_Attention"].ToString();
        var emailType = Convert.ToInt32(row["EmailType"]);
        var message = (emailType == 1) ? ("<br/>Tracking Number: " + row["Tracking_No"]) : ("");
        MailMessage msg = new MailMessage("DoNotReply@optoma.com",
                  to,
                  subject, 
                  string.Format("Dear {0},<br/><br/>{1}<br/>Your ISO ID is {2}{3}<br/><br/>Please examine the loaned items for this transaction:<br/><br/>{4}",
                                attention, body, isoId, message, sw));
    }

Answer 4

The precedence isn't what you expect -- + is being evaluated first. Your code should be in the form:

string + (true|false ? "somestring" : "anotherstring") + string

For your specific example:

MailMessage msg = new MailMessage("DoNotReply@optoma.com", myTbl.Rows[i]["Requester"].ToString().Trim(),
subject, "Dear " + myTbl.Rows[i]["Ship_Attention"].ToString() + ",<br/><br/>" +
body + "<br/>Your ISO ID is " + myTbl.Rows[i]["ISO_ID"].ToString() + (Convert.ToInt32(myTbl.Rows[i]["EmailType"]) == 1 ? ("<br/>Tracking Number: " + myTbl.Rows[i]["Tracking_No"].ToString()) : ("")) + "<br/><br/>Please examine the loaned items for this transaction:<br/><br/>" +
sw.ToString());

Note that this is a very long expression and should probably be broken down into several statements to make it more readable and maintainable.

Answer 5

The additive oprator (+) has higher priority than conditional (?:) as indicated here: http://msdn.microsoft.com/en-us/library/aa691323%28v=vs.71%29.aspx .

Therefore you need to put parenthesis around the whole condition:

string + ((true|false)?"somestring":"anotherstring") + string

I suggest you to divide your code into more lines, introduce some temporary variables and use string.format() to make it look clener. Finding errors in clean code is much easier.