String concatenation with ternary operator

Question

I am creating a SelectList of various contacts. I want the text shown to be a combination of FirstName, MiddleInit(if possible), and LastName.

IEnumerable<SelectListItem> items = contacts
.Select(r => new SelectListItem()
{
    Value = r.ContactID.ToString(),
    Text = r.FirstName + " " 
    + string.IsNullOrEmpty(r.MiddleInit) ? r.MiddleInit + ". " : ""
    + r.LastName
});

I am getting the error:

Error   4   Cannot implicitly convert type 'string' to 'bool'   C:\Users\cu551d\Documents\Visual Studio 2010\Projects\WVM\WVM\Controllers\SOWController.cs  181 15  WVM

I'm guessing it has something to do with my syntax. Am I able to do something like this in c#?

Also, should I include a Trim() statement on MiddleInit? I thought this error was very strange as IsNullOrEmpty does in fact return a bool. I also tried (string.IsNullOrEmpty(r.MiddleInit) == true ) ..

Answer 1

The problem is due to operator precedence causing the compiler to parse the expression in a way that you're not expecting. Just wrapping the ternary expression in parens will politely explain to the compiler what it is you mean to say.

As a side note, looking at the logic of the expression, I think the order of the result terms is incorrect. I've switched them in the code below.

IEnumerable<SelectListItem> items = contacts
.Select(r => new SelectListItem()
{
    Value = r.ContactID.ToString(),
    Text = r.FirstName + " " 
    + (string.IsNullOrEmpty(r.MiddleInit) ? "" : r.MiddleInit + ". ")
    + r.LastName
});

More explanation

Without (, + has a higher precedence than ?:, so the expression in your question is being parsed as if it said:

Test = (r.FirstName + " " + string.IsNullOrEmpty(r.MiddleInit)) ? (r.MiddleInit + ". ") : ("" + r.LastName)

So the source of your error is that the conditiional part of the ?: operator is actually a string - which will contain something like "Andrew true". Also note that r.LastName gets combine with the third term of the ?: operator.

The Trim() question

If there's a chance that r.MiddleInit could contain whitespace, then using Trim() would help, but it would be better to change the test to String.IsNullOrWhiteSpace(r.MiddleInit).

Answer 2

Try grouping in parenthesis. It also looks like you're doing the conditional operator backwards:

IEnumerable<SelectListItem> items = contacts
.Select(r => new SelectListItem()
{
    Value = r.ContactID.ToString(),
    Text = r.FirstName + " " +
        (string.IsNullOrEmpty(r.MiddleInit) ? "" : (r.MiddleInit + ". ")) +
        r.LastName
});

Another Technique

Since you are converting it to empty string anyway, you might consider this too:

Text = r.FirstName + " " + (r.MiddleInit ?? "") + r.LastName

The ?? is called the Null Coalescing Operator. It will return whatever is on the left if that value is not null, otherwise whatever is on the right.

http://msdn.microsoft.com/en-us/library/ms173224.aspx

EDIT: while cleaner, this won't quite work for your case since you need the period after the initial. But be aware it does exist!

Answer 3

Try wrapping your arguments in your ternary

IEnumerable<SelectListItem> items = contacts
.Select(r => new SelectListItem()
{
    Value = r.ContactID.ToString(),
    Text = r.FirstName + " " 
    + (string.IsNullOrEmpty(r.MiddleInit) ? (r.MiddleInit + ". ") : ("" + r.LastName))
});