Program With Dave

I had a situation today where I wanted to return a user’s name using Linq. Sure, I could have easily written some code for this, but for the sake of keeping my code simple and readable I wanted Linq to handle the building of the name.

Users Table

The Linq seemed easy:

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public static string GetNameOfUser(int userID)
        {
            DatabaseDataContext dc = new DatabaseDataContext();
 
            var user = from u in dc.Users
                       where u.UserID == userID
                       select u.FirstName + " " + u.LastName;
 
            return (user.Count() == 0) ? "Unknown User" : user.First();
        }

However my results were not correct. Bob was left out because he did not have a last name value (it’s NULL in the database.) You will get results like this if you run that for each user (one at a time, but I put them in a table here.)

Results

In SQL it is very common to use the COALESCE() function to handle problems with null values. But how can I do that in Linq? I didn’t want to write extra code to check for nulls and concatenate the values myself.

The answer is simple: ??. Yes, that’s right. Two question marks is the “coalesce” operator in Linq. It’s a good idea to wrap what you are testing in coalesce inside parenthesis as well.

What follows is my new function.

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public static string GetNameOfUser(int userID)
        {
            DatabaseDataContext dc = new DatabaseDataContext();
 
            var user = from u in dc.Users
                       where u.UserID == userID
                       select (u.FirstName ?? "") + " " + (u.LastName ?? "");
 
            return (user.Count() == 0) ? "Unknown User" : user.First();
        }

As you can see, I’ve added ?? “” after both FirstName and LastName. This translates to:

coalesce(FirstName, ”)
coalesce(LastName, ”)

Which means that if FirstName is null you will get ”, which adds correctly in SQL to another string. Same goes for LastName.

Here are my new results:

Results

This entry was posted on Monday, November 23rd, 2009 at 10:52 am and is filed under CSharp, SQL. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.