Linq的FirstOrDefault

Question

I am trying to move this foreach loop to linq:

compData = componentData[0];
foreach (var componentTraceData in componentData)
{
     if (!string.IsNullOrEmpty(componentTraceData.CompName))
     {
          compData = componentTraceData;
          break;
     }
}

And this is what I tried:

var tt = (from n in componentData 
          where !string.IsNullOrEmpty(n.CompName) 
          select n).FirstOrDefault();

How I can make it to take componentData[0] in case linq not found any results ?

Answer 1

Take the line you currently have and add ?? componentData[0] ?? componentData[0] at the end.

?? is the null coalescing operator. It's just shorthand for, "if what's to my left is null, return what's on my right. If it's not null, return that."

Answer 2

You can use Enumerable.DefaultIfEmpty and specify a custom default value:

var result = componentData.Where(n => !string.IsNullOrEmpty(n.CompName))
                          .DefaultIfEmpty(componentData.ElementAtOrDefault(0))
                          .First();

Note that you can now use First safely since the sequence cannot be empty anymore. ElementAtOrDefault will return default(T) (null for reference types) if the source sequence is empty. This will prevent an exception when you use the indexer of an IList<T> directly.

Answer 3

var tt = componentData.FirstOrDefault(n=!string.IsNullOrEmpty(n.CompName)) ?? componentData[0];

虽然您想要检查componentData为null，并且count / length也是大于零。