FirstOrDefault returns NullReferenceException if no match is found

Question

Here is my code:

string displayName = Dictionary.FirstOrDefault(x => x.Value.ID == long.Parse(options.ID)).Value.DisplayName; 

The code works fine if x.Value.ID matches options.ID. However, I get a NullReferenceException if it doesn't.

Answer 1

FirstOrDefault returns the default value of a type if no item matches the predicate. For reference types that is null. Thats the reason for the exception.

So you just have to check for null first:

string displayName = null; var keyValue = Dictionary     .FirstOrDefault(x => x.Value.ID == long.Parse(options.ID)); if(keyValue  != null) {     displayName = keyValue.Value.DisplayName; }  

But what is the key of the dictionary if you are searching in the values? A Dictionary<tKey,TValue> is used to find a value by the key. Maybe you should refactor it.

Another option is to provide a default value with DefaultIfEmpty:

string displayName = Dictionary     .Where(kv => kv.Value.ID == long.Parse(options.ID))     .Select(kv => kv.Value.DisplayName)   // not a problem even if no item matches     .DefaultIfEmpty("--Option unknown--") // or no argument -> null     .First();                             // cannot cause an exception 

Answer 2

You can use a combination of other LINQ methods to handle not matching condition:

var res = dictionary.Where(x => x.Value.ID == someID)                     .Select(x => x.Value.DisplayName)                     .DefaultIfEmpty("Unknown")                     .First();