Sometimes I need to get a single thing from a collection but I’m not very happy with the Single extension. It does not differentiate between empty and more then one element and throws the same InvalidOperationException in both cases. I find it’s an important disadvantage and it helps a lot if you know excactly why and exception was thrown.

I want to replace it with my own extension that I currently name Single2 as throwing two different exceptions.

public static T Single2<T>(this IEnumerable<T> source) {     var count = 0;     var single = default(T);     using (var enumerator = source.GetEnumerator())     {         while (enumerator.MoveNext() && count++ < 1)         {             single = enumerator.Current;         }     }      switch ((SearchResult)count)     {         case SearchResult.NotFound: throw new EmptySequenceException();         case SearchResult.SingleMatch: return single;         default: throw new MoreThenOneElementException();     } }  public enum SearchResult {     NotFound = 0,     SingleMatch = 1,     ManyMatches = 2, } 

The exceptions are very simple:

public class EmptySequenceException : Exception { } public class MoreThenOneElementException : Exception { } 

That’s all. What do you think of this new helper? Is there still room for improvement?

Example

This is one of the methods that already uses the new Single2. It’s from my ResourceReader and it searches for an embedded resource in an assembly.

public static string FindName<T>([NotNull] this IResourceReader resources, [NotNull] Expression<Func<string, bool>> predicateExpression) {     if (resources == null) throw new ArgumentNullException(nameof(resources));     if (predicateExpression == null) throw new ArgumentNullException(nameof(predicateExpression));      var predicate = predicateExpression.Compile();      try     {         return             resources                 .GetResourceNames(typeof(T).Assembly)                 .Where(predicate)                 .Single2();     }     catch (EmptySequenceException innerException)     {         throw DynamicException.Factory.CreateDynamicException(             $  "ResourceNotFound{nameof(Exception)}",             $  "Expression {predicateExpression.ToString().QuoteWith("'")} does not match any resource in the {typeof(T).Assembly.GetName().Name.QuoteWith("'")} assembly.",             innerException);     }     catch (MoreThenOneElementException innerException)     {         throw DynamicException.Factory.CreateDynamicException(             $  "MoreThenOneResourceFound{nameof(Exception)}",             $  "Expression {predicateExpression.ToString().QuoteWith("'")} matches more then one resource in the {typeof(T).Assembly.GetName().Name.QuoteWith("'")} assembly.",             innerException);     }             }