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An implementation of ToDictionary that provides information about which key had duplicate entries.
public static class EnumerableExtensions
public static Dictionary<TKey, TSource> ToDictionaryBetter<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
return source.GroupBy(keySelector).ToDictionary(group => group.Key, group =>
return group.Single();
catch (InvalidOperationException)
throw new InvalidOperationException(
string.Format("Duplicate item with the key '{0}'", keySelector(@group.First())));
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