Dictionary.FirstOrDefault() how to determine if a result was found

Question

I have (or wanted to have) some code like this:

IDictionary<string,int> dict = new Dictionary<string,int>(); // ... Add some stuff to the dictionary.  // Try to find an entry by value (if multiple, don't care which one). var entry = dict.FirstOrDefault(e => e.Value == 1); if ( entry != null ) {     // ^^^ above gives a compile error:    // Operator '!=' cannot be applied to operands of type 'System.Collections.Generic.KeyValuePair<string,int>' and '<null>' } 

I also tried changing the offending line like this:

if ( entry != default(KeyValuePair<string,int>) )  

But that also gives a compile error:

Operator '!=' cannot be applied to operands of type 'System.Collections.Generic.KeyValuePair<string,int>' and 'System.Collections.Generic.KeyValuePair<string,int>' 

What gives here?

Answer 1

Jon's answer will work with Dictionary<string, int>, as that can't have a null key value in the dictionary. It wouldn't work with Dictionary<int, string>, however, as that doesn't represent a null key value... the "failure" mode would end up with a key of 0.

Two options:

Write a TryFirstOrDefault method, like this:

public static bool TryFirstOrDefault<T>(this IEnumerable<T> source, out T value) {     value = default(T);     using (var iterator = source.GetEnumerator())     {         if (iterator.MoveNext())         {             value = iterator.Current;             return true;         }         return false;     } } 

Alternatively, project to a nullable type:

var entry = dict.Where(e => e.Value == 1)                 .Select(e => (KeyValuePair<string,int>?) e)                 .FirstOrDefault();  if (entry != null) {     // Use entry.Value, which is the KeyValuePair<string,int> }