Why does ConcurrentDictionary.TryRemove require a second out argument?

C#7 added discard syntactic sugar

So now you can write:

dictionary.TryRemove(entry.Key, out _); 

Reference

We allow "discards" as out parameters as well, in the form of a _, to let you ignore out parameters you don’t care about:

p.GetCoordinates(out var x, out _); // I only care about x

If you're not interested in the value that was removed, simply call IDictionary.Remove(key). It's shadowed, so you have to invoke it explicitly.

Example:

var dict = new ConcurrentDictionary<string, string>();
dict.AddOrUpdate("mykey", (val) => "test", (val1, val2) => "test");
((IDictionary)dict).Remove("mykey");

The TryRemove(key, out value) method is there to give you feedback whether the operation made any change. Use the one that best suits your needs.

You can create exactly the method you want:

public static class ConcurrentDictionaryEx {
  public static bool TryRemove<TKey, TValue>(
    this ConcurrentDictionary<TKey, TValue> self, TKey key) {
    TValue ignored;
    return self.TryRemove(key, out ignored);
  }
}

UPDATE: Or, as Dialecticus mentioned in the comments, just use Remove. But note that, since it's an explicit interface implementation, you'll need a reference to an IDictionary<TKey, TValue>, which leads you back to creating an extension method if you want to avoid casting a ConcurrentDictionary<TKey, TValue> reference:

public static class ConcurrentDictionaryEx {
  public static bool Remove<TKey, TValue>(
    this ConcurrentDictionary<TKey, TValue> self, TKey key) {
      return ((IDictionary<TKey, TValue>)self).Remove(key);
  }
}