Question

Let’s say I have a method as follows:

internal MyClass GetValue(long key)
{
  if (_myDictionary.ContainsKey(key))
    return _myDictionary[key];

  return null; // Not found
}

IDictionary<long,MyClass> _myDictionary=...

But the above code has two lookups in the dictionary:

1. The test for the existance of the key
2. The actual retrieval of the value

Is there a more optimal way to phrase such a function, so that only one lookup is performed, but the case of “not found” is still handled via a null return (i.e., not via a not found exception)?

For example, it would have been nice if the lookup for the key returned some sort of iterator that could be used to retrieve the value or an invalid iterator if the value was not found as is done in C++. Perhaps C# has a better way of doing this given its language features.

Answer 1

The TryGetValue method was designed for precisely this scenario – to avoid the two lookup operations.

This method combines the functionality of the ContainsKey method and
the Item property.

If the key is not found, then the value parameter gets the appropriate
default value for the value type TValue.

[…] Use the TryGetValue method if your code frequently attempts to access
keys that are not in the dictionary. Using this method is more
efficient than catching the KeyNotFoundException thrown by the Item
property.

internal MyClass GetValue(long key)
{
    MyClass maybeValue;

    // Normally, one would inspect the return value of this method
    // but in this case, it's not necessary since a failed lookup
    // will set the out argument to default(valType), which is null
    // for a reference type - exactly what you want.
   _myDictionary.TryGetValue(key, out maybeValue);

    return maybeValue; 
}