Why ConcurrentDictionary has AddOrUpdate and GetOrAdd, but Dictionary has not?

Question

In the .NET Framework, there is Dictionary and ConcurrentDictionary. These provide method like Add, Remove, and so on...

I know when we design a multi-thread program, we use ConcurrentDictionary to replace Dictionary for thread-safety.

I wonder why ConcurrentDictionary has AddOrUpdate, GetOrAdd and similar methods, while Dictionary has not.

We always like below code to get object from a Dictionary:

var dict = new Dictionary<string, object>();
object tmp;
if (dict.ContainsKey("key"))
{
       tmp = dict["key"];
}
else
{
       dict["key"] = new object();
       tmp = new object();
}

but when using ConcurrentDictionary, similar code is just one line only.

var conDict = new ConcurrentDictionary<string, object>();
var tmp = conDict.GetOrAdd("key", new object());

I expect .NET to have those methods, but why it doesn't?

Answer 1

Because such methods are:

1. A bare minimum for working in a concurrent context. You can't split the Get and Add in two separate steps without locking, and still yield correct results.

2. When implemented for Dictionary<TKey, TValue>, it implicitly indicates some level of thread-safety, as if Dictionary<TKey, TValue> can handle this in a correct matter. It can't, so it is just not implemented. That doesn't stop you for making an extension method to do something similar.

    public static TValue GetOrAdd<TKey, TValue>(this Dictionary<TKey, TValue> dict, TKey key, Func<TKey, TValue> valueGenerator)
    {
        //
        // WARNING: this method is not thread-safe and not intended as such.
        //
        if (!dict.TryGetValue(key, out TValue value))
        {
            value = valueGenerator(key);
   
            dict.Add(key, value);
        }
   
        return value;
    }