Updating fields of values in a ConcurrentDictionary

Question

I am trying to update entries in a ConcurrentDictionary something like this:

class Class1
{
    public int Counter { get; set; }
}

class Test
{
    private          


        

Answer 1

ConcurrentDictionary doesn't help you with accessing members of stored values concurrently, just with the elements themselves.

If multiple threads call TestIt, you should get a snapshot of the collection and lock the shared resources (which are the individual dictionary values):

foreach (KeyValuePair kvp in dict.ToArray())
{
    Class1 value = kvp.Value;
    lock (value)
    {
        value.Counter = value.Counter + 1;
    }
}

However, if you want to update the counter for a specific key, ConcurrentDictionary can help you with atomically adding a new key value pair if the key does not exist:

Class1 value = dict.GetOrAdd(42, key => new Class1());
lock (value)
{
    value.Counter = value.Counter + 1;
}

AddOrUpdate and TryUpdate indeed are for cases in which you want to replace the value for a given key in a ConcurrentDictionary. But, as you said, you don't want to change the value, you want to change a property of the value.