KeyedLock<TKey> Class

var keyedLock = new KeyedLock<string>(StringComparer.Ordinal);
var updateCounts = new ConcurrentDictionary<string, int>(StringComparer.Ordinal);

void ProcessUser(string userId)
{
    using (keyedLock.Lock(userId))
    {
        // Only one operation per userId at a time
        // Multiple different userIds can be processed concurrently
        var count = updateCounts.GetValueOrDefault(userId);
        updateCounts[userId] = count + 1;
    }
}

Parallel.Invoke(() => ProcessUser("alice"), () => ProcessUser("alice"), () => ProcessUser("bob"));
Assert.Equal(2, updateCounts["alice"]);
Assert.Equal(1, updateCounts["bob"]);