KeyedAsyncLock<TKey> Class

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

async Task ProcessUserAsync(string userId)
{
    using (await keyedLock.LockAsync(userId))
    {
        // Only one operation per userId at a time
        // Multiple different userIds can be processed concurrently
        var count = updateCounts.GetValueOrDefault(userId);
        await Task.Yield();
        updateCounts[userId] = count + 1;
    }
}

await Task.WhenAll(ProcessUserAsync("alice"), ProcessUserAsync("alice"), ProcessUserAsync("bob"));
Assert.Equal(2, updateCounts["alice"]);
Assert.Equal(1, updateCounts["bob"]);