ContainsKey为字符串返回false

Question

I'm basically attempting to cache some organizations in a dictionary with their id and name. I keep getting an error: Item with Same Key has already been added . This is strange to me since the first if block checks if the dictionary contains the key and exits the method if it does.

Why am I getting the error given the (simplified) code below?

    public async Task<IList<Stat>> GetStats(DateTime start, DateTime end, string role, long tenantId)
    {
    ...
        var tasks = new List<Task>();
        foreach (var org in query)
            tasks.Add(GetOrgName(org));

        await Task.WhenAll(tasks);

        return query;

    }

    private Dictionary<string, string> Orgs = new Dictionary<string, string>();

    private async Task GetOrgName(Organization org)
    {
        if (Orgs.ContainsKey(org.Id))
        {
            org.Name = Orgs[org.Id];
            return;
        }

        var result = await _directoryService.GetOrganization(org.Id);

        if (result != null)
            org.Name = result.DisplayName;
        else
            org.Name = org.Id;

        Orgs.Add(org.Id, org.Name);
    }

Answer 1

This code looks dangerous

    if (org != null)
        org.Name = org.DisplayName;
    else
        org.Name = org.Id;

If org == null I think your else clause will fire NullReference exception. But you don't get it

Problem may be in this function

    var org = await _directoryService.GetOrganization(org.Id);

Note that this command means org can be modified and It may not contain its original Id

For example: If this never returns null value, just simple blank object You may get Item with Same Key has already been added exception

I think this can be fixed by double check

    if (!Orgs.ContainsKey(org.Id))
    {
        Orgs.Add(org.Id, org.Name);
    }

Answer 2

So the problem is that the dictionary is accessed by multiple tasks at the same time. So while one task is checking if the name exists another is adding it. Also since dictionary is not thread safe you might run into random exceptions with your current implementation.

If you change this:

var tasks = new List<Task>();
foreach (var org in query)
    tasks.Add(GetOrgName(org))

to:

foreach (var org in query)
   await GetOrgName(org);

You wont have that problem.

Another option would be to use a thread safe collection for the cache. But if query contains lets say 1000 items, you code might make a thousand concurrent calls to _directoryService.GetOrganization . Which might be a problem?

Answer 3

Since the dictionary is accessed by multiple threads, you should use ConcurrentDictionary instead of Dictionary. Otherwise it can happen, that two threads are trying to add the same key at the same time, leading to the exception you mentioned. Moreover, Dictionary is not thread-safe and can be corrupted if accessed by mutiple threads.

Answer 4

With your latest edit, looks like it's not a multi-threaded problem but just a problem with multiple async calls - you're most likely awaiting and doing multiple queries to GetOrganization for the same org.Id when the first one hasn't finished yet (so the dictionary has not been updated yet). So you most likely don't need ConcurrentDictionary . You can do something like the following instead:

private Dictionary<string, Task> Orgs = new Dictionary<string, Task>();

private Task GetOrgName(Organization org)
{
    Task nameTask;
    if (!Orgs.TryGetValue(org.Id, out nameTask)
    {
        nameTask = GetOrganization(.org);
        Orgs.Add(org.Id, nameTask);
    }

    return nameTask;
}

private async Task GetOrganization(Organization org)
{
    // Consider using .ConfigureAwait(false) here...
    var result = await _directoryService.GetOrganization(org.Id);
    if (result != null)
        org.Name = result.DisplayName;
    else
        org.Name = org.Id;
}

Note the signature for the Dictionary .

Answer 5

Make dictionary searching and adding to case insensitive. You can refer following:

private async Task GetOrgName(Organization org)
{
    if (Orgs.ContainsKey(org.Id.ToUpper()))
    {
        org.Name = Orgs[org.Id.ToUpper()];
        return;
    }

    var result = await _directoryService.GetOrganization(org.Id.ToUpper());

    if (result != null)
        org.Name = result.DisplayName;
    else
        org.Name = org.Id;

    Orgs.Add(org.Id.ToUpper(), org.Name);
}