Use GraphServiceClient with HttpClient in muti-thread apps

If you use GraphServiceClient from Graph SDK for .Net in multi-thread app consider reusing it as static instance instead of creating own instances inside each thread. GraphServiceClient itself is thread safe (see here) and can be reused within multiple threads as is (without locks, synchronizations, etc).

Otherwise it will internally create multiple HttpClient instances and you may face with reaching sockets limit. Alternatively you may create static HttpClient and then pass it to the GraphServiceClient instances created per thread as it has special constructor for that:

public GraphServiceClient(HttpClient httpClient)
  : base("https://graph.microsoft.com/v1.0", httpClient)
{
}

If you create threads intensively there may be too many instances of HttpClient created and you will reach sockets limit. E.g. this is how it looked in Azure function app with queue-triggered Azure function when GraphServiceClient instances were created inside function call (own instance per thread):

I.e. there were 2K connection peaks and the following errors in the logs:

An operation on a socket could not be performed because the system lacked sufficient buffer space or because a queue was full

When I changed the code and reused static instance of GraphServiceClient connections count got stabilized: