Singleton httpclient vs creating new httpclient request

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Update: It seems that using a single static instance of HttpClient doesn’t respect DNS changes, so the solution is to use HttpClientFactory. See here for Microsoft docs about it.

To use the HttpClientFactory you have to use Microsoft’s dependency injection. This is the default for ASP.NET Core projects, but for others you will have to reference Microsoft.Extensions.Http and Microsoft.Extensions.DependencyInjection.

Then when you’re creating your service container, you simply call AddHttpClient():

var services = new ServiceCollection();
services.AddHttpClient()
var serviceProvider = services.BuildServiceProvider();

And then you can inject IHttpClientFactory into your services, and behind the scenes HttpClientFactory will maintain a pool of HttpClientHandler objects – keeping your DNS fresh and preventing problems with connection pool exhaustion.

Old answer:

Singleton is the correct way to use HttpClient. Please see this article for full details.

Microsoft docs state:

HttpClient is intended to be instantiated once and re-used throughout the life of an application. Instantiating an HttpClient class for every request will exhaust the number of sockets available under heavy loads. This will result in SocketException errors. Below is an example using HttpClient correctly.

And indeed, we found this in our application. We have code that can potentially make hundreds of API requests in a foreach loop, and for each iteration we were creating an HttpClient wrapped in a using. We soon started getting red herring errors from our MongoClient saying that it had timed out trying to connect to the database. After reading the linked article, we found that even after disposing of HttpClient, and realised that we were exhausting the available sockets.

The only thing to note is that things like DefaultRequestHeaders and BaseAddress will be applied anywhere that HttpClient is used. As a singleton, this is potentially throughout the application. You can still create multiple HttpClient instances in your application, but just be aware that each time you do, they create a new connection pool and, as such, should be created sparingly.

As pointed out by hvaughan3, you also can’t change the instance of HttpMessageHandler used by the HttpClient, so if this matters to you, you would need to use a separate instance with that handler.

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