How to Use HttpClient Properly in .NET

The HttpClient class lets you send HTTP requests and receive HTTP responses easily.

It’s essential to note that the HttpClient class implements the IDisposable interface. This means that when you create an HttpClient instance, you must either use the ‘using ‘keyword for automatic disposal or manually call the Dispose method to prevent resource leaks.

using HttpClient client = new HttpClient()
{
    BaseAddress = new Uri("http://localhost:5016")
};

But what if I tell you that this code is sometimes bad?

Socket Exhaustion Problem

As you know, for classes that implement an IDisposable interface, you must call the Dispose method. It disposes of unmanaged resources. 

However, the HttpClient class is different. The Dispose method doesn’t release the underlying socket immediately. So, if you initiate many HttpClients, you can get a socket exhaustion problem. You can read more about it in this blog post. 

In summary, if you instantiate a new HttpClient for every request in your application, be aware that this can rapidly consume all available sockets, especially under high loads. 

The suggestion is to create one instance of HttpClient for the whole application lifetime. However, it can lead to another problem. 

DNS Problem

From documentation:

HttpClient only resolves DNS entries when a connection is created. It does not track any time to live (TTL) durations specified by the DNS server. If DNS entries change regularly, which can happen in some scenarios, the client won’t respect those updates. 

One way to solve this issue is to limit the connection’s lifetime by setting the PooledConnectionLifetime property to repeat the DNS lookup when the connection is replaced. You can follow HttpClient Guidelines for that.

There is also another way to solve this issue.

IHttpClientFactory

.NET Core 2.1 introduced the IHttpClientFactory to create HttpClient instances for your application.

The IHttpClientFactory solves the issues mentioned above by managing the lifetime of HttpMessageHandler.

Using IHttpClientFactory, you can easily pre-configure the HttpClients for specific services and add Polly’s policies for resiliency.

The basic usage is super simple. You need to register IHttpClientFactory.

builder.Services.AddHttpClient();

Then, inject IHttpClientFactory and create a client.

app.MapGet("/test", async (IHttpClientFactory httpClientFactory) =>
{
    var httpClient = httpClientFactory.CreateClient();

    var response = await httpClient.GetFromJsonAsync<WeatherForecast[]>("/weatherforecast");
});

The factory supports the following clients:

• Named
• Typed
• Generated

You can register different pre-configured HttpClients by name. It’s helpful when you have many other services you want to communicate.

builder.Services.AddHttpClient("MyApi", httpClient =>
    {
        httpClient.BaseAddress = new Uri("http://localhost:5016");
    });

When you create an HttpClient, you need to pass its name.

var httpClient = httpClientFactory.CreateClient("MyApi");

The Typed clients are similar to Named, but you don’t need to use strings as names; you create types. 

A TypedClient must accept HttpClient in the constructor. 

public class MyApiService
{
    private readonly HttpClient _httpClient;

    public MyApiService(HttpClient httpClient)
    {
        _httpClient = httpClient;
        _httpClient.BaseAddress = new Uri("http://localhost:5016");
    }

    public async Task<WeatherForecast[]> GetWeatherForecasts() =>
        await _httpClient.GetFromJsonAsync<WeatherForecast[]>("/weatherforecast");
}

You use this type to register the HttpClient.

builder.Services.AddHttpClient<MyApiService>();

Then, you need to inject your service and use it.

app.MapGet("/test", async (MyApiService myApiService) =>
{
    var response = await myApiService.GetWeatherForecasts();
});

The IHttpClientFactory can also be used with third-party libraries, such as Refit.

You can read how to Boost Your .NET Development with Refit in another post.

Summary

The IHttpClientFactory is an excellent feature for managing HttpClients in .NET.
 
It solves sockets and DNS issues. 

It’s a centralized place for configuring many HTTP clients. 

It has many options to pre-configure your HttpClients, such as retry policies. You can register clients by name and type and use third-party libraries. 

And it’s easy to use.