每个请求创建全新的 HttpClient 是否便宜

Question

I always thought it's an antipattern since MS docs was always saying " HttpClient is intended to be instantiated once per application, rather than per-use". But the more I dive into modern C# development the more I see things changed. For example here is a tweet about IHttpClientFactory

A new HttpClient is created each time, but HttpClient is cheap to create.

The internal HTTP handler that the HttpClient uses to make the request (ie the part that does all the work and is expensive) is intelligently cached and reused by the factory.

Which says if you reuse HttpMessageHandler for multiple HttpClient s you're golden.

Assume we want to set some headers for each HTTP request. Should we do it the old fashion way with HttpRequestMessage interceptors or could I just create a per-request HttpClient and since I'm going to reuse the same HttpRequestMessage I'm completely fine? Is it the modern way to perform per-request message modifications?

Answer 1

HttpRequestMessage is not intended to be reused. HttpClient will actually modify the HttpRequestMessage when sending your request, so it's not a good idea to use it afterward.

In general, use IHttpClientFactory . It does more than just reusing a message handler, such as periodic recycling of connection pools.

One good use of creating multiple HttpClient instances against a single message handler is if you want to set different default headers for calling multiple APIs against the same endpoint. This allows for pooling of connections to that endpoint without needing to set headers on each message.