Fernando Correia Fernando Correia -4 years ago 277
C# Question

Do HttpClient and HttpClientHandler have to be disposed?

System.Net.Http.HttpClient and System.Net.Http.HttpClientHandler in .NET Framework 4.5 implement IDisposable (via System.Net.Http.HttpMessageInvoker).

The

using
statement documentation says:


As a rule, when you use an IDisposable object, you should declare and
instantiate it in a using statement.


This answer uses this pattern:

var baseAddress = new Uri("http://example.com");
var cookieContainer = new CookieContainer();
using (var handler = new HttpClientHandler() { CookieContainer = cookieContainer })
using (var client = new HttpClient(handler) { BaseAddress = baseAddress })
{
var content = new FormUrlEncodedContent(new[]
{
new KeyValuePair<string, string>("foo", "bar"),
new KeyValuePair<string, string>("baz", "bazinga"),
});
cookieContainer.Add(baseAddress, new Cookie("CookieName", "cookie_value"));
var result = client.PostAsync("/test", content).Result;
result.EnsureSuccessStatusCode();
}


But the most visible examples from Microsoft don't call
Dispose()
either explicitly or implicitly. For instance:



In the announcement's comments, someone asked the Microsoft employee:


After checking your samples, I saw that you didn't perform the dispose
action on HttpClient instance. I have used all instances of HttpClient
with using statement on my app and I thought that it is the right way
since HttpClient implements the IDisposable interface. Am I on the
right path?


His answer was:


In general that is correct although you have to be careful with
"using" and async as they dont' really mix in .Net 4, In .Net 4.5 you
can use "await" inside a "using" statement.

Btw, you can reuse the same HttpClient as many times are [as] you like so
typically you won't create/dispose them all the time.


The second paragraph is superflous to this question, which is not concerned about how many times you can use an HttpClient instance, but if it necessary to dispose it after you no longer need it.

(Update: in fact that second paragraph is the key to the answer, as provided below by @DPeden.)

So my questions are:


  1. Is it necessary, given the current implementation (.NET Framework 4.5), to call Dispose() on HttpClient and HttpClientHandler instances? Clarification: by "necessary" I mean if there are any negative consequences for not disposing, such as resource leakage or data corruption risks.

  2. If it's not necessary, would it be a "good practice" anyway, since they implement IDisposable?

  3. If it's necessary (or recommended), is this code mentioned above implementing it safely (for .NET Framework 4.5)?

  4. If these classes don't require calling Dispose(), why were they implemented as IDisposable?

  5. If they require, or if it's a recommended practice, are the Microsoft examples misleading or unsafe?


Answer Source

The general consensus is that you do not (should not) need to dispose of HttpClient.

Many people who are intimately involved in the way it works have stated this.

See Darrel Miller's blog post and a related SO post: HttpClient crawling results in memory leak for reference.

I'd also strongly suggest that you read the HttpClient chapter from Designing Evolvable Web APIs with ASP.NET for context on what is going on under the hood, particularly the "Lifecycle" section quoted here:

Although HttpClient does indirectly implement the IDisposable interface, the standard usage of HttpClient is not to dispose of it after every request. The HttpClient object is intended to live for as long as your application needs to make HTTP requests. Having an object exist across multiple requests enables a place for setting DefaultRequestHeaders and prevents you from having to re-specify things like CredentialCache and CookieContainer on every request as was necessary with HttpWebRequest.

Or even open up DotPeek.

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