samirahmed samirahmed - 17 days ago 7
C# Question

Retrying HttpClient Unsuccessful Requests

I am building a function that given an HttpContent Object, will issues request and retry on failure. However I get exceptions saying that HttpContent Object is disposed after issuing the request. Is there anyway to copy or duplicate the HttpContent Object so that I can issue multiple requests.

public HttpResponseMessage ExecuteWithRetry(string url, HttpContent content)
{
HttpResponseMessage result = null;
bool success = false;
do
{
using (var client = new HttpClient())
{
result = client.PostAsync(url, content).Result;
success = result.IsSuccessStatusCode;
}
}
while (!success);

return result;
}

// Works with no exception if first request is successful
ExecuteWithRetry("http://www.requestb.in/xfxcva" /*valid url*/, new StringContent("Hello World"));
// Throws if request has to be retried ...
ExecuteWithRetry("http://www.requestb.in/badurl" /*invalid url*/, new StringContent("Hello World"));


(Obviously I don't try indefinitely but the code above is essentially what i want).

It yields this exception

System.AggregateException: One or more errors occurred. ---> System.ObjectDisposedException: Cannot access a disposed object.
Object name: 'System.Net.Http.StringContent'.
at System.Net.Http.HttpContent.CheckDisposed()
at System.Net.Http.HttpContent.CopyToAsync(Stream stream, TransportContext context)
at System.Net.Http.HttpClientHandler.GetRequestStreamCallback(IAsyncResult ar)
--- End of inner exception stack trace ---
at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
at System.Threading.Tasks.Task`1.get_Result()
at Submission#8.ExecuteWithRetry(String url, HttpContent content)


Is there anyway to duplicate an HttpContent Object or reuse it?

Answer

Instead of implementing retry functionality that wraps the HttpClient, consider constructing the HttpClient with a HttpMessageHandler that performs the retry logic internally. For example:

public class RetryHandler : DelegatingHandler
{
    // Strongly consider limiting the number of retries - "retry forever" is
    // probably not the most user friendly way you could respond to "the
    // network cable got pulled out."
    private const int MaxRetries = 3;

    public RetryHandler(HttpMessageHandler innerHandler)
        : base(innerHandler)
    { }

    protected override async Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request,
        CancellationToken cancellationToken)
    {
        HttpResponseMessage response;
        for (int i = 0; i < MaxRetries; i++)
        {
            response = await base.SendAsync(request, cancellationToken);
            if (response.IsSuccessStatusCode) {
                return response;
            }
        }

        return response;
    }
}

public class BusinessLogic
{
    public void FetchSomeThingsSynchronously()
    {
        // ...

        // Consider abstracting this construction to a factory or IoC container
        using (var client = new HttpClient(new RetryHandler(new HttpClientHandler())))
        {
            myResult = client.PostAsync(yourUri, yourHttpContent).Result;
        }

        // ...
    }
}