c# - Why does waiting once on multiple calls to HttpClient.GetStringAsync() take as long as waiting individually on each call? -

using system.net.http;   using microsoft.visualstudio.testtools.unittesting;   using system.threading.tasks;   using system.diagnostics;    [testclass] public class unittest1 {     [testmethod]     public async task exceutemultiplerequestsinparallel()     {         stopwatch sw = new stopwatch();         sw.start();          httpclient client = new httpclient();         task<string> ms = client.getstringasync("http://www.microsoft.com");         task<string> msdn = client.getstringasync("http://msdn.microsoft.com");         task<string> blogs = client.getstringasync("http://blogs.msdn.com");         await task.whenall(ms, msdn, blogs);         sw.stop();         var result = sw.elapsed.tostring();     }      [testmethod]     public async task excecutemultiplerequests()     {         stopwatch sw = new stopwatch();         httpclient client = new httpclient();         string ms = await client.getstringasync("http://www.microsoft.com");         string msdn = await client.getstringasync("http://msdn.microsoft.com");         string blogs = await client.getstringasync("http://blogs.msdn.com");         sw.stop();         var result = sw.elapsed.tostring();     } } 

at first used 1 httpclient. expectation 1 await task.whenall(...) take time longest task take. expectation take measurably less calling each of 3 tasks await.

however, didn't seem case. had comment 1 otherwise cache distort things.

then used 3 httpclients , surprisingly method using 3 times await took less time 1 using await task.whenall(...). seems strange.

would please explain why not see same improvement in elapsed time using whenall() in first scenario (calling single httpclient 3 times) in second (calling 3 httpclient instances once each)?

[testmethod]     public async task exceutemultiplerequestsinparallel()     {         stopwatch sw = new stopwatch();         sw.start();         httpclient client = new httpclient();         httpclient client2 = new httpclient();         httpclient client3 = new httpclient();         task<string> ms = client.getstringasync("http://www.microsoft.com");         task<string> msdn = client2.getstringasync("http://msdn.microsoft.com");         task<string> blogs = client3.getstringasync("http://blogs.msdn.com");         await task.whenall(ms, msdn, blogs);         sw.stop();         var result = sw.elapsed.tostring();     }      [testmethod]     public async task excecutemultiplerequests()     {         stopwatch sw = new stopwatch();         sw.start();         httpclient client = new httpclient();         httpclient client2 = new httpclient();         httpclient client3 = new httpclient();         string ms = await client.getstringasync("http://www.microsoft.com");         string msdn = await client2.getstringasync("http://msdn.microsoft.com");         string blogs = await client3.getstringasync("http://blogs.msdn.com");         sw.stop();         var result = sw.elapsed.tostring();     } 

the behavior describe sounds normal me. see no reason expect single httpclient instance process more 1 http operation @ given time.

so when use single httpclient instance, if call getstringasync() again before first call has completed, class has no choice queue operation , not start 1 until previous 1 has completed. likewise third operation.

when use 3 different instances, each able proceed independently , 3 requests can happen concurrently.

the fact wait on 3 operations concurrently (i.e. call whenall()) cannot , not change order in executed internally httpclient. httpclient able give 3 different tasks wait on doesn't change fact single instance of able process 1 @ time. , since last operation cannot start until of enqueued operations have completed, same basic elapsed time if had waited each operation complete in turn before enqueuing next 1 yourself.