async-await中await是否阻塞当前方法？两段代码执行差异咨询

Great question—this is one of the most common "aha!" moments when learning async/await, so let’s break down exactly what’s happening in both code snippets.

First Code Snippet: Serial Execution #

var r1 = await httpClient.GetAsync(url1); 
var r2 = await httpClient.GetAsync(url2);

• When you use await directly on httpClient.GetAsync(url1), the method pauses right here until the entire request to url1 completes (including downloading the response content).
• Only after r1 is fully retrieved does the code move on to call httpClient.GetAsync(url2).
• So yes, the url2 request does wait for url1 to finish downloading before it starts. Total time here is roughly the sum of both request durations.

Second Code Snippet: Parallel Execution #

var r1 = httpClient.GetAsync(url1); 
var r2 = httpClient.GetAsync(url2); 
var response1 = await r1; 
var response2 = await r2;

• Here, you first call GetAsync(url1) and GetAsync(url2) back-to-back without awaiting them immediately. These calls start the HTTP requests right away (HttpClient’s async methods initiate the request and return a Task that represents the ongoing operation).
• The await r1 and await r2 lines just pause the method until each respective task completes—but since both requests were already started, they’re running in parallel.
• So yes, both URLs are requested simultaneously. The total time here is roughly the duration of the slower of the two requests, not their sum.

Key Difference Between the Two Snippets #

• First snippet: Requests run one after another (serial). You’re waiting for each to finish before starting the next.
• Second snippet: Requests run at the same time (parallel). You start both first, then wait for both to finish.

This is a critical distinction for performance—if you’re making multiple independent async calls, starting them all first before awaiting can drastically reduce total execution time.

内容的提问来源于stack exchange，提问作者leopik