How is async w/ await different from a synchronous call?

Question

I was reading about asynchronous function calls on http://msdn.microsoft.com/en-us/library/vstudio/hh191443.aspx?cs-save-lang=1&cs-lang=csharp.

At the first example, they do this, which I get:

Task<string> getStringTask = client.GetStringAsync("http://msdn.microsoft.com");  // You can do work here that doesn't rely on the string from GetStringAsync. DoIndependentWork();  string urlContents = await getStringTask; 

But then they explain that if there's not any work to be done in the mean time, you can just do it like this:

string urlContents = await client.GetStringAsync(); 

From what I understand, the await keyword will suspend the code flow until the function returns. So how is this different from:

string urlContents = client.GetString(); 

?

Answer 1

Calling await client.GetStringAsync() yields the execution to the calling method, which means it won't wait for the method to finish executing, and thus won't block the thread. Once it's done executing in the background, the method will continue from where it stopped.

If you just call client.GetString(), the thread's execution won't continue until this method finished executing, which will block the thread and may cause the UI to become unresponsive.

Example:

public void MainFunc() {     InnerFunc();     Console.WriteLine("InnerFunc finished"); }  public void InnerFunc() {     // This causes InnerFunc to return execution to MainFunc,     // which will display "InnerFunc finished" immediately.     string urlContents = await client.GetStringAsync();      // Do stuff with urlContents }  public void InnerFunc() {     // "InnerFunc finished" will only be displayed when InnerFunc returns,     // which may take a while since GetString is a costly call.     string urlContents = client.GetString();      // Do stuff with urlContents }