An entry point cannot be marked with the 'async' modifier

问题

I copied below code from this link.But when I am compiling this code I am getting an entry point cannot be marked with the 'async' modifier. How can I make this code compilable?

class Program
{
    static async void Main(string[] args)
    {
        Task<string> getWebPageTask = GetWebPageAsync("http://msdn.microsoft.com");

        Debug.WriteLine("In startButton_Click before await");
        string webText = await getWebPageTask;
        Debug.WriteLine("Characters received: " + webText.Length.ToString()); 
    }

    private static async Task<string> GetWebPageAsync(string url)
    {
        // Start an async task. 
        Task<string> getStringTask = (new HttpClient()).GetStringAsync(url);

        // Await the task. This is what happens: 
        // 1. Execution immediately returns to the calling method, returning a 
        //    different task from the task created in the previous statement. 
        //    Execution in this method is suspended. 
        // 2. When the task created in the previous statement completes, the 
        //    result from the GetStringAsync method is produced by the Await 
        //    statement, and execution continues within this method. 
        Debug.WriteLine("In GetWebPageAsync before await");
        string webText = await getStringTask;
        Debug.WriteLine("In GetWebPageAsync after await");

        return webText;
    }

    // Output: 
    //   In GetWebPageAsync before await 
    //   In startButton_Click before await 
    //   In GetWebPageAsync after await 
    //   Characters received: 44306
}

回答1:

The error message is exactly right: the Main() method cannot be async, because when Main() returns, the application usually ends.

If you want to make a console application that uses async, a simple solution is to create an async version of Main() and synchronously Wait() on that from the real Main():

static void Main()
{
    MainAsync().Wait();
}

static async Task MainAsync()
{
    // your async code here
}

This is one of the rare cases where mixing await and Wait() is a good idea, you shouldn't usually do that.

Update: Async Main is supported in C# 7.1.

回答2:

Starting from C# 7.1 there are 4 new signatures for Main method which allow to make it async(Source, Source 2, Source 3):

public static Task Main();
public static Task<int> Main();
public static Task Main(string[] args);
public static Task<int> Main(string[] args);

You can mark your Main method with async keyword and use await inside Main:

static async Task Main(string[] args)
{
    Task<string> getWebPageTask = GetWebPageAsync("http://msdn.microsoft.com");

    Debug.WriteLine("In startButton_Click before await");
    string webText = await getWebPageTask;
    Debug.WriteLine("Characters received: " + webText.Length.ToString()); 
}

C# 7.1 is available in Visual Studio 2017 15.3.

回答3:

The difference between the code in the link's example and yours, is that you're trying to mark the Main() method with an async modifier - this is not allowed, and the error says that exactly - the Main() method is the "entry point" to the application (it's the method that is executed when your application starts), and it's not allowed to be async.

回答4:

Wrap your async code in MainAsync() - which is an async function
then call MainAsync().GetAwaiter().GetResult();

来源：https://stackoverflow.com/questions/16712172/an-entry-point-cannot-be-marked-with-the-async-modifier

标签

async-await

c#-5.0