如何在 C# 异步构造函数中设置只读属性

Question

I have a service in an ASP.Net Core 2.2 Web API. The constructor is async because it calls an async method. But because the constructor is async, it's complaining about trying to initialize a property.

public class MyService
{
    private readonly IServiceScopeFactory _serviceScopeFactory;

    public async Task MyService(IServiceScopeFactory serviceScopeFactory)
    {
        this._serviceScopeFactory = serviceScopeFactory;
        await DoSomething();
    }
}

It gives me this error:

"A readonly field cannot be assigned to (except in a constructor or a variable initializer)"

Any ideas?

Answer 1

As users mentioned in the comments above, I was mistaken to think that I could make a constructor async.

Mathew Watson and Stephen Cleary provided me with a link with a good alternative to my problem: https://blog.stephencleary.com/2013/01/async-oop-2-constructors.html

Summary:

Factory Pattern Use a static creation method, making the type its own factory:

public sealed class MyClass
{
  private MyData asyncData;
  private MyClass() { ... }

  private async Task<MyClass> InitializeAsync()
  {
    asyncData = await GetDataAsync();
    return this;
  }

  public static Task<MyClass> CreateAsync()
  {
    var ret = new MyClass();
    return ret.InitializeAsync();
  }
}

public static async Task UseMyClassAsync()
{
  MyClass instance = await MyClass.CreateAsync();
  ...
}

Answer 2

One common example to solve your problem is to create a static method on the class and call the async method from there and well as the constructor.

public class MyService
{
    private readonly IServiceScopeFactory _serviceScopeFactory;

    public static async Task<MyService> BuildMyService(IServiceScopeFactory serviceScopeFactory)
    {
        await DoSomething();
        return new MyService(serviceScopeFactory);
    }

    public MyService(IServiceScopeFactory serviceScopeFactory)
    {
        this._serviceScopeFactory = serviceScopeFactory;
    }
}