[英]How to set a readonly property in a C# async constructor
我在 ASP.Net Core 2.2 Web API 中有一项服务。 构造函数是异步的,因为它调用异步方法。 但是因为构造函数是异步的,所以它会抱怨试图初始化一个属性。
public class MyService
{
private readonly IServiceScopeFactory _serviceScopeFactory;
public async Task MyService(IServiceScopeFactory serviceScopeFactory)
{
this._serviceScopeFactory = serviceScopeFactory;
await DoSomething();
}
}
它给了我这个错误:
“不能将只读字段分配给(构造函数或变量初始化程序除外)”
有任何想法吗?
正如用户在上面的评论中提到的,我错误地认为我可以使构造函数异步。
Mathew Watson 和 Stephen Cleary 为我提供了一个很好的解决问题的链接: https://blog.stephencleary.com/2013/01/async-oop-2-constructors.html
概括:
工厂模式使用 static 创建方法,使类型成为自己的工厂:
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();
...
}
解决您的问题的一个常见示例是在 class 上创建一个 static 方法,并从那里调用async
方法以及构造函数。
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;
}
}
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