[英]How to set a readonly property in a C# async constructor
我在 ASP.Net Core 2.2 Web API 中有一項服務。 構造函數是異步的,因為它調用異步方法。 但是因為構造函數是異步的,所以它會抱怨試圖初始化一個屬性。
public class MyService
{
private readonly IServiceScopeFactory _serviceScopeFactory;
public async Task MyService(IServiceScopeFactory serviceScopeFactory)
{
this._serviceScopeFactory = serviceScopeFactory;
await DoSomething();
}
}
它給了我這個錯誤:
“不能將只讀字段分配給(構造函數或變量初始化程序除外)”
有任何想法嗎?
正如用戶在上面的評論中提到的,我錯誤地認為我可以使構造函數異步。
Mathew Watson 和 Stephen Cleary 為我提供了一個很好的解決問題的鏈接: https://blog.stephencleary.com/2013/01/async-oop-2-constructors.html
概括:
工廠模式使用 static 創建方法,使類型成為自己的工廠:
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();
...
}
解決您的問題的一個常見示例是在 class 上創建一個 static 方法,並從那里調用async
方法以及構造函數。
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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