C# doesn't allow async constructors. Constructors are meant to return fast after some brief initialization. You don't expect and you don't want to wait for an instance i.e. the constructor to return. Therefore, even if async constructors were possible, a constructor is not a place for long-running operations or starting background threads. The only purpose of a constructor is initialization of instance or class members to a default value or the captured constructor parameters. You always create the instance and then call DoSomething() on this instance. Async operations are no exception. You always defer expensive initialization of members.
有一些解决方案可以避免对异步构造函数的需求。
使用Lazy<T>或AsyncLazy<T>的简单替代解决方案(需要通过NuGet包管理器安装Microsoft.VisualStudio.Threading包)。Lazy<T>允许延迟实例化或昂贵资源的分配。
public class OrderService
{
public List<object> Orders => this.OrdersInitializer.GetValue();
private AsyncLazy<List<object>> OrdersInitializer { get; }
public OrderService()
=> this.OrdersInitializer = new AsyncLazy<List<object>>(InitializeOrdersAsync, new JoinableTaskFactory(new JoinableTaskContext()));
private async Task<List<object>> InitializeOrdersAsync()
{
await Task.Delay(TimeSpan.FromSeconds(5));
return new List<object> { 1, 2, 3 };
}
}
public static void Main()
{
var orderService = new OrderService();
// Trigger async initialization
orderService.Orders.Add(4);
}
可以使用方法而不是属性公开数据
public class OrderService
{
private List<object> Orders { get; set; }
public async Task<List<object>> GetOrdersAsync()
{
if (this.Orders == null)
{
await Task.Delay(TimeSpan.FromSeconds(5));
this.Orders = new List<object> { 1, 2, 3 };
}
return this.Orders;
}
}
public static async Task Main()
{
var orderService = new OrderService();
// Trigger async initialization
List<object> orders = await orderService.GetOrdersAsync();
}
使用InitializeAsync方法，在使用实例之前必须调用该方法
public class OrderService
{
private List<object> orders;
public List<object> Orders
{
get
{
if (!this.IsInitialized)
{
throw new InvalidOperationException();
}
return this.orders;
}
private set
{
this.orders = value;
}
}
public bool IsInitialized { get; private set; }
public async Task<List<object>> InitializeAsync()
{
if (this.IsInitialized)
{
return;
}
await Task.Delay(TimeSpan.FromSeconds(5));
this.Orders = new List<object> { 1, 2, 3 };
this.IsInitialized = true;
}
}
public static async Task Main()
{
var orderService = new OrderService();
// Trigger async initialization
await orderService.InitializeAsync();
}
通过将昂贵的参数传递给构造函数来实例化实例
public class OrderService
{
public List<object> Orders { get; }
public async Task<List<object>> OrderService(List<object> orders)
=> this.Orders = orders;
}
public static async Task Main()
{
List<object> orders = await GetOrdersAsync();
// Instantiate with the result of the async operation
var orderService = new OrderService(orders);
}
private static async Task<List<object>> GetOrdersAsync()
{
await Task.Delay(TimeSpan.FromSeconds(5));
return new List<object> { 1, 2, 3 };
}
使用工厂方法和私有构造函数
public class OrderService
{
public List<object> Orders { get; set; }
private OrderServiceBase()
=> this.Orders = new List<object>();
public static async Task<OrderService> CreateInstanceAsync()
{
var instance = new OrderService();
await Task.Delay(TimeSpan.FromSeconds(5));
instance.Orders = new List<object> { 1, 2, 3 };
return instance;
}
}
public static async Task Main()
{
// Trigger async initialization
OrderService orderService = await OrderService.CreateInstanceAsync();
}
