异步方法返回任务＆LT; T＆GT;在C＃泛型约束 [英] Async method returning Task<T> with generic constraint in C#

问题描述

我实现了在一个项目，我的工作。这是pretty目前大部分结构：

I've implemented a command pattern in a project I'm working on. This is pretty much the current structure:

public class Response
{
    public bool Success { get; private set; }

    public static Response CreateErrorResponse()
    {
        return new Response { Success = false };
    }
}

public interface ICommand<T> where T : Response
{
    Task<T> ExecuteAsync();
}

public abstract CommandBase : ICommand<T> where T: Response
{
    protected abstract Uri BuildUrl();
    protected abstract Task<T> HandleResponseAsync();

    public async override Task<T> ExecuteAsync()
    {
        var url = BuildUrl();
        var httpClient = new HttpClient();

        var response = await httpClient.GetAsync(url);
        return await HandleResponseAsync(response);
    }
}

我要处理可能由HttpClient的抛出的异常，所以我想CommandBase.ExecuteAsync改变这样的事情...

I want to handle any exceptions that could be thrown by the HttpClient, so I want to change CommandBase.ExecuteAsync to something like this...

public async override Task<T> ExecuteAsync()
{
    var url = BuildUrl();
    var httpClient = new HttpClient();

    try
    {
        var response = await httpClient.GetAsync(url);
        return await HandleResponseAsync(response);
    }
    catch (HttpRequestException hex)
    {
        return Response.CreateErrorResponse(); // doesn't compile
    }
}

编译错误我得到的是无法将响应式异步返回类型T。我不能使用 T.CreateErrorResponse（），所概述<一href=\"http://stackoverflow.com/questions/196661/calling-a-static-method-on-a-generic-type-parameter\">in这个问题。

我如何解决此问题？

编辑downvoters：你是否与在这样的图书馆捕获异常同意，问题仍然有效。

Edit to downvoters: whether or not you agree with catching exceptions in a library like this, the question still stands!

推荐答案

虽然我不知道这是最好的解决办法（或在您的特定使用案例可行的），你可以做的是：

Although I am not sure this is the best solution (or feasible in your specific use case), what you can do is:

public class Response
{
    public bool Success { get; private set; }
    public ExceptionDispatchInfo ErrorInfo { get; private set; }
    public bool HasFailed
    {
        get { return !Success; }
    }

    public static T CreateErrorResponse<T>(ExceptionDispatchInfo errorInfo) where T : Response, new()
    {
        var response = new T();
        response.Success = false;
        response.ErrorInfo = errorInfo;
        return response;
    }
}

用法：

catch (HttpRequestException hex)
{
    return Response.CreateErrorResponse<T>(ExceptionDispatchInfo.Capture(hex)); // should compile (I did not check)
}

这篇关于异步方法返回任务＆LT; T＆GT;在C＃泛型约束的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！