在 C# 中返回具有通用约束的 Task<T> 的异步方法

Question

我在a project I'm working on 中实现了一个命令模式。这几乎是当前的结构：

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);
    }
}

我想处理 HttpClient 可能抛出的任何异常，所以我想将 CommandBase.ExecuteAsync 更改为类似这样的内容...

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
    }
}

我得到的编译错误是“无法将类型响应转换为异步返回类型 T”。如in this question 所述，我不能使用T.CreateErrorResponse()。

我该如何解决这个问题？

编辑反对票：无论您是否同意在这样的库中捕获异常，问题仍然存在！

Answer 1

虽然我不确定这是最好的解决方案（或在您的特定用例中是否可行），但您可以做的是：

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)
}

Answer 2

您可以将响应转换为 T。编辑：添加完整源代码

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

    public async Task<T> ExecuteAsync()
{
    var url = BuildUrl();
    var httpClient = new System.Net.Http.HttpClient();

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

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 (T)Response.CreateErrorResponse(); // compiles on liqpad
    }
}