c# - 异步 lambda 表达式转换为 'Task' 返回委托(delegate)无法返回值

Question

我在 ASP.NET Core 应用程序中有以下内容( Startup.cs ， Configure 方法):

我刚刚添加了 async关键字，因为我需要 await一...所以现在，我得到以下信息:

Error CS8031 Async lambda expression converted to a 'Task' returning delegate cannot return a value. Did you intend to return 'Task<T>'?

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory, ITableRepositories repository)
{
    // ...  
    app.UseStaticFiles();    
    app.UseCookieAuthentication();

    app.UseOpenIdConnectAuthentication(new OpenIdConnectOptions
    {
        ClientId = Configuration["..."],
        Authority = Configuration["..."],
        CallbackPath = Configuration["..."],
        Events = new OpenIdConnectEvents
        {
            OnAuthenticationFailed = context => { return Task.FromResult(0); },
            OnRemoteSignOut = context => { return Task.FromResult(0); },
            OnTicketReceived = async context =>
            {
                var user = (ClaimsIdentity)context.Ticket.Principal.Identity;
                if (user.IsAuthenticated)
                {
                    var firstName = user.FindFirst(ClaimTypes.GivenName).Value;
                    // ...
                    List<Connection> myList = new List<Connection>() { c };
                    var results = await repository.InsertOrMergeAsync(myList);
                    var myConnection = (results.First().Result as Connection);                         
                }
                return Task.FromResult(0); // <<< ERROR HERE ....... !!!
            },
            OnTokenValidated = context => { return Task.FromResult(0); },
            OnUserInformationReceived = context => { return Task.FromResult(0); },
        }
    });

    app.UseMvc(routes =>        { ...        });
}

在那种情况下我应该返回什么？我试过 return 0; , 但错误信息没有改变...

附言。 OnTicketRecieved签名

namespace Microsoft.AspNetCore.Authentication
{
    public class RemoteAuthenticationEvents : IRemoteAuthenticationEvents
    {
        public Func<TicketReceivedContext, Task> OnTicketReceived { get; set; }

Answer 1

错误信息实际上是不言自明的:

Async lambda expression converted to a 'Task' returning delegate cannot return a value. Did you intend to return 'Task<T>'?

所以你有一个异步 lambda 表达式，它应该返回一个 Task —不是Task<T>对于任何 T .

但是当你做 return 0你返回一个 int，所以你的异步方法的返回类型是 Task<int> , 不是 Task .编译器希望您做的是返回根本没有值。

OnTicketReceived = async context =>
{
    await Task.Delay(100);
    return;
}