具有自定义任务的 C# 任务重写函数

Question

我有一个这样声明的函数：

public Task<PossibleUser> FindByNameAsync(string userName)
{
    Task<PossibleUser> task = context.TabelaStudenti.Where(
        apu => apu.NrMatricol == userName)
        .FirstOrDefaultAsync();

    return task;
}

问题是我现在想return那个函数的其他东西，例如，我试过：

 public Task<PossibleUser> FindByNameAsync(string userName)
    {
        Student studentUser = context.TabelaStudenti.Where(st => st.NrMatricol == userName).FirstOrDefault();
        Task<PossibleUser> someTask= new Task<PossibleUser>(() =>
        {
            if (studentUser == null)
            {
                return new PossibleUser()
                {
                    Password = studentUser.Parola,
                    UserName = studentUser.UserName
                };
            }
            else
            {
                //just return something else
                return new PossibleUser();
            }
        });
    return someTask;
    }

但它似乎不起作用，我没有收到该功能的响应。 我如何修改第一个函数以便 return 我自己的自定义任务？

我必须说明这个函数已经像这样被调用了：

PossibleUser user = await userManager.FindAsync(context.UserName, context.Password);

它是 ASP.NET Web API Owin 身份验证的一部分，其中 UserManager 是

    UserManagerFactory = () => new UserManager<PossibleUser>(new StudentStore()) { PasswordHasher = new MyPasswordHasher() };

Answer 1

您只是忘记启动新创建的任务：

public Task<PossibleUser> FindByNameAsync(string userName)
{
    Student studentUser = context.TabelaStudenti.Where(st => st.NrMatricol == userName).FirstOrDefault();
    Task<PossibleUser> someTask= new Task<PossibleUser>(() =>
    {
        if (studentUser == null)
        {
            return new PossibleUser()
            {
                Password = studentUser.Parola,
                UserName = studentUser.UserName
            };
        }
        else
        {
            //just return something else
            return new PossibleUser();
        }
    });

    // start your task
    someTask.Start();
    return someTask;
}

或者由于 C# 编译器足够智能，您可以直接 return PossibleUser 来自代码：

 public async Task<PossibleUser> FindByNameAsync(string userName)
 {
    Task<PossibleUser> task = context.TabelaStudenti.Where(apu => apu.NrMatricol == userName)
    .FirstOrDefaultAsync();
    // get result from async method
    var studentUser = await task;

    if (studentUser == null)
    {
        // Direct return without creating Task
        return new PossibleUser()
        {
            Password = studentUser.Parola,
            UserName = studentUser.UserName
        };
    }
    else
    {
        //just return something else
        return new PossibleUser();
    }
    return someTask;
 }

Answer 2

您可以使用 ContinueWith 这样的东西：

public Task<PossibleUser> FindByNameAsync(string userName)
{
    return context.TabelaStudenti
        .FirstOrDefaultAsync(st => st.NrMatricol == userName)
        .ContinueWith(task =>
        {
            var studentUser = task.Result;
            if (studentUser != null)
            {
                return new PossibleUser
                {
                    Password = studentUser.Parola,
                    UserName = studentUser.UserName
                };
            }
            //just return something else
            return new PossibleUser();
        });
}

Answer 3

永远不要使用任务构造函数。只需使用 async/await:

public async Task<PossibleUser> FindByNameAsync(string userName)
{
  Student studentUser = await context.TabelaStudenti.FirstOrDefaultAsync(st => st.NrMatricol == userName);
  if (studentUser != null)
  {
    return new PossibleUser()
    {
      Password = studentUser.Parola,
      UserName = studentUser.UserName
    };
  }
  else
  {
    //just return something else
    return new PossibleUser();
  }
}

具有自定义任务的 C# 任务重写函数

C# Task rewrite function with custom task

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