操作员！ 不能应用于Task类型的操作数 <bool>

Question

I have following implementation and added a Async Task<bool> operation in the ChangeDate() method, previously it was just bool .

In the following line if (!ChangeDate())

Operator ! cannot be applied to operand of type Task

 public DateTime Date
 {
    get { return _date; }
    set
    {
      if (!ChangeDate())
      {
         return;
      }

      _date = value.Date;

    }
} 

private async Task<bool> ChangeDate()
{
  if (IsSave)
  {
     await Mvx.Resolve<IUserDialogs>().ConfirmAsync(new ConfirmConfig
     {
        Message = "Are you sure ?",
        OnConfirm = b =>
        {
            if (b)
            {
              Save();
            }
         }
      });
   }
   return true;
 }

Answer 1

Maybe you want something like this ... ?

public DateTime Date { get; private set; }

public Task SetDateIfUserConfirmsAsync( DateTime proposedDate) 
{ 
     var confirmConfig = new ConfirmConfig() {
         Message = "Are you sure ?",
         OnConfirm = b => { if (b) { this.Date = proposedDate; } }
     }
     await Mvx.Resolve<IUserDialogs>().ConfirmAsync( confirmConfig);
}

You haven't said what UI you are using but likely you are in a button callback, which has a synchronous signature (void return). In that case you want to make sure that you catch any exceptions because otherwise you will never see them (exceptions are not propagated back to the caller of an async void method). A bit like this:

public async void OnButtonClick()
{ 
    try {
        DateTime proposedDate = ...;
        await SetDateIfUserConfirmsAsync( proposedDate)
    } catch (Exception e) {
        // display or log the exception
    }
 }

PS On the error you saw, "operator ! cannot be applied to operand of type Task<bool>", it was because you did not await the task. If t has type Task<bool> then "await t" has type bool.

Answer 2

Found this answer: How to make an Asynchronous Method return a value?

You'll need to make an async helper method, and call that instead of calling just your setter, because as @Ron Beyer points out, the await call will not work in a property.

Replace:

if(!ChangeDate())

With:

if( !(await ChangeDate()) )