Async Command Wpf C#

Question

The program has an update button which sends a request to the server. My task is that when you click on the button, the request to the server is started in a separate asynchronous thread, and on the user interface (wpf) a image "spinner-loading" appears. I get an error:

"Cannot convert lambda expression to intended delegate type because some of the return types in the block are not implicitly convertible to the delegate return type".

How can I combine async and void or, can I do something any better?

ViewModel:

public CarasViewModel()
    {
      ...
     AddCommand = new AsyncCommand<Task>(() => Add());
      ...
    }

public IAsyncCommand AddCommand { get; private set; }
    async Task Add()
    {
        await Task.Run(() => OnAdd());
    }

void OnAdd()
    {
        var Result = Helper.Get(Configuration.Settings);
        if (Result != null)
        {
            var SelectionViewModel = new SelectionViewModel(Result);
            if (DialogService.ShowModalWindow(selectionViewModel))
            {
                ...
            }
        }
        else
        {
            MessageBoxService.ShowError("Check your connection settings.");
        }
    }

code snippet AsyncCommand:

public class AsyncCommand<TResult> : AsyncCommandBase, INotifyPropertyChanged
{
    private readonly Func<Task<TResult>> _command;
    private NotifyTaskCompletion<TResult> _execution;

    public AsyncCommand(Func<Task<TResult>> command)
    {
        _command = command;
    }
...

Answer 1

You're confused about TResult and Task. As you have already AsyncCommand<TResult>

public class AsyncCommand<TResult> : AsyncCommandBase, INotifyPropertyChanged
{
}

A class static AsyncCommand like a factory to create the command

    public static class AsyncCommand
    {
        public static AsyncCommand<object> Create(Func<Task> command)
        {
            return new AsyncCommand<object>(async () => { await command(); return null; });
        }

        public static AsyncCommand<TResult> Create<TResult>(Func<Task<TResult>> command)
        {
            return new AsyncCommand<TResult>(command);
        }
    }

Next, you create your command by the factory AsyncCommand like this:

public async Task<string> Add()
{
    //you can put whatever you want, in my project, it's string containing the message
    //your stuff
    var Result = Helper.Get(Configuration.Settings);
}

private AsyncCommand<string> _addCommand;
public AsyncCommand<string> AddCommand
        {
            get
            {
                if (_addCommand!= null)
                    return _addCommand;

                _addCommand= AsyncCommand.Create(Add);
                return _addCommand;
            }
        }

XAML

I show the progress bar if the task is in execution

<Grid VerticalAlignment="Center" Visibility="{Binding AddCommand.Execution, Converter={StaticResource NullToVisibilityConverter}}">
                <!--Busy indicator-->
                <ProgressBar HorizontalAlignment="Stretch" Foreground="{Binding AccentBaseColor}" 
                                           Visibility="{Binding AddCommand.Execution.IsNotCompleted,
                             Converter={StaticResource BooleanToVisibilityConverter}}"
                             IsIndeterminate="True"
                             Value="100" />
</Grid>

Answer 2

We believe that this is probably a Semantic model bug. Semantic model should be showing a Type of null for all lambda expressions.

You should use dispatcher on calling command.

public CarasViewModel()
{
 AddCommand = new ICommand<Task>(() => Add());
}

public ICommand AddCommand { get; private set; }

async Task Add()
{
   Thread longRunningThread = new Thread(new ThreadStart(delegate (){ Thread.Sleep(10000); Application.Current.Dispatcher.BeginInvoke(System.Windows.Threading.DispatcherPriority.Normal, new CalculateTimeElapsed(OnAdd)); }));
longRunningThread.Start();}
void OnAdd{}