class ModalDialog<TItem> {

  private TaskCompletionSource<TItem> taskSource = new TaskCompletionSource<TItem>();

  public Task<Result> ExecuteModal() {
    this.Show();
    return taskSource.Task;
  }

  private void OkButtonClicked() {
    taskSource.SetResult(state);
  } 
}

class SomeView {

  private async void GetUserSelection() {
      var dialog = new ModalDialog { ... };
      var userSelection = await dialog.ExecuteModal();

      // Doing something with result
  }
}

  private async void OnMessageArrived() {  // Вызывается не из потока ui
    InvokeAsync(() => { // Передаём выполнение в поток UI
      taskCompletionSource.SetResult(someResult); // сигнализируем о завершении диалога
    });
  }

private async void GetUserSelection() {
      var dialog = new ModalDialog { ... };
      var userSelection = await dialog.ExecuteModal();

      // Это раскроется во что-то типа такого
      var sc = SyncronizationContext.Current; // Захватили UI-ный контекст синхронизации
      var task = dialog.ExecuteModal();
      task.GetAwaiter().OnCompleted = () => {
            sc.Post(()=>{ // Исполняем continuation на UI-ном контексте
                  var userSelection = task.Result;
                  // Doing something with result
              });
      }    
  }
}

S>class ModalDialog<TItem> {

S>  private TaskCompletionSource<TItem> taskSource = new TaskCompletionSource<TItem>();

S>  public Task<Result> ExecuteModal() {
S>    this.Show();
S>    return taskSource.Task;
S>  }

S>  private void OkButtonClicked() {
S>    taskSource.SetResult(state);
S>  } 
S>}
S>

 public Task<Result> ExecuteModal() {
    this.Show();
    return taskSource.Task;
  }

    private async void OnDoubleClick(DhcpServerLease lease)
    {
        var newIp = await ModalService.ShowModal<MM.Pages.Modals.AssignAddress, string>("Назначение нового IP-адреса", new Dictionary<string, object>
        {
            { "IpAddress", lease.Address },
        });

        if (newIp != default(string))
        {
            Device.ReassignHost(lease.Address, newIp);
            await Device.ReloadContextAsync();
        }
    }

        public Task<ResultType> ShowModal<ComponentType, ResultType>(string title, Dictionary<string, object> parameters = null) where ComponentType : ComponentBase, IModalDialog<ResultType>
        {
            if (OnShowDialog == null)
            {
                throw new InvalidOperationException("ModalHost is not instantiated");
            }

            var taskSource = new TaskCompletionSource<ResultType>();
            IModalDialog<ResultType> modalDialog = null;

            var content = new RenderFragment(rf =>
            {
                int idx = 1;

                rf.OpenComponent(idx++, typeof(ComponentType));
                if (parameters != null)
                {
                    foreach (var param in parameters)
                    {
                        rf.AddAttribute(idx++, param.Key, param.Value);
                    }
                }
                rf.AddComponentReferenceCapture(idx++, component => { 
                    modalDialog = component as IModalDialog<ResultType>;
                    modalDialog.DialogHasChanged = DialogHasChanged;
                    modalDialog.CompleteModal = state => 
                        { 
                            taskSource.TrySetResult(state ? modalDialog.GetDialogResult() : default(ResultType));
                            OnClose?.Invoke();
                        };
                });
                rf.CloseComponent();
            });

            OnShowDialog(title,
                content,
                state => { taskSource.TrySetResult(state ? modalDialog.GetDialogResult() : default(ResultType)); },
                () => modalDialog?.ValidateDialog() ?? false
                );

            return taskSource.Task;
        }

await ModalService.ShowModal<MM.Pages.Modals.AssignAddress>("Назначение нового IP-адреса", new Dictionary<string, object>{...});

await ModalService.ShowModal<MM.Pages.Modals.AssignAddress, string>("Назначение нового IP-адреса", new Dictionary<string, object>{...});

class AssignAddress: ComponentBase, IModalDialog<string> {...}

K>>> public Task<Result> ExecuteModal() {
K>>>    this.Show();
K>>>    return taskSource.Task;
K>>>  }
K>>>

    private async void OnDoubleClick(DhcpServerLease lease)
    {
        var newIp = await ModalService.ShowModal<MM.Pages.Modals.AssignAddress, string>("Назначение нового IP-адреса", new Dictionary<string, object>
        {
            { "IpAddress", lease.Address },
        });

        if (newIp != default)
        {
            ...
        }
    }

S>    private async void OnDoubleClick(DhcpServerLease lease)
S>    {
S>        var newIp = await ModalService.ShowModal<MM.Pages.Modals.AssignAddress, string>("Назначение нового IP-адреса", new Dictionary<string, object>
S>        {
S>            { "IpAddress", lease.Address },
S>        });

S>        if (newIp != default(string))
S>        {
S>            Device.ReassignHost(lease.Address, newIp);
S>            await Device.ReloadContextAsync();
S>        }
S>    }

private void OnDoubleClick(DhcpServerLease lease)
{
  var newIp = new AssignAddressDialog().Show();
  if (newIp != default(string)) 
  {
    Device.ReassignHost(lease.Address, newIp);
    Device.ReloadContextAsync(); // Запустили таску и вышли из обработчика, пусть рефрешится в фоне
  }
}