从PRESENTATION层（MVVM）访问VIEW中的数据以获取datagrid项源

Question

In my presentation layer (PrintViewModel.cs) I have the following code where I expose my dataset that will be used to populate a datagrid.

    public const string ViewFullRecipeGroupingPropertyName = "ViewFullRecipeGrouping";
    public List<ViewFullRecipe> _viewFullRecipeGrouping = new List<ViewFullRecipe>();
    public List<ViewFullRecipe> ViewFullRecipeGrouping
    {
        get { return _viewFullRecipeGrouping; }
        set { Set(ViewFullRecipeGroupingPropertyName, ref _viewFullRecipeGrouping, value, true); }
    }

Now in my view layer (PrintPage.xaml.cs) I am creating a datagrid programatically and when done I need to set the itemsource, something like this:

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
    var datagrid = new DataGrid();
    datagrid.ItemsSource = PrintViewModel.ViewFullRecipeGrouping;
    }

However this is generating the following error:
An object reference is required for the non-static field, method, or property 'Presentation.Print.PrintViewModel.ViewFullRecipeGrouping.get'

I know my dataset is ok because if I set it directly in the XAML it works perfectly fine (for my test datagrid created in the XAML itself).

So, I guess the issue is on HOW I am accessing ViewFullRecipeGrouping from the PRESENTATION layer (in my VIEW layer).

This is how the PrintViewModel is instantiated:

public class ViewModelLocator
{
    static ViewModelLocator()
    {
        SimpleIoc.Default.Register<PrintViewModel>(true);
    }
    public PrintViewModel Print
    {
        get
        {
            return ServiceLocator.Current.GetInstance<PrintViewModel>();
        }
    }

Any ideas or suggestions on either how to get this work or a better way (am I violating MVVM here?) Thanks,

Answer 1

I believe the reason why you are getting this error:

An object reference is required for the non-static field, method, or property 'Presentation.Print.PrintViewModel.ViewFullRecipeGrouping.get'

is because you're trying to access the property as if it is static. Without knowing too much about your ViewModel class, is the class static itself? If it's not, the way you are trying to access it won't work.. you will need to first instantiate the class, then access the property like so: (you may also need to set the data context)

  private PrintViewModel _viewModel = new PrintViewModel();
  private void Window_Loaded(object sender, RoutedEventArgs e)
  {
    this.DataContext = _viewModel;
    var datagrid = new DataGrid();
    datagrid.ItemsSource = _viewModel.ViewFullRecipeGrouping;
  }

I would also like to ask, why are you programmatically creating the DataGrid? Why not define it via XAML and and use DataBinding for the ItemsSource.

Also, I'd like to note that the point of Properties is for encapsulation. You are using a "getter" for a public member variable.. The member variable should actually be private:

private List<ViewFullRecipe> _viewFullRecipeGrouping = new List<ViewFullRecipe>();
public List<ViewFullRecipe> ViewFullRecipeGrouping
{
    get { return _viewFullRecipeGrouping; }
    set { Set(ViewFullRecipeGroupingPropertyName, ref _viewFullRecipeGrouping, value, true); }
}

Edit: Okay, since you are using a "factory" to get what looks like a singelton instance of the ViewModel, update the code to:

  private ViewModelLocator _locator = new ViewModelLocator();
  private void Window_Loaded(object sender, RoutedEventArgs e)
  {
    var viewModel = _locator.Print; // your ViewModel instance
    var datagrid = new DataGrid();
    datagrid.ItemsSource = viewModel.ViewFullRecipeGrouping;
  }

or try setting the DataBinding of the GridView

  private ViewModelLocator _locator = new ViewModelLocator();
  private void Window_Loaded(object sender, RoutedEventArgs e)
  {
    var viewModel = _locator.Print; // your ViewModel instance
    this.DataContext = viewModel;

    var datagrid = new DataGrid();
    var binding = new Binding("ViewFullRecipeGrouping");
    BindingOperations.SetBinding(datagrid, DataGrid.ItemsSource, binding);
  }

Answer 2

Without seeing the rest of your code I can only guess, but it appears that this assignment code is contained in a static method.

public static void AssignSource() // I'm guessing at the name here.
{
    var datagrid = new DataGrid();
    //ViewFullRecipeGrouping is not static so will throw the exception.
    datagrid.ItemsSource = PrintViewModel.ViewFullRecipeGrouping;
}

If that is the case, you will need to either make the assignment method non-static (most likely easiest fix), put your ViewFullRecipeGrouping property in its own static method, or create an instance of the of the property within the context of the assignment code, ie.

public static void AssignSource() // I'm guessing at the name here.
{
    PrintViewModel.ViewFullRecipeGrouping = new List<ViewFullRecipe>();
    PrintViewModel.ViewFullRecipeGrouping.Add(new ViewFullRecipe());
    var datagrid = new DataGrid();
    datagrid.ItemsSource = PrintViewModel.ViewFullRecipeGrouping;
}