How to access public variables from another View Model - datacontext

Question

I have a public variable in one View Model that I want to be able to access (get and set) from another Views code-behind. I have a MainWindowViewModel and a SettingsViewModel. The code-behind I want to be able to access this variable already has a DataContext definition for MainWindowViewModel, which I think may be the problem, since as far as I know I cant define two DataContext for the same view.
As an example, SettingsViewModel has a variable called LowerLimit which I have bound to the selected value of a NumericUpDown. I initialized a variable to hold the DataContext as SettingsViewModel so I can access the variable in that ViewModel. I did this with the code
var settingsContext = DataContext as SettingsViewModel;
But for some reason this throwing a NullRefereceException, saying settingsContext is null.
SettingsViewModel.cs:
public double[] LowerLimit { get; set; } = { 0, 0, 0, 0 }; //LowerLimit NumericUpDown
MainWindow.xaml.cs:
public MainWindow()
{
InitializeComponent();
DataContext = new MainWindowViewModel();
}
private void Run_Clicked(object sender, RoutedEventArgs args) //Button click event
{
...
var settingsContext = DataContext as SettingsViewModel;
lowerLimitCol = (int)settingsContext.LowerLimit[generalIndex];
}
I have only provided the relevant code to the problem. If I am mistaken and more code is required to answer the question then I can provide it.
Expected result: settingsContext gets DataContext from SettingsViewModel, allowing me to access the LowerLimit variable from MainWindow.xaml.cs and assign it to a local variable.
Actual result: NullReferenceException, settingsContext was null

Answer 1

You are initializing your DataContext to an instance of MainWindowViewModel:
DataContext = new MainWindowViewModel();
And then you're trying to cast DataContext to SettingsViewModel:
var settingsContext = DataContext as SettingsViewModel;
This obviously won't work because you set DataContext to an instance of MainWindowViewModel and not SettingsViewModel.
One way around this would be to expose your SettingsViewModel as a property of MainWindowViewModel so you could then do:
var settingsContext = ((MainWindowViewModel)DataContext).Settings