Change property of class 1 from class 2 using MVVM

Question

I am trying to change the property "MyText" of the class "MainViewModel" from the class "ClassA" under MVVM. For this I use the classes as singleton. Unfortunately it does not work for me. It should be a simple call. Does anyone have any idea what might be doing wrong?

<Window x:Class="MoreClasses.MainWindow"  
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"  
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"  
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"  
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"  
        xmlns:local="clr-namespace:MoreClasses"  
        mc:Ignorable="d"  
        Title="MainWindow" Height="450" Width="800">  
  
    <Window.DataContext>  
        <local:MainViewModel/>  
    </Window.DataContext>  
    <Grid>  
        <StackPanel Orientation="Vertical">  
            <TextBlock Text="{Binding MyText}"  
                       Margin="50"/>  
            <Button Content="Main Class"  
                    Width="150"  
                    Height="70"  
                    Margin="50"  
                    Command="{Binding Cmd}"  
                    CommandParameter="MainClass"/>  
  
            <Button Content="Class A"  
                    Width="150"  
                    Height="70"  
                    Command="{Binding Cmd}"  
                    CommandParameter="ClassA" />  
        </StackPanel>  
    </Grid>  
</Window>  


namespace MoreClasses  
{  
    public class MainViewModel : BaseViewModel  
    {  
        #region Singelton  
        private static MainViewModel instance;  
  
        static MainViewModel()  
        {  
            instance = new MainViewModel();  
        }  
  
        public static MainViewModel Instance  
        {  
            get => instance;  
        }  
        #endregion Singelton  
  
        private string myText = string.Empty;  
        public string MyText  
        {  
            get  
            {  
                return myText;  
            }  
            set  
            {  
                myText = value;  
                OnPropertyChanged();  
            }  
        }  
  
        public RelayCommand Cmd { get => new RelayCommand(CmdExec); }  
        private void CmdExec(object parameter)  
        {  
            switch (parameter.ToString())  
            {  
                case "MainClass":  
                    try  
                    {  
                        MyText = "I am from Class -> MainViewModel!";  
                    }  
                    catch (Exception ex)  
                    {  
                        MessageBox.Show(ex.Message);  
                    }  
                    break;  
  
                case "ClassA":  
                    try  
                    {  
                        ClassA.Instance.ChangeText();  
                    }  
                    catch (Exception ex)  
                    {  
                        MessageBox.Show(ex.Message);  
                    }  
                    break;  
  
                default:  
                    break;  
            }  
        }  
    }  
}  


namespace MoreClasses  
{  
    public class ClassA  
    {  
  
        #region Singelton  
        private static ClassA instance;  
  
        static ClassA()  
        {  
            instance = new ClassA();  
        }  
  
        public static ClassA Instance  
        {  
            get => instance;  
        }  
        #endregion Singelton  
  
        public void ChangeText()  
        {  
            MainViewModel.Instance.MyText = "I'am from Class -> ClassA!";  
        }  
    }  
}  

Answer 1

Hi Sarah,
please, change your code to use only one instance of MainViewModel:

 namespace MoreClasses  
 {  
     public class MainViewModel : BaseViewModel  
     {  
         #region Singelton  
         private static MainViewModel instance;  
      
         public MainViewModel()  
         {  
             instance = this;  
         }  
      
         public static MainViewModel Instance  
         {  
             get => instance;  
         }  
         #endregion Singelton  
...  

Answer 2

You could provide classes with private constructors to ensure that only one instance of a class can be created.

267123-singleton.txt

Get rid of

<Window.DataContext>  
     <local:MainViewModel/>  
 </Window.DataContext>   

Because DataContext is set to another instance of MainViewModel.

Method 1:

 public MainWindow()  
    {  
        InitializeComponent();  
        DataContext = MainViewModel.Instance;  
    }  

or Method 2:

<Window ...  
        DataContext="{x:Static local:MainViewModel.Instance}">  

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