C#•3mo ago

✅ How does the button know what to do?

Hey, new to C# and wrapping my head around following MVVM. Can someone help me understand how the button knows what to do here? When adding a new button or data binding, they all trigger each other (if that makes sense...). Will any button trigger all data bindings unless otherwise set with like a command relay or something? XAML:

        <TextBlock Text="{Binding DisplayText}" Background="LightGray" Margin="10"/>          
        <StackPanel Grid.Row="1" Orientation="Horizontal" HorizontalAlignment="Center">
            <TextBox Width="550" Height="30" Margin="10, 0,0,0" Text="{Binding DisplayText}"/>  
            <Button Content="Set" Width="200" Height="30" />
        </StackPanel>

        <TextBlock Text="{Binding DisplayText}" Background="LightGray" Margin="10"/>          
        <StackPanel Grid.Row="1" Orientation="Horizontal" HorizontalAlignment="Center">
            <TextBox Width="550" Height="30" Margin="10, 0,0,0" Text="{Binding DisplayText}"/>  
            <Button Content="Set" Width="200" Height="30" />
        </StackPanel>

Code Behind:

    public partial class MainWindow : Window {
        public MainWindow() {
            InitializeComponent();
            MainWindow_ViewModel vm = new MainWindow_ViewModel();
            DataContext = vm;
        }
    }

    public partial class MainWindow : Window {
        public MainWindow() {
            InitializeComponent();
            MainWindow_ViewModel vm = new MainWindow_ViewModel();
            DataContext = vm;
        }
    }

View Model:

    class MainWindow_ViewModel
    {
        private string displayText;
        public string DisplayText {
            get { return displayText; }
            set { displayText = value; }
        }
    }

    class MainWindow_ViewModel
    {
        private string displayText;
        public string DisplayText {
            get { return displayText; }
            set { displayText = value; }
        }
    }

25 Replies

aodpi•3mo ago

In this case nothing will display in data bindings because your ViewModel doesn’t implement the ‘INotifyPropertyChanged’ interface

sibber•3mo ago

wdym? the button doesnt do anything

intee_OP•3mo ago

But it is working :\

sibber•3mo ago

also

-       private string displayText;
-       public string DisplayText {
-           get { return displayText; }
-           set { displayText = value; }
-       }
+       public string DisplayText { get; set; }

-       private string displayText;
-       public string DisplayText {
-           get { return displayText; }
-           set { displayText = value; }
-       }
+       public string DisplayText { get; set; }

intee_OP•3mo ago

I am using a base for my view mode that is using INotify but it's not being inherited anywhere

sibber•3mo ago

wdym by working?

intee_OP•3mo ago

If I type into the text box and press the button, the text is displayed in the textblock

intee_OP•3mo ago

And I just checked, I haven't even changed my ViewModel bases namespace from another project I copied it from.

sibber•3mo ago

huh so are you raising a change notification anywhere?

intee_OP•3mo ago

I'm going to say no but not 100% sure. The only place I have an event is in the base

sibber•3mo ago

can you show you entire viewmodel?

intee_OP•3mo ago

This is the base if it matters, but yeah NAmespace is wrong and I'm not inheriting it as far as I know.

namespace Basic_Tutorial_Rerun.MVVM {

    internal class ViewModel_Base : INotifyPropertyChanged {

        public event PropertyChangedEventHandler? PropertyChanged;

        protected void OnPropertyChanged([CallerMemberName] string propertyName = null) {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

    }
}

namespace Basic_Tutorial_Rerun.MVVM {

    internal class ViewModel_Base : INotifyPropertyChanged {

        public event PropertyChangedEventHandler? PropertyChanged;

        protected void OnPropertyChanged([CallerMemberName] string propertyName = null) {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

    }
}

namespace NoGoogle_MVVM_Test.ViewModel {
    class MainWindow_ViewModel
    {
        private string displayText;

        public string DisplayText {
            get { return displayText; }
            set { displayText = value; }
        }

    }
}

namespace NoGoogle_MVVM_Test.ViewModel {
    class MainWindow_ViewModel
    {
        private string displayText;

        public string DisplayText {
            get { return displayText; }
            set { displayText = value; }
        }

    }
}

sibber•3mo ago

your viewmodel doesnt inherit from your base? also the underscores in names are very unconventional

intee_OP•3mo ago

Yeah, it's muscel memory still haha.

sibber•3mo ago

oh yeah wtf any random button makes the binding update somehow

intee_OP•3mo ago

I tried putting it into a different grid as well (Within the main one, whatever it's called) and same thing. That is about all I tried if I'm being honest though haha.

sibber•3mo ago

ohh i see whats happening here its weird wpf defaults the default binding mode for text boxes in wpf is two way, and the default UpdateSourceTrigger is LostFocus so when you type in the textbox, it sets the property, and when you press the button, the focus trigger is triggered and it reads the new property

intee_OP•3mo ago

Ohh, so whenever I click set, I'm jsut losing focus. Ahhh, thanks so much mate!

sibber•3mo ago

yeah, whenever the textbox loses focus

intee_OP•3mo ago

I would never have figured that out haha.

sibber•3mo ago

yeah thats why i like to explicitly set these things, not leave the defaults

intee_OP•3mo ago

Yeah, I'll keep defaults in mind from now on for sure haha. Thanks again!

sibber•3mo ago

no worries also keep in mind that the defaults may vary between controls

The default value varies for each dependency property. In general, user-editable control properties, such as those of text boxes and check boxes, default to two-way bindings, whereas most other properties default to one-way bindings.

https://learn.microsoft.com/en-us/dotnet/api/system.windows.data.bindingmode?view=windowsdesktop-8.0#system-windows-data-bindingmode-default $close @intee_

MODiX•3mo ago

If you have no further questions, please use /close to mark the forum thread as answered

intee_OP•3mo ago

Legend man. thanks for all the help! It's making so much more sense now hahah..

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✅ How does the button know what to do?