How to set Multiple Datacontext in a window in wpf

Question

In the application I wanted to bind data from 2 different viewmodels. Which is I have to set datacontext to two different viewmodels.

When I did with two different viewmodels, only the last set datacontext is taking precedence.

Hence, I have a CombinedDataContext class with other viewmodel classes.

I could able to update to viewmodel, but it is not getting displayed onto views.

ViewModel1

public class Data1 : INotifyPropertyChanged
    {
        private string _string1;

        public string String1
        {
            get { return _string1; }
            set { _string1 = value; onPropertyChnaged("String1"); }
        }

        private void onPropertyChnaged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

    }

ViewModel2

public class Data2 : INotifyPropertyChanged
    {
        private string _string2;

        public string String2
        {
            get { return _string2; }
            set { _string2 = value; onPropertyChnaged("String2");}
        }

        private void onPropertyChnaged(string propertyName)
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

    }

Views:

public partial class MainWindow : Window
    {
        Data12 d12 = new Data12 { };
        public MainWindow()
        {
            InitializeComponent();
            DataContext = d12;
        }

        private void button_Click(object sender, RoutedEventArgs e)
        {
            ViewModel_1();
            ViewModel_2();
        }

        private void ViewModel_1()
        {
            var viewmodel = DataContext as Data12;
            viewmodel.Data1.String1 = textBox.Text;
        }
        private void ViewModel_2()
        {
            var viewmodel = DataContext as Data12;
            viewmodel.Data1.String1 = textBox.Text;
        }
}

Combined ViewModel:

class Data12
    {
        public Data1 Data1 = new Data1();
        public Data2 Data2= new Data2();
    }

Xaml file:

<Grid>

        <Label x:Name="label" HorizontalAlignment="Left" Margin="92,61,0,0" VerticalAlignment="Top" Content="{Binding Data1.String1}"  />

        <Label x:Name="label1" Content="{Binding Data2.String2}" HorizontalAlignment="Left" Margin="349,61,0,0" VerticalAlignment="Top" />
        <TextBox x:Name="textBox" HorizontalAlignment="Left" Height="23" Margin="65,153,0,0" TextWrapping="Wrap" Text="TextBox" VerticalAlignment="Top" Width="120"/>
        <TextBox x:Name="textBox1" HorizontalAlignment="Left" Height="23" Margin="360,153,0,0" TextWrapping="Wrap" Text="TextBox" VerticalAlignment="Top" Width="120"/>
        <Button x:Name="button" Content="Button" HorizontalAlignment="Left" Margin="205,246,0,0" VerticalAlignment="Top" Width="75" Click="button_Click" />
</Grid>

There is no Nullreference, but binding is not working. I tried even by instantiating inner properties, but did not work. Please help me on this.

Look at the output window for binding errors. WPF bindings are infamous for swallowing exceptions and errors and hiding them in the output window. — Alejandro, Nov 02 '17 at 19:32
A good starting point for you: [Why does WPF support binding to properties of an object, but not fields?](https://stackoverflow.com/questions/842575/why-does-wpf-support-binding-to-properties-of-an-object-but-not-fields) — grek40, Nov 02 '17 at 19:35
Output: BindingExpression path error:'Data1' property not found on 'object' ''Data12' (HashCode=15767257)'. BindingExpression: Path =Data1.String1; DataItem ='Data12'(HashCode=15767257); target element is 'Label' (Name='label'); target property is 'Content' (type 'Object') — srinivas, Nov 02 '17 at 19:40

grek40 · Accepted Answer · 2017-11-03T06:30:56.320

2

You can't bind on Fields, so just turn them into properties and it should work:

class Data12
{
    public Data12()
    {
        this.Data1 = new Data1();
        this.Data2 = new Data2();
    }
    public Data1 Data1 { get; private set; }
    public Data2 Data2 { get; private set; }
}

As @Clemens commented, this can be shortened to

class Data12
{
    public Data1 Data1 { get; } = new Data1();
    public Data2 Data2 { get; } = new Data2();
}

edited Nov 03 '17 at 06:30

answered Nov 02 '17 at 19:42

grek40

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Please note that you could also write `public Data1 Data1 { get; } = new Data1();`, thus avoiding the need for a constructor. – Clemens Nov 03 '17 at 06:25
@Clemens thanks, I was considering it, but I'm stuck with VS 2012 (don't ask me why^^) and didn't want to provide untested code. – grek40 Nov 03 '17 at 06:30
what special you have done? i followed same way but my UI is not updating. – Ashish-BeJovial Dec 30 '19 at 17:54
@AshishJain if you want to update, you probably need to implement `INotifyPropertyChanged`. This question was written in a way, where pre-initialized readonly properties where sufficient. If you add a setter that's used anywhere outside of the constructor, add a property changed handling too. – grek40 Jan 01 '20 at 04:21

How to set Multiple Datacontext in a window in wpf

1 Answers1