WPF Binding: How to set Binding Source inside UserControl XAML

Question

I'd like to set UserControl's Property like below. How can I achieve this?

<HierarchicalDataTemplate DataType="{x:Type src:Class}">
      <UserControls:ClassBlock classInstance="{Binding PropertyFromClass}"/>
</HierarchicalDataTemplate>

Answer 1

You need to set the DataContext of the user control. It currently does not know where to fetch PropertyFromClass. You can do it like such:

<HierarchicalDataTemplate DataType="{x:Type src:Class}">
    <UserControls:ClassBlock classInstance="{Binding PropertyFromClass}">
        <UserControls:ClassBlock.DataContext>
            <MyViewModels:SomeViewModelHavingPropertyFromClass />
        </UserControls:ClassBlock.DataContext>
    </UserControls:ClassBlock
</HierarchicalDataTemplate>

If your property is a property in your code-behind ClassBlock.Xaml.cs file, which it sounds like it might be, you must implement the INotifyPropertyChanged interface on the property.

UserControl

public partial class ClassBlock: UserControl, INotifyPropertyChanged
{        
    private string classInstance;

    public ClassBlock()
    {
        this.InitializeComponent();
    }

    public string ClassInstance
    {
        get
        {
            return this.classInstance;
        }

        set
        {
            this.classInstance= value;
            this.OnPropertyChanged();
        }
    }

    public void OnPropertyChanged([CallerMemberName] string propertyName = "")
    {
        if (this.PropertyChanged != null)
        {
            // Invoke the event handlers attached by other objects.
            this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

XAML

<HierarchicalDataTemplate DataType="{x:Type src:Class}">
    <!-- Upper case ClassInstance -->
    <UserControls:ClassBlock ClassInstance="{Binding PropertyFromClass}" />
</HierarchicalDataTemplate>

That would fix the issue, although that really should be in your View Model . Take a look at the MVVM Pattern documentation to help out.