WPF binding OneWayToSource sets source property to “” when the DataContext is changed

Question

I have a OneWayToSource binding that is not behaving as I expected when I set the DataContext of the target control. The property of the source is being set to default inste

Answer 1

TextBox has a Binding in it's TextProperty and when you set TextBox's DataContext, TextBox will update it's source (viewmodel.Text) , no matter which type of the UpdateSourceTrigger.

It's said that the first output in viewmodel

"ViewModel.Text (old value=, new value=)"

is not triggered by UpdateSourceTrigger=PropertyChanged.

It's just a process of init:

private string _Text;
public string Text
{
    get { return _Text; }
    set
    {
        Debug.Print(
           "ViewModel.Text (old value=" + (_Text ?? "") +
           ", new value=" + (value ?? "") + ")");
        _Text = value;
    }
}

Because it's not triggered by UpdateSourceTrigger=PropertyChanged, the viewmodel will not know the value of TextBox.Text.

When you type "Y",the trigger of PropertyChanged will working,so the viewmodel read text of TextBox.