WPF custom UserControl - Property Binding

I try to develop a UserControl look like a TextBox white two different changes.

UserControl

TextBox

First of all the new TextBox has to display a "PlaceholderText" if the TextBox text value is empty. My solution for this implementation includes a second TextBox white the "PlaceholderText" as simply Text Attribute. At last I changed the visibility an the focus to the other TextBox.

TextBox

TextBox

TextBox

TextBox

An when the Textbox ValidationResult Object return false they display a TextBlock white an "ErrorMessage"

TextBlock

They tow implementations are already working and existent. For my new TextBox I copied all the TextBox specific properties into my new control and passed them to the original TextBox.

TextBox

TextBox

TextBox

Now I tried to bind the Text property from my new control to a DependencyPropery Object (in the ViewModel).

Text

My implementation looks this:

Custom TextBox Text property

public string Text
{
get => TbSource.Text;
set => TbSource.Text = value;
}


ViewModel propdp

public static DependencyProperty PersonProperty =
DependencyProperty.Register(nameof(Person), typeof(Person), typeof(PersonViewModel));

public Person Person
{
get => (Person)GetValue(PersonProperty);
set => SetValue(PersonProperty, value);
}


And my view

<customControl:NiceTextBox Grid.Row="0" Grid.Column="1" IsPlaceholderAktive="True" PlaceholderText="Enter first name" ErrorMessage="The given first name isn't valid." Text="{Binding Person.Name}" />


Now in the implementation in the View I became follow message:

Has anyone an idea how to fix it? I tried to change my Text property to a dependency property but then I can't pass the input and output from the TbSource.

2 Answers
2

The Text property of your custom control - the target property - must be a dependency property for you to be able to bind to it like this in XAML:

Text

<customControl:NiceTextBox ... Text="{Binding Person.Name}" />


But the Person property in the view model - the source property - shouldn't be defined as a dependency property.

Person

So you have defined the dependency property in the wrong class. Only target properties must be defined as dependency property for you to be able to bind them to some source property.

A control inherits from a DependencyObject class where the GetValue and SetValue methods are defined but a view model generally doesn't.

DependencyObject

GetValue

SetValue

Make your UserControl Text property as DependencyProperty and the property in ViewModel as a normal CLR property and bind it.

Text

DependencyProperty

ViewModel

UserControl

public string Text
{
get { return (string)GetValue(TextProperty); }
set { SetValue(TextProperty, value); }
}
public static readonly DependencyProperty TextProperty =
DependencyProperty.Register("Text", typeof(string), typeof(NiceTextBox), new PropertyMetadata(string.Empty));


ViewModel

public Person Person { get; set; }

public Person Person { get; set; }

XAML

<customControl:NiceTextBox ... Text="{Binding Person.Name}" />


By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.