Setting and displaying a property in a WPF UserControl

Question

Am i missing something here? I have created a usercontrol with a property and for arguments sake it has a text box in it.

<UserControl x:Class="Isd.Utility.SystemMonitorWpf.Bar"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"&gt;

<TextBlock x:Name="txtExpected" Grid.Column="0" Grid.ColumnSpan="2" Width="auto" Height="auto" FontSize="10" LayoutTransform="{StaticResource Rotate}" HorizontalAlignment="Center" VerticalAlignment="Center" FontFamily="Tahoma" Foreground="Red" Panel.ZIndex="100" Margin="5,5,5,5"/>

Then in the code behind i have

public partial class Bar : UserControl
{
    private string _PropTest;

    public string PropTest
    {
        get { return _PropTest; }
        set { _PropTest = value; }
    } 

public Bar()
    {
        InitializeComponent();
        txtExpected.Text = PropTest;
    }
}

Then i drop the usercontrol into the xaml and set the property to a value

<local:Bar PropTest="test"></local:Bar>

In this example, when the usercontrol is displayed the text is showing as null, its like the property PropTest never got set. Am I missing something obvious here? Thanks in advance.

Answer 1

When used as an attribute like you have it, PropTest gets set after the constructor is called, so it doesn't get set when you apply the property to the text box.

You'd be better attaching an event to the property changing, or use the TextBox as the backing value for the property.

Answer 2

What happens when you add Text attribute like so:

<TextBlock x:Name="txtExpected" Text="{Binding PropTest}" />

and eliminate the line

txtExpected.Text = PropTest;

from the constructor?

Answer 3

Delegate value assignment in PropTest property to TextBox:

public string PropTest
{
    get { return txtExpected.Text; }
    set { txtExpected.Text = value; }
} 

Answer 4

You don't appear to be doing anything in the setter of PropTest. It won't be set prior to construction, so it will be null when you do:

txtExpected.Text = PropTest;

Inside your constructor. If you do this:

public string PropTest
{
    get { return _PropTest; }
    set 
    { 
        _PropTest = value; 
        txtExpected.Text = PropTest;
    }
} 

It should work. It's not what I'd call an "ideal" way to do things though, you might want to take a look at Dependency Properties, INotifyPropertyChanged and Binding.

Answer 5

It's because the value of the attribute will never set on the Text-Property of the txtExpected-Control. At the time when the constructor is called, the property PropTest still null.

So, you have to change the implementation of your property:

public string PropTest
{
    get { return txtExpected.Text; }
    set { txtExptected.Text = value; }
}

Answer 6

You should use DependencyProperties, so you can bind your control properties via xaml

On your class declaration:

    public static readonly DependencyProperty MyProperty = DependencyProperty.Register(
        "MyProperty",                  //Property name
        typeof(string),                //Property type
        typeof(MyControl),             //Type of the dependency property provider
        new PropertyMetadata(MyPropertyChanged));//Callback invoked on property value has changes

    public string MyProperty
    {
        set
        {
            this.SetValue(MyProperty, value);
        }
        get
        {
            return (string)this.GetValue(MyProperty);
        }
    }

    private static void MyPropertyChanged( object sender, DependencyPropertyChangedEventArgs args )
    {
      // update your control inner elements properties
    }

Edited a few times because of typos :P