How to read a passed parameter in a WPF UserControl?

Question

I created a user control in WPF:

<UserControl x:Class="TestUserControl.Controls.GetLatest"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
        <TextBlock Name="theTextBlock"/>
</UserControl>

The code behind has a parameter called "FirstMessage" which it sets as the text of my user control TextBlock:

public partial class GetLatest : UserControl
{
    public string FirstMessage { get; set; }

    public GetLatest()
    {
        InitializeComponent();
        theTextBlock.Text = this.FirstMessage;
    }
}

In my main code I can set the FirstMessage parameter in my user control with intellisense:

<Window x:Class="TestUserControl.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1" Height="300" Width="300"
    xmlns:controls="clr-namespace:TestUserControl.Controls"
    >
    <StackPanel>
        <controls:GetLatest FirstMessage="This is the title"/>
    </StackPanel>
</Window>

However, it still doesn't set the text. I've tried Text="{Binding Path=FirstMessage}" and other syntaxes I have found but nothing works.

How can I access the FirstMessage value in my user control?

Answer 1

Your approach to binding doesn't work because your property FirstMessage doesn't notify when it gets updated. Use Dependency Properties for that. See below:

public partial class GetLatest : UserControl
{
    public static readonly DependencyProperty FirstMessageProperty = DependencyProperty.Register("FirstMessage", typeof(string), typeof(GetLatest));

    public string FirstMessage
    {
        get { return (string)GetValue(FirstMessageProperty); }
        set { SetValue(FirstMessageProperty, value); }
    }

    public GetLatest()
    {
        InitializeComponent();
        this.DataContext = this;
    }

}

XAML:

<UserControl x:Class="TestUserControl.Controls.GetLatest"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <TextBlock Text="{Binding FirstMessage}" />
</UserControl>

Whenever the FirstMessage property changes, your text block will update itself.

Answer 2

FirstMessage is set after the constructor has been called. You should change your Text from the setter of FirstMessage.

When initializing object from XAML, first the default constructor is called, then the properties are set on the object.

Answer 3

This quick example won't use any binding because the value isn't set up until after the Default Constructor is called, but here's how you can get the text to show up.

<UserControl x:Class="TestUserControl.Controls.GetLatest"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Loaded="GetLatest_Loaded">
        <TextBlock Name="theTextBlock"/>
</UserControl>

Then, just modify your cs file to this:

public partial class GetLatest : UserControl
{
    public string FirstMessage { get; set; }

    public GetLatest()
    {
        InitializeComponent();
        theTextBlock.Text = this.FirstMessage;
    }

    private void GetLatest_Loaded(object sender, RoutedEventArgs e)
    {
        theTextBlock.Text = this.FirstMessage;
    }
}

I recommend working on setting up a Binding instead, as this is fairly spaghetti-like code.

Answer 4

You can also use:

public partial class GetLatest : UserControl
{
    private string _firstMessage;
    public string FirstMessage 
    {
        get { return _firstMessage; }
        set { _firstMessage = value; theTextBlock.Text = value; }
    }

    public GetLatest()
    {
        InitializeComponent();
    }
}