How do I Access Buttons inside a UserControl from xaml?

Question

At work I have several pages, each with buttons in the same places, and with the same properties. Each page also has minor differences. To that end, we created a userControl

Answer 1

You could register a Dependency Property Button on your UserControland handle the initialization in its PropertyChangedCallback.

Template.xaml.cs

using System;
using System.Windows;
using System.Windows.Controls;
using System.Collections.Generic;
using System.Windows.Markup.Primitives;

namespace TemplateCode
{
    public partial class Template : UserControl
    {
        public Template()
        {
            InitializeComponent();
        }

        public static readonly DependencyProperty ButtonProperty =
            DependencyProperty.Register("Button", typeof(Button), typeof(Template),
                new UIPropertyMetadata(new PropertyChangedCallback(ButtonChangedCallback)));

        public Button Button
        {
            get { return (Button)GetValue(ButtonProperty); }
            set { SetValue(ButtonProperty, value); }
        }

        public static List GetDependencyProperties(Object element)
        {
            List properties = new List();
            MarkupObject markupObject = MarkupWriter.GetMarkupObjectFor(element);
            if (markupObject != null)
            {
                foreach (MarkupProperty mp in markupObject.Properties)
                {
                    if (mp.DependencyProperty != null)
                    {
                        properties.Add(mp.DependencyProperty);
                    }
                }
            }
            return properties;
        }

        private static void ButtonChangedCallback(object sender, DependencyPropertyChangedEventArgs args)
        {
            // Get button defined by user in MainWindow
            Button userButton     = (Button)args.NewValue;
            // Get template button in UserControl
            UserControl template  = (UserControl)sender;
            Button templateButton = (Button)template.FindName("button");
            // Get userButton props and change templateButton accordingly
            List properties = GetDependencyProperties(userButton);
            foreach(DependencyProperty property in properties)
            {
                if (templateButton.GetValue(property) != userButton.GetValue(property))
                {
                    templateButton.SetValue(property, userButton.GetValue(property));
                }
            }
        }
    }
}

Template.xaml

UserControl DataContext is inherited from parent, no need not to set it explicitly

MainWindow.xaml

You were setting Button.Content instead of Button

EDIT - Binding Button.Content

3 ways to do this:

1. Dependency Properties

By far the best method. Creating UserControl DP's for every property on the Button is certainly overkill, but for those you want bound to the ViewModel / MainWindow DataContext it makes sense.

Adding in Template.xaml.cs

public static readonly DependencyProperty TextProperty =
    DependencyProperty.Register("Text", typeof(string), typeof(Template));

public string Text
{
    get { return (string)GetValue(TextProperty); }
    set { SetValue(TextProperty, value); }
}

Template.xaml

MainWindow.xaml

Or

Value precedence: UserButton Content > DP Text, so setting the Content in Resources wins.

2. Creating the Button in your ViewModel

MVVM purists won't like this, but you could use the Binding mark up instead of StaticResource.

MainWindow.xaml

3. Setting the binding in code

As you already noticed, a ViewModel prop (e.g. Txt) can't be referenced in Resources because of the order everything is initialized. You can still do it in code later, but it gets a bit messy with the error to prove.

System.Windows.Data Error: 4 : Cannot find source for binding with reference 'RelativeSource FindAncestor, AncestorType='System.Windows.Window', AncestorLevel='1''. BindingExpression:Path=DataContext.Txt; DataItem=null; target element is 'Button' (Name=''); target property is 'Content' (type 'Object')

Note you need to define the full path on the Content property (setting DataContext on parent won't do).

MainWindow.xaml

Template.xaml.cs

private static void ButtonChangedCallback(object sender, DependencyPropertyChangedEventArgs args)
{
    // Get button defined by user in MainWindow
    Button userButton = (Button)args.NewValue;
    // Get template button in UserControl
    UserControl template = (UserControl)sender;
    Button templateButton = (Button)template.FindName("button");
    // Get userButton props and change templateButton accordingly
    List properties = GetDependencyProperties(userButton);
    foreach (DependencyProperty property in properties)
    {
    if (templateButton.GetValue(property) != userButton.GetValue(property))
        templateButton.SetValue(property, userButton.GetValue(property));
    }
    // Set Content binding
    BindingExpression bindingExpression = userButton.GetBindingExpression(Button.ContentProperty);
    if (bindingExpression != null)
        templateButton.SetBinding(Button.ContentProperty, bindingExpression.ParentBinding);
}