WPF set from DependencyProperty not set a value if DP is binded in xaml

Question

I have this code inside class inherited from DependencyObject

   public float VolumeStartLeft
    {
        get { return (float)GetValue(VolumeStartLeftProperty); }
        set
        {
            if (mifp_VolumeStartLeft != null) mifp_VolumeStartLeft(value, e_side.left); // this is never called
            SetValue(VolumeStartLeftProperty, value);
        }
    }

    public static readonly DependencyProperty VolumeStartLeftProperty = DependencyProperty.Register(
        "VolumeStartLeft", typeof(float), typeof(c_delay_dp)
        , new PropertyMetadata(0.0f)); // , VolumeStartLeftCallback

inside xaml I bind this property into slider value.

    <Slider x:Name="SliderLeft" Value="{Binding VolumeStartLeft, Mode=TwoWay}" />

Problem is, set code is never called. When I add code (from property set) if (mifp_VolumeStartLeft != null) mifp_VolumeStartLeft(value, e_side.left); into DependencyPropertyCallback function and slider is changed, Callback is called, but never from set inside property. I wanted avoid to write callback functions for every DependencyProperty, how can I ensure that set will be called when value is changed from slider?

Answer 1

Hi,
If you use static and instance properties you must set instance property in callback method:

XAML MainWindow:

<Window x:Class="WpfApp1.Window88"  
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"  
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"  
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"  
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"  
        xmlns:local="clr-namespace:WpfApp88"  
        mc:Ignorable="d"  
        Title="Slider Binding" Height="450" Width="800">  
  <Window.DataContext>  
    <local:ViewModel/>  
  </Window.DataContext>  
  <StackPanel DataContext="{Binding Obj}">  
    <TextBox Text="{Binding VolumeStartLeft}"/>  
    <Slider Value="{Binding VolumeStartLeft, Mode=TwoWay}"/>  
  </StackPanel>  
</Window>  

ViewModel:

using System;  
using System.ComponentModel;  
using System.Diagnostics;  
using System.Runtime.CompilerServices;  
using System.Windows;  
  
namespace WpfApp88  
{  
  public class ViewModel  
  {  
    public c_delay_dp Obj { get; set; } = new c_delay_dp();  
  }  
  
  public class c_delay_dp : FrameworkElement, INotifyPropertyChanged  
  {  
    public float VolumeStartLeft  
    {  
      get { return (float)GetValue(VolumeStartLeftProperty); }  
      set  
      {  
        string stamp = DateTime.Now.ToString("HH:mm.fff");  
        Debug.WriteLine($"{stamp} - Setter VolumeStartLeft");  
        SetValue(VolumeStartLeftProperty, value);  
      }  
    }  
  
    public static readonly DependencyProperty VolumeStartLeftProperty = DependencyProperty.Register(  
        "VolumeStartLeft", typeof(float), typeof(c_delay_dp)  
        , new PropertyMetadata(1.0f,propCallBack)); // , VolumeStartLeftCallback  
  
    private static void propCallBack(DependencyObject d, DependencyPropertyChangedEventArgs e)  
    {  
      var dep = d as c_delay_dp;  
      if (dep == null) return;  
      dep.VolumeStartLeft = (float)e.NewValue;  
      dep.OnPropertyChanged(nameof(VolumeStartLeft));  
    }  
  
    public event PropertyChangedEventHandler PropertyChanged;  
    private void OnPropertyChanged([CallerMemberName] string propName = "") =>  
      PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));  
  }  
}  

Result:

Answer 2

Hi,
I didn't unterstand how you want synchronize static and instance properties without code. You can use this approach:

XAML MainWindow:

<Window x:Class="WpfApp1.Window89"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp89"
        mc:Ignorable="d"
        Title="Slider Binding" Height="450" Width="800">
  <Window.DataContext>
    <local:ViewModel/>
  </Window.DataContext>
  <StackPanel>
    <Slider Value="{Binding VolumeStartLeft, Mode=TwoWay}"/>
  </StackPanel>
</Window>

ViewModel and class:

using System;
using System.Diagnostics;
using System.Windows;

namespace WpfApp89
{
  public class ViewModel:FrameworkElement
  {
    public float VolumeStartLeft
    {
      get { return (float)GetValue(VolumeStartLeftProperty); }
      set
      {
        string stamp = DateTime.Now.ToString("HH:mm.fff");
        Debug.WriteLine($"{stamp} - Setter VolumeStartLeft");
        SetValue(VolumeStartLeftProperty, value);
      }
    }

    public static readonly DependencyProperty VolumeStartLeftProperty = DependencyProperty.Register(
        "VolumeStartLeft", typeof(float), typeof(c_delay_dp)
        , new PropertyMetadata(1.0f)); // , VolumeStartLeftCallback
  }

  public class c_delay_dp : FrameworkElement
  {
    public static readonly DependencyProperty VolumeStartLeftProperty = DependencyProperty.Register(
        "VolumeStartLeft", typeof(float), typeof(c_delay_dp)
        , new PropertyMetadata(1.0f)); // , VolumeStartLeftCallback
  }
}