CheckBox's checked state doesn't change

It seems that bound properties have priority over styles. If you want to set the initial value of IsChecked to True, then try this:

IsChecked="{Binding State, Mode=OneWayToSource, FallbackValue=True }"

Maybe it is also possible to bind FallbackValue to the value of the style.

One workaround is override OnApplyTemplate and set IsChecked=true there and if I do so and keep Mode=OneWayToSource in binding, it hits set twice, first with false and then with true. If I remove the Mode in binding, it hits set once with true.

In some other custom controls I've set default style with Setter Property ... Value ..., for example:

<Setter Property="TextSearch.TextPath" Value="Name"/>
<Setter Property="IsSynchronizedWithCurrentItem" Value="True"/>
<Setter Property="FocusVisualStyle" Value="{x:Null}"/>
<Setter Property="Focusable" Value="False"/>
<Setter Property="BorderThickness" Value="0 0 1 0"/>
<Setter Property="HorizontalContentAlignment" Value="Stretch"/>
....

and all these works normally, I don't have to override OnApplyTemplate BUT in case of CheckBox, I've to, who knows why!

I can't reproduce your error , could you reproduce this issue based on my sample code?And could you tell me what framwork did your project target?
MainWindow.xaml.cs:

  public partial class MainWindow : Window, INotifyPropertyChanged  
    {  
        public MainWindow()  
        {  
            InitializeComponent();  
            //State = true;  
            var test = tstFilter.IsChecked;  
        }  
        bool state;  
  
        protected void OnPropertyChanged(string propertyName)  
        {  
            if (PropertyChanged != null)  
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));  
        }  
        public event PropertyChangedEventHandler PropertyChanged;  
  
        public bool State  
        {  
            get { return state; }  
            set { state = value;   
                //this.OnPropertyChanged("State");   
            }  
        }  
        public override void OnApplyTemplate()  
        {  
            tstFilter.IsChecked = true;  
            base.OnApplyTemplate();  
        }  
    }  

MainWindow.Xaml

<Window x:Class="teststate.MainWindow"  
        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:teststate"  
        mc:Ignorable="d"  
        Title="MainWindow" Height="450" Width="800">  
    <Grid>  
        <local:filter Content="Test" x:Name="tstFilter" IsChecked="{Binding State, Mode=OneWayToSource}" >  
        </local:filter>  
        <TextBlock Text="{Binding IsChecked, ElementName=tstFilter}" Height="50" />  
        <TextBlock Text="{Binding State}" Foreground="Red" Height="50" Margin="0,226,0,146.5" />  
    </Grid>  
</Window>  
  

Custom Control filter.cs:

 public class filter : CheckBox  
    {  
        static filter()  
        {  
            DefaultStyleKeyProperty.OverrideMetadata(typeof(filter), new FrameworkPropertyMetadata(typeof(filter)));  
        }  
    }  

Generic.xaml:

 <Style TargetType="{x:Type local:filter}">  
        <Setter Property="Template">  
            <Setter.Value>  
                <ControlTemplate TargetType="{x:Type local:filter}">  
                    <Grid>  
                    <Border Background="Black"  
                            BorderBrush="{TemplateBinding BorderBrush}"  
                            BorderThickness="{TemplateBinding BorderThickness}"  
                            Width="100" Height="100">  
                    </Border>  
                    <TextBlock Text="test"></TextBlock>  
                    </Grid>  
                </ControlTemplate>  
            </Setter.Value>  
        </Setter>  
        <Setter Property="IsThreeState" Value="False"/>  
        <Setter Property="IsChecked" Value="True"/>  
    </Style>  

If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.