Added the CheckBoxSample.

CheckBoxSample/App.xaml

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+<?xml version = "1.0" encoding = "UTF-8" ?>
+<Application xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             xmlns:local="clr-namespace:CheckBoxSample"
+             x:Class="CheckBoxSample.App">
+    <Application.Resources>
+        <ResourceDictionary>
+            <ResourceDictionary.MergedDictionaries>
+                <ResourceDictionary Source="Resources/Styles/Colors.xaml" />
+                <ResourceDictionary Source="Resources/Styles/Styles.xaml" />
+            </ResourceDictionary.MergedDictionaries>
+        </ResourceDictionary>
+    </Application.Resources>
+</Application>

CheckBoxSample/App.xaml.cs

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+namespace CheckBoxSample
+{
+    public partial class App : Application
+    {
+        public App()
+        {
+            InitializeComponent();
+
+            MainPage = new AppShell();
+        }
+    }
+}

CheckBoxSample/AppShell.xaml

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+<?xml version="1.0" encoding="UTF-8" ?>
+<Shell
+    x:Class="CheckBoxSample.AppShell"
+    xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+    xmlns:local="clr-namespace:CheckBoxSample"
+    Shell.FlyoutBehavior="Disabled"
+    Title="CheckBoxSample">
+
+    <ShellContent
+        Title="Home"
+        ContentTemplate="{DataTemplate local:MainPage}"
+        Route="MainPage" />
+
+</Shell>

CheckBoxSample/AppShell.xaml.cs

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+namespace CheckBoxSample
+{
+    public partial class AppShell : Shell
+    {
+        public AppShell()
+        {
+            InitializeComponent();
+        }
+    }
+}

CheckBoxSample/CheckBoxSample.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+	<PropertyGroup>
+		<TargetFrameworks>net8.0-android;net8.0-ios;net8.0-maccatalyst</TargetFrameworks>
+		<TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('windows'))">$(TargetFrameworks);net8.0-windows10.0.19041.0</TargetFrameworks>
+		<!-- Uncomment to also build the tizen app. You will need to install tizen by following this: https://github.com/Samsung/Tizen.NET -->
+		<!-- <TargetFrameworks>$(TargetFrameworks);net8.0-tizen</TargetFrameworks> -->
+
+		<!-- Note for MacCatalyst:
+		The default runtime is maccatalyst-x64, except in Release config, in which case the default is maccatalyst-x64;maccatalyst-arm64.
+		When specifying both architectures, use the plural <RuntimeIdentifiers> instead of the singular <RuntimeIdentifier>.
+		The Mac App Store will NOT accept apps with ONLY maccatalyst-arm64 indicated;
+		either BOTH runtimes must be indicated or ONLY macatalyst-x64. -->
+		<!-- For example: <RuntimeIdentifiers>maccatalyst-x64;maccatalyst-arm64</RuntimeIdentifiers> -->
+
+		<OutputType>Exe</OutputType>
+		<RootNamespace>CheckBoxSample</RootNamespace>
+		<UseMaui>true</UseMaui>
+		<SingleProject>true</SingleProject>
+		<ImplicitUsings>enable</ImplicitUsings>
+		<Nullable>enable</Nullable>
+
+		<!-- Display name -->
+		<ApplicationTitle>CheckBoxSample</ApplicationTitle>
+
+		<!-- App Identifier -->
+		<ApplicationId>com.companyname.checkboxsample</ApplicationId>
+
+		<!-- Versions -->
+		<ApplicationDisplayVersion>1.0</ApplicationDisplayVersion>
+		<ApplicationVersion>1</ApplicationVersion>
+
+		<SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'ios'">11.0</SupportedOSPlatformVersion>
+		<SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'maccatalyst'">13.1</SupportedOSPlatformVersion>
+		<SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'android'">21.0</SupportedOSPlatformVersion>
+		<SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'windows'">10.0.17763.0</SupportedOSPlatformVersion>
+		<TargetPlatformMinVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'windows'">10.0.17763.0</TargetPlatformMinVersion>
+		<SupportedOSPlatformVersion Condition="$([MSBuild]::GetTargetPlatformIdentifier('$(TargetFramework)')) == 'tizen'">6.5</SupportedOSPlatformVersion>
+	</PropertyGroup>
+
+	<ItemGroup>
+		<!-- App Icon -->
+		<MauiIcon Include="Resources\AppIcon\appicon.svg" ForegroundFile="Resources\AppIcon\appiconfg.svg" Color="#512BD4" />
+
+		<!-- Splash Screen -->
+		<MauiSplashScreen Include="Resources\Splash\splash.svg" Color="#512BD4" BaseSize="128,128" />
+
+		<!-- Images -->
+		<MauiImage Include="Resources\Images\*" />
+		<MauiImage Update="Resources\Images\dotnet_bot.png" Resize="True" BaseSize="300,185" />
+
+		<!-- Custom Fonts -->
+		<MauiFont Include="Resources\Fonts\*" />
+
+		<!-- Raw Assets (also remove the "Resources\Raw" prefix) -->
+		<MauiAsset Include="Resources\Raw\**" LogicalName="%(RecursiveDir)%(Filename)%(Extension)" />
+	</ItemGroup>
+
+	<ItemGroup>
+		<PackageReference Include="Microsoft.Maui.Controls" Version="$(MauiVersion)" />
+		<PackageReference Include="Microsoft.Maui.Controls.Compatibility" Version="$(MauiVersion)" />
+		<PackageReference Include="Microsoft.Extensions.Logging.Debug" Version="8.0.0" />
+		<PackageReference Include="Syncfusion.Maui.Buttons" Version="*" />
+	</ItemGroup>
+
+</Project>

CheckBoxSample/CheckBoxSample.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.9.34723.18
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "CheckBoxSample", "CheckBoxSample.csproj", "{0E80D0B9-1DC8-426D-B392-A1FB9047EB47}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{0E80D0B9-1DC8-426D-B392-A1FB9047EB47}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{0E80D0B9-1DC8-426D-B392-A1FB9047EB47}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{0E80D0B9-1DC8-426D-B392-A1FB9047EB47}.Debug|Any CPU.Deploy.0 = Debug|Any CPU
+		{0E80D0B9-1DC8-426D-B392-A1FB9047EB47}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{0E80D0B9-1DC8-426D-B392-A1FB9047EB47}.Release|Any CPU.Build.0 = Release|Any CPU
+		{0E80D0B9-1DC8-426D-B392-A1FB9047EB47}.Release|Any CPU.Deploy.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {C734F631-A02A-4536-83B8-E11F2310FF13}
+	EndGlobalSection
+EndGlobal

CheckBoxSample/MainPage.xaml

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+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="CheckBoxSample.MainPage"
+             xmlns:buttons="clr-namespace:Syncfusion.Maui.Buttons;assembly=Syncfusion.Maui.Buttons">
+
+    <StackLayout Padding="20">
+        <Label x:Name="label" Margin="10" Text="Pizza Toppings"/>
+        <buttons:SfCheckBox x:Name="selectAll" Text="Select All" IsChecked="{x:Null}" IsThreeState="True" StateChanged="SelectAll_StateChanged"/>
+        <buttons:SfCheckBox x:Name="pepperoni" Text="Pepperoni" StateChanged="CheckBox_StateChanged" Margin="30,0"/>
+        <buttons:SfCheckBox x:Name="beef" Text="Beef" IsChecked="True" StateChanged="CheckBox_StateChanged" Margin="30,0"/>
+        <buttons:SfCheckBox x:Name="mushroom" Text="Mushrooms" StateChanged="CheckBox_StateChanged" Margin="30,0"/>
+        <buttons:SfCheckBox x:Name="onion" Text="Onions" IsChecked="True" StateChanged="CheckBox_StateChanged" Margin="30,0"/>
+    </StackLayout>
+
+</ContentPage>

CheckBoxSample/MainPage.xaml.cs

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+namespace CheckBoxSample
+{
+    public partial class MainPage : ContentPage
+    {
+        public MainPage()
+        {
+            InitializeComponent();
+        }
+        bool skip = false;
+        private void SelectAll_StateChanged(object sender, Syncfusion.Maui.Buttons.StateChangedEventArgs e)
+        {
+            if (!skip)
+            {
+                skip = true;
+                pepperoni.IsChecked = beef.IsChecked = mushroom.IsChecked = onion.IsChecked = e.IsChecked;
+                skip = false;
+            }
+        }
+
+        private void CheckBox_StateChanged(object sender, Syncfusion.Maui.Buttons.StateChangedEventArgs e)
+        {
+            if (!skip)
+            {
+                skip = true;
+                if (pepperoni.IsChecked.Value && beef.IsChecked.Value && mushroom.IsChecked.Value && onion.IsChecked.Value)
+                    selectAll.IsChecked = true;
+                else if (!pepperoni.IsChecked.Value && !beef.IsChecked.Value && !mushroom.IsChecked.Value && !onion.IsChecked.Value)
+                    selectAll.IsChecked = false;
+                else
+                    selectAll.IsChecked = null;
+                skip = false;
+            }
+        }
+    }
+
+}

CheckBoxSample/MauiProgram.cs

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+using Microsoft.Extensions.Logging;
+using Syncfusion.Maui.Core.Hosting;
+
+namespace CheckBoxSample
+{
+    public static class MauiProgram
+    {
+        public static MauiApp CreateMauiApp()
+        {
+            var builder = MauiApp.CreateBuilder();
+            builder
+                .UseMauiApp<App>()
+                .ConfigureSyncfusionCore()
+                .ConfigureFonts(fonts =>
+                {
+                    fonts.AddFont("OpenSans-Regular.ttf", "OpenSansRegular");
+                    fonts.AddFont("OpenSans-Semibold.ttf", "OpenSansSemibold");
+                });
+
+#if DEBUG
+    		builder.Logging.AddDebug();
+#endif
+
+            return builder.Build();
+        }
+    }
+}

CheckBoxSample/Platforms/Android/AndroidManifest.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android">
+	<application android:allowBackup="true" android:icon="@mipmap/appicon" android:roundIcon="@mipmap/appicon_round" android:supportsRtl="true"></application>
+	<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
+	<uses-permission android:name="android.permission.INTERNET" />
+</manifest>

CheckBoxSample/Platforms/Android/MainActivity.cs

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+using Android.App;
+using Android.Content.PM;
+using Android.OS;
+
+namespace CheckBoxSample
+{
+    [Activity(Theme = "@style/Maui.SplashTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation | ConfigChanges.UiMode | ConfigChanges.ScreenLayout | ConfigChanges.SmallestScreenSize | ConfigChanges.Density)]
+    public class MainActivity : MauiAppCompatActivity
+    {
+    }
+}

CheckBoxSample/Platforms/Android/MainApplication.cs

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+using Android.App;
+using Android.Runtime;
+
+namespace CheckBoxSample
+{
+    [Application]
+    public class MainApplication : MauiApplication
+    {
+        public MainApplication(IntPtr handle, JniHandleOwnership ownership)
+            : base(handle, ownership)
+        {
+        }
+
+        protected override MauiApp CreateMauiApp() => MauiProgram.CreateMauiApp();
+    }
+}

CheckBoxSample/Platforms/Android/Resources/values/colors.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <color name="colorPrimary">#512BD4</color>
+    <color name="colorPrimaryDark">#2B0B98</color>
+    <color name="colorAccent">#2B0B98</color>
+</resources>

CheckBoxSample/Platforms/MacCatalyst/AppDelegate.cs

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+using Foundation;
+
+namespace CheckBoxSample
+{
+    [Register("AppDelegate")]
+    public class AppDelegate : MauiUIApplicationDelegate
+    {
+        protected override MauiApp CreateMauiApp() => MauiProgram.CreateMauiApp();
+    }
+}

CheckBoxSample/Platforms/MacCatalyst/Entitlements.plist

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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+    <!-- See https://aka.ms/maui-publish-app-store#add-entitlements for more information about adding entitlements.-->
+    <dict>
+        <!-- App Sandbox must be enabled to distribute a MacCatalyst app through the Mac App Store. -->
+        <key>com.apple.security.app-sandbox</key>
+        <true/>
+        <!-- When App Sandbox is enabled, this value is required to open outgoing network connections. -->
+        <key>com.apple.security.network.client</key>
+        <true/>
+    </dict>
+</plist>
+

CheckBoxSample/Platforms/MacCatalyst/Info.plist

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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <!-- The Mac App Store requires you specify if the app uses encryption. -->
+    <!-- Please consult https://developer.apple.com/documentation/bundleresources/information_property_list/itsappusesnonexemptencryption -->
+    <!-- <key>ITSAppUsesNonExemptEncryption</key> -->
+    <!-- Please indicate <true/> or <false/> here. -->
+
+    <!-- Specify the category for your app here. -->
+    <!-- Please consult https://developer.apple.com/documentation/bundleresources/information_property_list/lsapplicationcategorytype -->
+    <!-- <key>LSApplicationCategoryType</key> -->
+    <!-- <string>public.app-category.YOUR-CATEGORY-HERE</string> -->
+	<key>UIDeviceFamily</key>
+	<array>
+		<integer>2</integer>
+	</array>
+	<key>UIRequiredDeviceCapabilities</key>
+	<array>
+		<string>arm64</string>
+	</array>
+	<key>UISupportedInterfaceOrientations</key>
+	<array>
+		<string>UIInterfaceOrientationPortrait</string>
+		<string>UIInterfaceOrientationLandscapeLeft</string>
+		<string>UIInterfaceOrientationLandscapeRight</string>
+	</array>
+	<key>UISupportedInterfaceOrientations~ipad</key>
+	<array>
+		<string>UIInterfaceOrientationPortrait</string>
+		<string>UIInterfaceOrientationPortraitUpsideDown</string>
+		<string>UIInterfaceOrientationLandscapeLeft</string>
+		<string>UIInterfaceOrientationLandscapeRight</string>
+	</array>
+	<key>XSAppIconAssets</key>
+	<string>Assets.xcassets/appicon.appiconset</string>
+</dict>
+</plist>

CheckBoxSample/Platforms/MacCatalyst/Program.cs

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+using ObjCRuntime;
+using UIKit;
+
+namespace CheckBoxSample
+{
+    public class Program
+    {
+        // This is the main entry point of the application.
+        static void Main(string[] args)
+        {
+            // if you want to use a different Application Delegate class from "AppDelegate"
+            // you can specify it here.
+            UIApplication.Main(args, null, typeof(AppDelegate));
+        }
+    }
+}

CheckBoxSample/Platforms/Tizen/Main.cs

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+using Microsoft.Maui;
+using Microsoft.Maui.Hosting;
+using System;
+
+namespace CheckBoxSample
+{
+    internal class Program : MauiApplication
+    {
+        protected override MauiApp CreateMauiApp() => MauiProgram.CreateMauiApp();
+
+        static void Main(string[] args)
+        {
+            var app = new Program();
+            app.Run(args);
+        }
+    }
+}

CheckBoxSample/Platforms/Tizen/tizen-manifest.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<manifest package="maui-application-id-placeholder" version="0.0.0" api-version="8" xmlns="http://tizen.org/ns/packages">
+  <profile name="common" />
+  <ui-application appid="maui-application-id-placeholder" exec="CheckBoxSample.dll" multiple="false" nodisplay="false" taskmanage="true" type="dotnet" launch_mode="single">
+    <label>maui-application-title-placeholder</label>
+    <icon>maui-appicon-placeholder</icon>
+    <metadata key="http://tizen.org/metadata/prefer_dotnet_aot" value="true" />
+  </ui-application>
+  <shortcut-list />
+  <privileges>
+    <privilege>http://tizen.org/privilege/internet</privilege>
+  </privileges> 
+  <dependencies />
+  <provides-appdefined-privileges />
+</manifest>