private fields which are never re-assigned outside the constructor were made readonly

Material.Demo/Models/SideSheetDemoViewModel.cs

@@ -3,7 +3,7 @@
 namespace Material.Demo.Models {
     // Data context for sidesheet
     public class SideSheetData : ViewModelBase {
-        private string _header = "SideSheet";
+        private readonly string _header = "SideSheet";
         public string Header => _header;
 
         public string ContentHeader { get; set; } = "What is Lorem Ipsum?";
@@ -14,7 +14,7 @@ public class SideSheetData : ViewModelBase {
 
     // Data context for SideSheet demo page
     public class SideSheetDemoViewModel : ViewModelBase {
-        private SideSheetData _information = new SideSheetData();
+        private readonly SideSheetData _information = new SideSheetData();
 
         private bool _sideInfoOpened = false;
         public SideSheetData Information => _information;

Material.Demo/Pages/ProgressIndicatorDemo.axaml.cs

@@ -7,8 +7,8 @@ namespace Material.Demo.Pages {
     public partial class ProgressIndicatorDemo : UserControl {
         private int caseProgress;
 
-        private Context context;
-        private Timer timer;
+        private readonly Context context;
+        private readonly Timer timer;
 
         public ProgressIndicatorDemo() {
             this.InitializeComponent();

Material.Demo/ViewModels/ExpanderDemoViewModel.cs

@@ -6,7 +6,7 @@ namespace Material.Demo.ViewModels
     public class ExpanderDemoViewModel : ViewModelBase
     {
         public ObservableCollection<string> LoremText => _loremText;
-        private ObservableCollection<string> _loremText;
+        private readonly ObservableCollection<string> _loremText;
 
         public ExpanderDemoViewModel()
         {

Material.Demo/ViewModels/RelayCommand.cs

@@ -8,8 +8,8 @@ namespace Material.Demo.ViewModels {
     /// If you want to use this, you should copy all whole code and paste them to your new RelayCommand.cs source file.
     /// </summary>
     public class RelayCommand : ICommand {
-        private Func<object?, bool>? canExecute;
-        private Action<object?> execute;
+        private readonly Func<object?, bool>? canExecute;
+        private readonly Action<object?> execute;
 
         public RelayCommand(Action<object?> execute, Func<object?, bool>? canExecute = null) {
             this.execute = execute;

Material.Dialog/Commands/MaterialDialogRelayCommand.cs

@@ -10,8 +10,8 @@ namespace Material.Dialog.Commands
     /// </summary>
     public class MaterialDialogRelayCommand : ICommand
     {
-        private Action<object> execute;
-        private Func<object, bool> canExecute;
+        private readonly Action<object> execute;
+        private readonly Func<object, bool> canExecute;
         public event EventHandler CanExecuteChanged;
 
         public MaterialDialogRelayCommand(Action<object> execute, Func<object, bool> canExecute = null)

Material.Dialog/ViewModels/Elements/DialogButtonViewModel.cs

@@ -17,7 +17,7 @@ public DialogButtonViewModel(DialogWindowViewModel parent, object content, IComm
             _command = command;
         }
 
-        private DialogWindowViewModel _parent;
+        private readonly DialogWindowViewModel _parent;
 
         public DialogWindowViewModel Parent => _parent;