CmdPal: Null pattern matching based on is expression rather than overridable operators (#40972)

What the title says. 😄 

Rather than relying on the potentially overloaded `!=` or `==` operators
when checking for null, now we'll use the `is` expression (possibly
combined with the `not` operator) to ensure correct checking. Probably
overkill for many of these classes, but decided to err on the side of
consistency. Would matter more on classes that may be inherited or
extended.

Using `is` and `is not` will provide us a guarantee that no
user-overloaded equality operators (`==`/`!=`) is invoked when a
`expression is null` is evaluated.

In code form, changed all instances of:

```c#
something != null

something == null
```

to:

```c#
something is not null

something is null
```

The one exception was checking null on a `KeyChord`. `KeyChord` is a
struct which is never null so VS will raise an error when trying this
versus just providing a warning when using `keyChord != null`. In
reality, we shouldn't do this check because it can't ever be null. In
the case of a `KeyChord` it **would** be a `KeyChord` equivalent to:

```c#
KeyChord keyChord = new ()
{
    Modifiers = 0,
    Vkey = 0,
    ScanCode = 0
};
```

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/AliasManager.cs

@@ -35,7 +35,7 @@ public partial class AliasManager : ObservableObject
             try
             {
                 var topLevelCommand = _topLevelCommandManager.LookupCommand(alias.CommandId);
-                if (topLevelCommand != null)
+                if (topLevelCommand is not null)
                 {
                     WeakReferenceMessenger.Default.Send<ClearSearchMessage>();
 
@@ -88,7 +88,7 @@ public partial class AliasManager : ObservableObject
         }
 
         // If we already have _this exact alias_, do nothing
-        if (newAlias != null &&
+        if (newAlias is not null &&
             _aliases.TryGetValue(newAlias.SearchPrefix, out var existingAlias))
         {
             if (existingAlias.CommandId == commandId)
@@ -113,7 +113,7 @@ public partial class AliasManager : ObservableObject
             _aliases.Remove(alias.SearchPrefix);
         }
 
-        if (newAlias != null)
+        if (newAlias is not null)
         {
             AddAlias(newAlias);
         }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/AppStateModel.cs

@@ -55,7 +55,7 @@ public partial class AppStateModel : ObservableObject
 
             var loaded = JsonSerializer.Deserialize<AppStateModel>(jsonContent, JsonSerializationContext.Default.AppStateModel);
 
-            Debug.WriteLine(loaded != null ? "Loaded settings file" : "Failed to parse");
+            Debug.WriteLine(loaded is not null ? "Loaded settings file" : "Failed to parse");
 
             return loaded ?? new();
         }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/CommandProviderWrapper.cs

@@ -15,7 +15,7 @@ namespace Microsoft.CmdPal.UI.ViewModels;
 
 public sealed class CommandProviderWrapper
 {
-    public bool IsExtension => Extension != null;
+    public bool IsExtension => Extension is not null;
 
     private readonly bool isValid;
 
@@ -188,14 +188,14 @@ public sealed class CommandProviderWrapper
 
             return topLevelViewModel;
         };
-        if (commands != null)
+        if (commands is not null)
         {
             TopLevelItems = commands
                 .Select(c => makeAndAdd(c, false))
                 .ToArray();
         }
 
-        if (fallbacks != null)
+        if (fallbacks is not null)
         {
             FallbackItems = fallbacks
                 .Select(c => makeAndAdd(c, true))

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/CommandSettingsViewModel.cs

@@ -18,18 +18,18 @@ public partial class CommandSettingsViewModel(ICommandSettings? _unsafeSettings,
     public bool Initialized { get; private set; }
 
     public bool HasSettings =>
-        _model.Unsafe != null && // We have a settings model AND
-        (!Initialized || SettingsPage != null); // we weren't initialized, OR we were, and we do have a settings page
+        _model.Unsafe is not null && // We have a settings model AND
+        (!Initialized || SettingsPage is not null); // we weren't initialized, OR we were, and we do have a settings page
 
     private void UnsafeInitializeProperties()
     {
         var model = _model.Unsafe;
-        if (model == null)
+        if (model is null)
         {
             return;
         }
 
-        if (model.SettingsPage != null)
+        if (model.SettingsPage is not null)
         {
             SettingsPage = new CommandPaletteContentPageViewModel(model.SettingsPage, mainThread, provider.ExtensionHost);
             SettingsPage.InitializeProperties();

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Commands/CreatedExtensionForm.cs

@@ -30,7 +30,7 @@ internal sealed partial class CreatedExtensionForm : NewExtensionFormBase
     public override ICommandResult SubmitForm(string inputs, string data)
     {
         var dataInput = JsonNode.Parse(data)?.AsObject();
-        if (dataInput == null)
+        if (dataInput is null)
         {
             return CommandResult.KeepOpen();
         }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Commands/LogMessagesPage.cs

@@ -23,7 +23,7 @@ public partial class LogMessagesPage : ListPage
 
     private void LogMessages_CollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
     {
-        if (e.Action == NotifyCollectionChangedAction.Add && e.NewItems != null)
+        if (e.Action == NotifyCollectionChangedAction.Add && e.NewItems is not null)
         {
             foreach (var item in e.NewItems)
             {

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Commands/MainListPage.cs

@@ -203,7 +203,7 @@ public partial class MainListPage : DynamicListPage,
 
             // If we don't have any previous filter results to work with, start
             // with a list of all our commands & apps.
-            if (_filteredItems == null)
+            if (_filteredItems is null)
             {
                 _filteredItems = commands;
                 _filteredItemsIncludesApps = _includeApps;

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Commands/NewExtensionForm.cs

@@ -98,7 +98,7 @@ internal sealed partial class NewExtensionForm : NewExtensionFormBase
     public override CommandResult SubmitForm(string payload)
     {
         var formInput = JsonNode.Parse(payload)?.AsObject();
-        if (formInput == null)
+        if (formInput is null)
         {
             return CommandResult.KeepOpen();
         }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Commands/NewExtensionPage.cs

@@ -14,7 +14,7 @@ public partial class NewExtensionPage : ContentPage
 
     public override IContent[] GetContent()
     {
-        return _resultForm != null ? [_resultForm] : [_inputForm];
+        return _resultForm is not null ? [_resultForm] : [_inputForm];
     }
 
     public NewExtensionPage()
@@ -28,13 +28,13 @@ public partial class NewExtensionPage : ContentPage
 
     private void FormSubmitted(NewExtensionFormBase sender, NewExtensionFormBase? args)
     {
-        if (_resultForm != null)
+        if (_resultForm is not null)
         {
             _resultForm.FormSubmitted -= FormSubmitted;
         }
 
         _resultForm = args;
-        if (_resultForm != null)
+        if (_resultForm is not null)
         {
             _resultForm.FormSubmitted += FormSubmitted;
         }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/ContentMarkdownViewModel.cs

@@ -20,7 +20,7 @@ public partial class ContentMarkdownViewModel(IMarkdownContent _markdown, WeakRe
     public override void InitializeProperties()
     {
         var model = Model.Unsafe;
-        if (model == null)
+        if (model is null)
         {
             return;
         }
@@ -47,7 +47,7 @@ public partial class ContentMarkdownViewModel(IMarkdownContent _markdown, WeakRe
     protected void FetchProperty(string propertyName)
     {
         var model = Model.Unsafe;
-        if (model == null)
+        if (model is null)
         {
             return; // throw?
         }
@@ -66,7 +66,7 @@ public partial class ContentMarkdownViewModel(IMarkdownContent _markdown, WeakRe
     {
         base.UnsafeCleanup();
         var model = Model.Unsafe;
-        if (model != null)
+        if (model is not null)
         {
             model.PropChanged -= Model_PropChanged;
         }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/ContentTreeViewModel.cs

@@ -30,13 +30,13 @@ public partial class ContentTreeViewModel(ITreeContent _tree, WeakReference<IPag
     public override void InitializeProperties()
     {
         var model = Model.Unsafe;
-        if (model == null)
+        if (model is null)
         {
             return;
         }
 
         var root = model.RootContent;
-        if (root != null)
+        if (root is not null)
         {
             RootContent = ViewModelFromContent(root, PageContext);
             RootContent?.InitializeProperties();
@@ -82,7 +82,7 @@ public partial class ContentTreeViewModel(ITreeContent _tree, WeakReference<IPag
     protected void FetchProperty(string propertyName)
     {
         var model = Model.Unsafe;
-        if (model == null)
+        if (model is null)
         {
             return; // throw?
         }
@@ -91,7 +91,7 @@ public partial class ContentTreeViewModel(ITreeContent _tree, WeakReference<IPag
         {
             case nameof(RootContent):
                 var root = model.RootContent;
-                if (root != null)
+                if (root is not null)
                 {
                     RootContent = ViewModelFromContent(root, PageContext);
                 }
@@ -119,7 +119,7 @@ public partial class ContentTreeViewModel(ITreeContent _tree, WeakReference<IPag
             foreach (var item in newItems)
             {
                 var viewModel = ViewModelFromContent(item, PageContext);
-                if (viewModel != null)
+                if (viewModel is not null)
                 {
                     viewModel.InitializeProperties();
                     newContent.Add(viewModel);
@@ -153,7 +153,7 @@ public partial class ContentTreeViewModel(ITreeContent _tree, WeakReference<IPag
 
         Children.Clear();
         var model = Model.Unsafe;
-        if (model != null)
+        if (model is not null)
         {
             model.PropChanged -= Model_PropChanged;
             model.ItemsChanged -= Model_ItemsChanged;

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/HotkeyManager.cs

@@ -29,7 +29,7 @@ public partial class HotkeyManager : ObservableObject
             }
         }
 
-        _commandHotkeys.RemoveAll(item => item.Hotkey == null);
+        _commandHotkeys.RemoveAll(item => item.Hotkey is null);
 
         foreach (var item in _commandHotkeys)
         {

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/Models/ExtensionService.cs

@@ -90,7 +90,7 @@ public partial class ExtensionService : IExtensionService, IDisposable
         }).Result;
         var isExtension = isCmdPalExtensionResult.IsExtension;
         var extension = isCmdPalExtensionResult.Extension;
-        if (isExtension && extension != null)
+        if (isExtension && extension is not null)
         {
             CommandPaletteHost.Instance.DebugLog($"Installed new extension app {extension.DisplayName}");
 
@@ -152,7 +152,7 @@ public partial class ExtensionService : IExtensionService, IDisposable
             {
                 var (cmdPalProvider, classId) = await GetCmdPalExtensionPropertiesAsync(extension);
 
-                return new(cmdPalProvider != null && classId.Count != 0, extension);
+                return new(cmdPalProvider is not null && classId.Count != 0, extension);
             }
         }
 
@@ -237,7 +237,7 @@ public partial class ExtensionService : IExtensionService, IDisposable
     {
         var (cmdPalProvider, classIds) = await GetCmdPalExtensionPropertiesAsync(extension);
 
-        if (cmdPalProvider == null || classIds.Count == 0)
+        if (cmdPalProvider is null || classIds.Count == 0)
         {
             return [];
         }
@@ -352,12 +352,12 @@ public partial class ExtensionService : IExtensionService, IDisposable
     {
         var propSetList = new List<string>();
         var singlePropertySet = GetSubPropertySet(activationPropSet, CreateInstanceProperty);
-        if (singlePropertySet != null)
+        if (singlePropertySet is not null)
         {
             var classId = GetProperty(singlePropertySet, ClassIdProperty);
 
             // If the instance has a classId as a single string, then it's only supporting a single instance.
-            if (classId != null)
+            if (classId is not null)
             {
                 propSetList.Add(classId);
             }
@@ -365,7 +365,7 @@ public partial class ExtensionService : IExtensionService, IDisposable
         else
         {
             var propertySetArray = GetSubPropertySetArray(activationPropSet, CreateInstanceProperty);
-            if (propertySetArray != null)
+            if (propertySetArray is not null)
             {
                 foreach (var prop in propertySetArray)
                 {
@@ -375,7 +375,7 @@ public partial class ExtensionService : IExtensionService, IDisposable
                     }
 
                     var classId = GetProperty(propertySet, ClassIdProperty);
-                    if (classId != null)
+                    if (classId is not null)
                     {
                         propSetList.Add(classId);
                     }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/ProviderSettings.cs

@@ -35,7 +35,7 @@ public class ProviderSettings
     public void Connect(CommandProviderWrapper wrapper)
     {
         ProviderId = wrapper.ProviderId;
-        IsBuiltin = wrapper.Extension == null;
+        IsBuiltin = wrapper.Extension is null;
 
         ProviderDisplayName = wrapper.DisplayName;
 

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/ProviderSettingsViewModel.cs

@@ -33,7 +33,7 @@ public partial class ProviderSettingsViewModel(
         Resources.builtin_disabled_extension;
 
     [MemberNotNullWhen(true, nameof(Extension))]
-    public bool IsFromExtension => _provider.Extension != null;
+    public bool IsFromExtension => _provider.Extension is not null;
 
     public IExtensionWrapper? Extension => _provider.Extension;
 
@@ -76,7 +76,7 @@ public partial class ProviderSettingsViewModel(
     {
         get
         {
-            if (_provider.Settings == null)
+            if (_provider.Settings is null)
             {
                 return false;
             }
@@ -100,7 +100,7 @@ public partial class ProviderSettingsViewModel(
     {
         get
         {
-            if (_provider.Settings == null)
+            if (_provider.Settings is null)
             {
                 return null;
             }
@@ -126,7 +126,7 @@ public partial class ProviderSettingsViewModel(
     {
         get
         {
-            if (field == null)
+            if (field is null)
             {
                 field = BuildTopLevelViewModels();
             }
@@ -149,7 +149,7 @@ public partial class ProviderSettingsViewModel(
     {
         get
         {
-            if (field == null)
+            if (field is null)
             {
                 field = BuildFallbackViewModels();
             }
@@ -173,7 +173,7 @@ public partial class ProviderSettingsViewModel(
 
     private void InitializeSettingsPage()
     {
-        if (_provider.Settings == null)
+        if (_provider.Settings is null)
         {
             return;
         }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/RecentCommandsManager.cs

@@ -30,7 +30,7 @@ public partial class RecentCommandsManager : ObservableObject
             // These numbers are vaguely scaled so that "VS" will make "Visual Studio" the
             // match after one use.
             // Usually it has a weight of 84, compared to 109 for the VS cmd prompt
-            if (entry.Item != null)
+            if (entry.Item is not null)
             {
                 var index = entry.Index;
 
@@ -61,7 +61,7 @@ public partial class RecentCommandsManager : ObservableObject
             var entry = History
             .Where(item => item.CommandId == commandId)
             .FirstOrDefault();
-            if (entry == null)
+            if (entry is null)
             {
                 var newitem = new HistoryItem() { CommandId = commandId, Uses = 1 };
                 History.Insert(0, newitem);

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/SettingsModel.cs

@@ -95,7 +95,7 @@ public partial class SettingsModel : ObservableObject
 
             var loaded = JsonSerializer.Deserialize<SettingsModel>(jsonContent, JsonSerializationContext.Default.SettingsModel);
 
-            Debug.WriteLine(loaded != null ? "Loaded settings file" : "Failed to parse");
+            Debug.WriteLine(loaded is not null ? "Loaded settings file" : "Failed to parse");
 
             return loaded ?? new();
         }

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/TopLevelCommandManager.cs

@@ -249,7 +249,7 @@ public partial class TopLevelCommandManager : ObservableObject,
             _extensionCommandProviders.Clear();
         }
 
-        if (extensions != null)
+        if (extensions is not null)
         {
             await StartExtensionsAndGetCommands(extensions);
         }
@@ -283,7 +283,7 @@ public partial class TopLevelCommandManager : ObservableObject,
         var startTasks = extensions.Select(StartExtensionWithTimeoutAsync);
 
         // Wait for all extensions to start
-        var wrappers = (await Task.WhenAll(startTasks)).Where(wrapper => wrapper != null).Select(w => w!).ToList();
+        var wrappers = (await Task.WhenAll(startTasks)).Where(wrapper => wrapper is not null).Select(w => w!).ToList();
 
         lock (_commandProvidersLock)
         {
@@ -293,7 +293,7 @@ public partial class TopLevelCommandManager : ObservableObject,
         // Load the commands from the providers in parallel
         var loadTasks = wrappers.Select(LoadCommandsWithTimeoutAsync);
 
-        var commandSets = (await Task.WhenAll(loadTasks)).Where(results => results != null).Select(r => r!).ToList();
+        var commandSets = (await Task.WhenAll(loadTasks)).Where(results => results is not null).Select(r => r!).ToList();
 
         lock (TopLevelCommands)
         {

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/TopLevelViewModel.cs

@@ -240,7 +240,7 @@ public sealed partial class TopLevelViewModel : ObservableObject, IListItem
     private void FetchAliasFromAliasManager()
     {
         var am = _serviceProvider.GetService<AliasManager>();
-        if (am != null)
+        if (am is not null)
         {
             var commandAlias = am.AliasFromId(Id);
             if (commandAlias is not null)
@@ -254,7 +254,7 @@ public sealed partial class TopLevelViewModel : ObservableObject, IListItem
     private void UpdateHotkey()
     {
         var hotkey = _settings.CommandHotkeys.Where(hk => hk.CommandId == Id).FirstOrDefault();
-        if (hotkey != null)
+        if (hotkey is not null)
         {
             _hotkey = hotkey.Hotkey;
         }
@@ -264,12 +264,12 @@ public sealed partial class TopLevelViewModel : ObservableObject, IListItem
     {
         List<Tag> tags = [];
 
-        if (Hotkey != null)
+        if (Hotkey is not null)
         {
             tags.Add(new Tag() { Text = Hotkey.ToString() });
         }
 
-        if (Alias != null)
+        if (Alias is not null)
         {
             tags.Add(new Tag() { Text = Alias.SearchPrefix });
         }