Use 'is not null' in some System.Windows.Forms files (#6112)

## Proposed changes

- Contributes to #3459

src/System.Windows.Forms/src/System/Windows/Forms/AutomationMessages.cs

@@ -33,7 +33,7 @@ public static IntPtr WriteAutomationText(string text)
             IntPtr fileId = IntPtr.Zero;
             string fullFileName = GenerateLogFileName(ref fileId);
 
-            if (fullFileName != null)
+            if (fullFileName is not null)
             {
                 try
                 {
@@ -96,9 +96,9 @@ private static string GenerateLogFileName(ref IntPtr fileId)
             string fullFileName = null;
 
             string filePath = System.Environment.GetEnvironmentVariable("TEMP");
-            Debug.Assert(filePath != null, "Could not get value of the TEMP environment variable");
+            Debug.Assert(filePath is not null, "Could not get value of the TEMP environment variable");
 
-            if (filePath != null)
+            if (filePath is not null)
             {
                 if (fileId == IntPtr.Zero) // Create id
                 {

src/System.Windows.Forms/src/System/Windows/Forms/Control.Annotated.cs

@@ -26,6 +26,6 @@ public partial class Control
         internal bool TryGetService<T>(
             [NotNullWhen(true)] out T? service)
             where T : class
-            => (service = GetService(typeof(T)) as T) != null;
+            => (service = GetService(typeof(T)) as T) is not null;
     }
 }

src/System.Windows.Forms/src/System/Windows/Forms/Design/ComponentEditorForm.PageSelector.cs

@@ -204,7 +204,7 @@ private unsafe void OnCustomDraw(ref Message m)
                     case ComCtl32.CDDS.ITEMPREPAINT:
                         {
                             TreeNode itemNode = TreeNode.FromHandle(this, nmtvcd->nmcd.dwItemSpec);
-                            if (itemNode != null)
+                            if (itemNode is not null)
                             {
                                 int state = STATE_NORMAL;
                                 ComCtl32.CDIS itemState = nmtvcd->nmcd.uItemState;

src/System.Windows.Forms/src/System/Windows/Forms/Design/ComponentEditorForm.cs

@@ -167,10 +167,10 @@ private void OnButtonClick(object sender, EventArgs e)
         private void OnConfigureUI()
         {
             Font uiFont = Control.DefaultFont;
-            if (component.Site != null)
+            if (component.Site is not null)
             {
                 IUIService uiService = (IUIService)component.Site.GetService(typeof(IUIService));
-                if (uiService != null)
+                if (uiService is not null)
                 {
                     uiFont = (Font)uiService.Styles["DialogFont"];
                 }
@@ -200,7 +200,7 @@ private void OnConfigureUI()
 
             int selectorWidth = MIN_SELECTOR_WIDTH;
 
-            if (pageSites != null)
+            if (pageSites is not null)
             {
                 // Add the nodes corresponding to the pages
                 for (int n = 0; n < pageSites.Length; n++)
@@ -225,7 +225,7 @@ private void OnConfigureUI()
 
             string caption = string.Empty;
             ISite site = component.Site;
-            if (site != null)
+            if (site is not null)
             {
                 caption = string.Format(SR.ComponentEditorFormProperties, site.Name);
             }
@@ -242,7 +242,7 @@ private void OnConfigureUI()
             grayStrip.Bounds = new Rectangle(pageHostBounds.X, BUTTON_PAD,
                                              pageHostBounds.Width, STRIP_HEIGHT);
 
-            if (pageSites != null)
+            if (pageSites is not null)
             {
                 Rectangle pageBounds = new Rectangle(0, 0, pageHostBounds.Width, pageHostBounds.Height);
                 for (int n = 0; n < pageSites.Length; n++)

src/System.Windows.Forms/src/System/Windows/Forms/Design/ComponentEditorPage.cs

@@ -207,7 +207,7 @@ protected virtual void ReloadComponent()
         /// </summary>
         protected virtual void SetDirty()
         {
-            if (!IsLoading() && PageSite != null)
+            if (!IsLoading() && PageSite is not null)
             {
                 PageSite.SetDirty();
             }

src/System.Windows.Forms/src/System/Windows/Forms/DomainUpDown.cs

@@ -411,7 +411,7 @@ private void SelectIndex(int index)
         {
             // Sanity check index
 
-            Debug.Assert(_domainItems != null, "Domain values array is null");
+            Debug.Assert(_domainItems is not null, "Domain values array is null");
             Debug.Assert(index < _domainItems.Count && index >= -1, "SelectValue: index out of range");
             if (_domainItems is null || index < -1 || index >= _domainItems.Count)
             {
@@ -453,7 +453,7 @@ private void SortDomainItems()
                 // Sanity check
                 Debug.Assert(_sorted, "Sorted == false");
 
-                if (_domainItems != null)
+                if (_domainItems is not null)
                 {
                     // Sort the domain values
                     ArrayList.Adapter(_domainItems).Sort(new DomainUpDownItemCompare());
@@ -482,7 +482,7 @@ public override string ToString()
         {
             string s = base.ToString();
 
-            if (Items != null)
+            if (Items is not null)
             {
                 s += ", Items.Count: " + Items.Count.ToString(CultureInfo.CurrentCulture);
                 s += ", SelectedIndex: " + SelectedIndex.ToString(CultureInfo.CurrentCulture);

src/System.Windows.Forms/src/System/Windows/Forms/DrawListViewItemEventArgs.cs

@@ -95,7 +95,7 @@ public void DrawText(TextFormatFlags flags)
         private Rectangle UpdateBounds(Rectangle originalBounds, bool drawText)
         {
             Rectangle resultBounds = originalBounds;
-            if (Item.ListView != null && Item.ListView.View == View.Details)
+            if (Item.ListView is not null && Item.ListView.View == View.Details)
             {
                 // Note: this logic will compute the bounds so they align w/ the system drawn bounds only
                 // for the default font.

src/System.Windows.Forms/src/System/Windows/Forms/DropTarget.cs

@@ -62,7 +62,7 @@ HRESULT Ole32.IDropTarget.DragEnter(object pDataObj, uint grfKeyState, Point pt,
         {
             Debug.WriteLineIf(CompModSwitches.DragDrop.TraceInfo, "OleDragEnter received");
             Debug.WriteLineIf(CompModSwitches.DragDrop.TraceInfo, "\t" + (pt.X) + "," + (pt.Y));
-            Debug.Assert(pDataObj != null, "OleDragEnter didn't give us a valid data object.");
+            Debug.Assert(pDataObj is not null, "OleDragEnter didn't give us a valid data object.");
             DragEventArgs? drgevent = CreateDragEventArgs(pDataObj, grfKeyState, pt, pdwEffect);
 
             if (drgevent is not null)

src/System.Windows.Forms/src/System/Windows/Forms/KeyboardToolTipStateMachine.cs

@@ -103,7 +103,7 @@ public void NotifyAboutMouseEnter(IKeyboardToolTip sender)
 
         private bool IsToolTracked(IKeyboardToolTip sender)
         {
-            return _toolToTip[sender] != null;
+            return _toolToTip[sender] is not null;
         }
 
         public void NotifyAboutLostFocus(IKeyboardToolTip sender)
@@ -246,7 +246,7 @@ private void Transit(SmEvent @event, IKeyboardToolTip source)
             try
             {
                 ToolTip toolTip = _toolToTip[source];
-                if ((_currentTool is null || _currentTool.CanShowToolTipsNow()) && toolTip != null)
+                if ((_currentTool is null || _currentTool.CanShowToolTipsNow()) && toolTip is not null)
                 {
                     _currentState = Transition(source, toolTip, @event);
                 }
@@ -290,10 +290,10 @@ private void HidePersistent()
 
         private SmState FullFsmReset()
         {
-            if (_currentState == SmState.Shown && _currentTool != null)
+            if (_currentState == SmState.Shown && _currentTool is not null)
             {
                 ToolTip currentToolTip = _toolToTip[_currentTool];
-                if (currentToolTip != null)
+                if (currentToolTip is not null)
                 {
                     currentToolTip.HideToolTip(_currentTool);
                 }
@@ -312,7 +312,7 @@ private void ResetTimer()
 
         private void Reset(ToolTip toolTipToReset)
         {
-            if (toolTipToReset is null || (_currentTool != null && _toolToTip[_currentTool] == toolTipToReset))
+            if (toolTipToReset is null || (_currentTool is not null && _toolToTip[_currentTool] == toolTipToReset))
             {
                 FullFsmReset();
             }
@@ -332,7 +332,7 @@ private void StopTracking(IKeyboardToolTip tool, ToolTip toolTip)
 
         private void OnFormDeactivation(ToolTip sender)
         {
-            if (_currentTool != null && _toolToTip[_currentTool] == sender)
+            if (_currentTool is not null && _toolToTip[_currentTool] == sender)
             {
                 FullFsmReset();
             }

src/System.Windows.Forms/src/System/Windows/Forms/KeysConverter.cs

@@ -180,7 +180,7 @@ public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo c
                             obj = Enum.Parse(typeof(Keys), tokens[i]);
                         }
 
-                        if (obj != null)
+                        if (obj is not null)
                         {
                             Keys currentKey = (Keys)obj;
 

src/System.Windows.Forms/src/System/Windows/Forms/Layout/CommonProperties.cs

@@ -169,7 +169,7 @@ internal static Rectangle GetSpecifiedBounds(IArrangedElement element)
         internal static void ResetPadding(IArrangedElement element)
         {
             object value = element.Properties.GetObject(_paddingProperty);
-            if (value != null)
+            if (value is not null)
             {
                 element.Properties.RemoveObject(_paddingProperty);
             }

src/System.Windows.Forms/src/System/Windows/Forms/Layout/DockAndAnchorLayout.cs

@@ -299,7 +299,7 @@ private static void LayoutAnchoredControls(IArrangedElement container)
                 IArrangedElement element = children[i];
                 if (CommonProperties.GetNeedsAnchorLayout(element))
                 {
-                    Debug.Assert(GetAnchorInfo(element) != null, "AnchorInfo should be initialized before LayoutAnchorControls().");
+                    Debug.Assert(GetAnchorInfo(element) is not null, "AnchorInfo should be initialized before LayoutAnchorControls().");
                     SetCachedBounds(element, GetAnchorDestination(element, displayRectangle, /*measureOnly=*/false));
                 }
             }
@@ -403,7 +403,7 @@ private static Size LayoutDockedControls(IArrangedElement container, bool measur
                 }
 
                 // Treat the MDI client specially, since it's supposed to blend in with the parent form
-                if (mdiClient != null)
+                if (mdiClient is not null)
                 {
                     SetCachedBounds(mdiClient, remainingBounds);
                 }
@@ -641,7 +641,7 @@ private static void UpdateAnchorInfo(IArrangedElement element)
             Debug.Indent();
             Debug.WriteLineIf(CompModSwitches.RichLayout.TraceInfo, element.Container is null ? "No parent" : "Parent");
 
-            if (CommonProperties.GetNeedsAnchorLayout(element) && element.Container != null)
+            if (CommonProperties.GetNeedsAnchorLayout(element) && element.Container is not null)
             {
                 Rectangle bounds = GetCachedBounds(element);
                 AnchorInfo oldAnchorInfo = new AnchorInfo
@@ -750,11 +750,11 @@ public static void SetAnchor(IArrangedElement container, IArrangedElement elemen
                     SetAnchorInfo(element, null);
                 }
 
-                if (element.Container != null)
+                if (element.Container is not null)
                 {
                     bool rightReleased = IsAnchored(oldValue, AnchorStyles.Right) && !IsAnchored(value, AnchorStyles.Right);
                     bool bottomReleased = IsAnchored(oldValue, AnchorStyles.Bottom) && !IsAnchored(value, AnchorStyles.Bottom);
-                    if (element.Container.Container != null && (rightReleased || bottomReleased))
+                    if (element.Container.Container is not null && (rightReleased || bottomReleased))
                     {
                         // If the right or bottom anchor is being released, we have a special case where the element's
                         // margin may affect preferredSize where it didn't previously. Rather than do an expensive
@@ -810,7 +810,7 @@ public static void ScaleAnchorInfo(IArrangedElement element, SizeF factor)
             AnchorInfo anchorInfo = GetAnchorInfo(element);
 
             // some controls don't have AnchorInfo, i.e. Panels
-            if (anchorInfo != null)
+            if (anchorInfo is not null)
             {
                 anchorInfo.Left = (int)((float)anchorInfo.Left * factor.Width);
                 anchorInfo.Top = (int)((float)anchorInfo.Top * factor.Height);
@@ -823,13 +823,13 @@ public static void ScaleAnchorInfo(IArrangedElement element, SizeF factor)
 
         private static Rectangle GetCachedBounds(IArrangedElement element)
         {
-            if (element.Container != null)
+            if (element.Container is not null)
             {
                 IDictionary dictionary = (IDictionary)element.Container.Properties.GetObject(s_cachedBoundsProperty);
-                if (dictionary != null)
+                if (dictionary is not null)
                 {
                     object bounds = dictionary[element];
-                    if (bounds != null)
+                    if (bounds is not null)
                     {
                         return (Rectangle)bounds;
                     }
@@ -841,7 +841,7 @@ private static Rectangle GetCachedBounds(IArrangedElement element)
 
         private static bool HasCachedBounds(IArrangedElement container)
         {
-            return container != null && container.Properties.GetObject(s_cachedBoundsProperty) != null;
+            return container is not null && container.Properties.GetObject(s_cachedBoundsProperty) is not null;
         }
 
         private static void ApplyCachedBounds(IArrangedElement container)
@@ -858,7 +858,7 @@ private static void ApplyCachedBounds(IArrangedElement container)
             }
 
             IDictionary dictionary = (IDictionary)container.Properties.GetObject(s_cachedBoundsProperty);
-            if (dictionary != null)
+            if (dictionary is not null)
             {
 #if DEBUG
                 // In debug builds, we need to modify the collection, so we add a break and an

src/System.Windows.Forms/src/System/Windows/Forms/Layout/FlowLayout.cs

@@ -329,7 +329,7 @@ private void Debug_VerifyAlignment(IArrangedElement container, FlowDirection flo
             // We cannot apply any of these checks @ design-time since dragging new children into a FlowLayoutPanel
             // will attempt to set the children at the mouse position when the child was dropped - we rely on the control
             // to reposition the children once added.
-            if (container is Control flp && flp.Site != null && flp.Site.DesignMode)
+            if (container is Control flp && flp.Site is not null && flp.Site.DesignMode)
             {
                 return;
             }

src/System.Windows.Forms/src/System/Windows/Forms/Layout/LayoutTransaction.cs

@@ -43,7 +43,7 @@ public LayoutTransaction(Control? controlToLayout, IArrangedElement? controlCaus
             _controlToLayout = controlToLayout;
 
             _resumeLayout = resumeLayout;
-            if (_controlToLayout != null)
+            if (_controlToLayout is not null)
             {
 #if DEBUG
                 _layoutSuspendCount = _controlToLayout.LayoutSuspendCount;
@@ -62,7 +62,7 @@ public LayoutTransaction(Control? controlToLayout, IArrangedElement? controlCaus
 
         public void Dispose()
         {
-            if (_controlToLayout != null)
+            if (_controlToLayout is not null)
             {
                 _controlToLayout.ResumeLayout(_resumeLayout);
 
@@ -90,17 +90,17 @@ public static IDisposable CreateTransactionIf(bool condition, Control? controlTo
 
         public static void DoLayout(IArrangedElement? elementToLayout, IArrangedElement? elementCausingLayout, string? property)
         {
-            if (elementCausingLayout != null)
+            if (elementCausingLayout is not null)
             {
                 CommonProperties.xClearPreferredSizeCache(elementCausingLayout);
-                if (elementToLayout != null)
+                if (elementToLayout is not null)
                 {
                     CommonProperties.xClearPreferredSizeCache(elementToLayout);
                     elementToLayout.PerformLayout(elementCausingLayout, property);
                 }
             }
 
-            Debug.Assert(elementCausingLayout != null, "LayoutTransaction.DoLayout - elementCausingLayout is null, no layout performed - did you mix up your parameters?");
+            Debug.Assert(elementCausingLayout is not null, "LayoutTransaction.DoLayout - elementCausingLayout is null, no layout performed - did you mix up your parameters?");
         }
 
         // This overload should be used when a property has changed that affects preferred size,
@@ -110,7 +110,7 @@ public static void DoLayoutIf(bool condition, IArrangedElement? elementToLayout,
         {
             if (!condition)
             {
-                if (elementCausingLayout != null)
+                if (elementCausingLayout is not null)
                 {
                     CommonProperties.xClearPreferredSizeCache(elementCausingLayout);
                 }

src/System.Windows.Forms/src/System/Windows/Forms/Layout/LayoutUtils.cs

@@ -45,7 +45,7 @@ internal partial class LayoutUtils
         public static Size OldGetLargestStringSizeInCollection(Font? font, ICollection? objects)
         {
             Size largestSize = Size.Empty;
-            if (objects != null)
+            if (objects is not null)
             {
                 foreach (object obj in objects)
                 {

src/System.Windows.Forms/src/System/Windows/Forms/Layout/TableLayout.ContainerInfo.cs

@@ -205,7 +205,7 @@ public TableLayoutRowStyleCollection RowStyles
                 set
                 {
                     _rowStyles = value;
-                    if (_rowStyles != null)
+                    if (_rowStyles is not null)
                     {
                         _rowStyles.EnsureOwnership(_container);
                     }
@@ -226,7 +226,7 @@ public TableLayoutColumnStyleCollection ColumnStyles
                 set
                 {
                     _colStyles = value;
-                    if (_colStyles != null)
+                    if (_colStyles is not null)
                     {
                         _colStyles.EnsureOwnership(_container);
                     }
@@ -348,7 +348,7 @@ public bool HasMultiplePercentColumns
             {
                 get
                 {
-                    if (_colStyles != null)
+                    if (_colStyles is not null)
                     {
                         bool foundAny = false;
                         foreach (ColumnStyle style in _colStyles)