Changes to fix issues in DemoConsole

src/System.Windows.Forms.Design/src/System/ComponentModel/Design/Serialization/ResourceCodeDomSerializer.SerializationResourceManager.cs

@@ -301,7 +301,7 @@ public IDictionaryEnumerator GetMetadataEnumerator()
                 _mergedMetadata = metaData;
             }
 
-            return _mergedMetadata.GetEnumerator();
+            return _mergedMetadata?.GetEnumerator();
         }
 
         /// <summary>
@@ -392,8 +392,7 @@ private Dictionary<string, object> GetResourceSet(CultureInfo culture)
         {
             Debug.Assert(culture is not null, "null parameter");
             Dictionary<string, object> resourceSet = null;
-            object objRs = ResourceTable[culture];
-            if (objRs is null)
+            if (!ResourceTable.TryGetValue(culture, out Dictionary<string, object> objRs))
             {
                 IResourceService resSvc = (IResourceService)_manager.GetService(typeof(IResourceService));
                 TraceIf(TraceLevel.Error, resSvc is null, "IResourceService is not available.  We will not be able to load resources.");
@@ -423,7 +422,7 @@ private Dictionary<string, object> GetResourceSet(CultureInfo culture)
             }
             else
             {
-                resourceSet = objRs as Dictionary<string, object>;
+                resourceSet = objRs;
                 if (resourceSet is null)
                 {
                     // the resourceSets hash table may contain our "this" pointer as a sentinel value

src/System.Windows.Forms.Design/src/System/Windows/Forms/Design/Behavior/GlyphCollection.cs

@@ -117,6 +117,10 @@ public void Insert(int index, Glyph value)
     /// </summary>
     public void Remove(Glyph value)
     {
-        List.Remove(value);
+        // Its possible that the value doesn't exist in the list in case of an `Undo` operation
+        if (List.Contains(value))
+        {
+            List.Remove(value);
+        }
     }
 }

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

@@ -102,16 +102,21 @@ internal SplitterPanel? Selected
         {
             if (_selectedPanel is not null)
             {
-                var panelDesigner1 = (SplitterPanelDesigner)_designerHost!.GetDesigner(_selectedPanel)!;
-                panelDesigner1.Selected = false;
+                if (_designerHost!.GetDesigner(_selectedPanel) is SplitterPanelDesigner panelDesigner1)
+                {
+                    panelDesigner1.Selected = false;
+                }
+
                 _selectedPanel = null;
             }
 
             if (value is not null)
             {
-                var panelDesigner = (SplitterPanelDesigner)_designerHost!.GetDesigner(value)!;
                 _selectedPanel = value;
-                panelDesigner.Selected = true;
+                if (_designerHost!.GetDesigner(value) is SplitterPanelDesigner panelDesigner)
+                {
+                    panelDesigner.Selected = true;
+                }
             }
         }
     }
@@ -126,7 +131,7 @@ public override ICollection AssociatedComponents
             List<Control> components = new();
             foreach (SplitterPanel panel in _splitContainer!.Controls)
             {
-                components.AddRange((IEnumerable<Control>)panel.Controls);
+                components.AddRange(panel.Controls.Cast<Control>());
             }
 
             return components;

src/System.Windows.Forms.Design/src/System/Windows/Forms/Design/StringCollectionEditor.StringCollectionForm.cs

@@ -2,8 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-#nullable disable
-
 using System.ComponentModel;
 using System.ComponentModel.Design;
 
@@ -36,7 +34,7 @@ public StringCollectionForm(CollectionEditor editor)
             HookEvents();
         }
 
-        private void Edit1_keyDown(object sender, KeyEventArgs e)
+        private void Edit1_keyDown(object? sender, KeyEventArgs e)
         {
             if (e.KeyCode != Keys.Escape)
             {
@@ -47,13 +45,13 @@ private void Edit1_keyDown(object sender, KeyEventArgs e)
             e.Handled = true;
         }
 
-        private void StringCollectionEditor_HelpButtonClicked(object sender, CancelEventArgs e)
+        private void StringCollectionEditor_HelpButtonClicked(object? sender, CancelEventArgs e)
         {
             e.Cancel = true;
             _editor.ShowHelp();
         }
 
-        private void Form_HelpRequested(object sender, HelpEventArgs e)
+        private void Form_HelpRequested(object? sender, HelpEventArgs e)
         {
             _editor.ShowHelp();
         }
@@ -69,6 +67,11 @@ private void HookEvents()
         ///  NOTE: The following code is required by the form designer.
         ///  It can be modified using the form editor.  Do not modify it using the code editor.
         /// </summary>
+        [MemberNotNull(nameof(_instruction))]
+        [MemberNotNull(nameof(_textEntry))]
+        [MemberNotNull(nameof(_okButton))]
+        [MemberNotNull(nameof(_cancelButton))]
+        [MemberNotNull(nameof(_overarchingLayoutPanel))]
         private void InitializeComponent()
         {
             ComponentResourceManager resources = new ComponentResourceManager(typeof(StringCollectionEditor));
@@ -135,79 +138,53 @@ private void InitializeComponent()
         /// <summary>
         ///  Commits the changes to the editor.
         /// </summary>
-        private void OKButton_click(object sender, EventArgs e)
+        private void OKButton_click(object? sender, EventArgs e)
         {
-            char[] delims = new char[] { '\n' };
-            char[] trims = new char[] { '\r' };
+            // Split the text into array of lines.
+            string[] lines = _textEntry.Text.Split('\n');
 
-            string[] strings = _textEntry.Text.Split(delims);
-            object[] curItems = Items;
+            // Remove trailing carriage return characters.
+            for (int i = 0; i < lines.Length; i++)
+            {
+                lines[i] = lines[i].TrimEnd('\r');
+            }
 
-            int nItems = strings.Length;
-            for (int i = 0; i < nItems; i++)
+            // Check if the content has changed.
+            if (lines.Length != Items.Length)
             {
-                strings[i] = strings[i].Trim(trims);
+                UpdateItems(lines);
+                return;
             }
 
-            bool dirty = true;
-            if (nItems == curItems.Length)
+            for (int i = 0; i < lines.Length; ++i)
             {
-                int i;
-                for (i = 0; i < nItems; ++i)
+                if (!lines[i].Equals(Items[i]?.ToString()))
                 {
-                    if (!strings[i].Equals((string)curItems[i]))
-                    {
-                        break;
-                    }
+                    UpdateItems(lines);
+                    return;
                 }
-
-                if (i == nItems)
-                    dirty = false;
             }
 
-            if (!dirty)
-            {
-                DialogResult = DialogResult.Cancel;
-                return;
-            }
+            DialogResult = DialogResult.Cancel;
 
-            // If the final line is blank, we don't want to create an item from it
-            if (strings.Length > 0 && strings[strings.Length - 1].Length == 0)
+            void UpdateItems(string[] newLines)
             {
-                nItems--;
-            }
+                // If the last line is empty, we don't want to create an item from it.
+                if (newLines[^1].Length == 0)
+                {
+                    Array.Resize(ref newLines, newLines.Length - 1);
+                }
 
-            object[] values = new object[nItems];
-            for (int i = 0; i < nItems; i++)
-            {
-                values[i] = strings[i];
+                // Assign newLines to Items.
+                Items = newLines;
             }
-
-            Items = values;
         }
 
         /// <summary>
         ///  This is called when the value property in the CollectionForm has changed.
         ///  In it you should update your user interface to reflect the current value.
         /// </summary>
         protected override void OnEditValueChanged()
-        {
-            object[] items = Items;
-            string text = string.Empty;
-
-            for (int i = 0; i < items.Length; i++)
-            {
-                if (items[i] is string)
-                {
-                    text += (string)items[i];
-                    if (i != items.Length - 1)
-                    {
-                        text += "\r\n";
-                    }
-                }
-            }
-
-            _textEntry.Text = text;
-        }
+            => _textEntry.Text = string.Join(Environment.NewLine, Items);
     }
 }

src/System.Windows.Forms/src/System/Windows/Forms/ComboBox.ObjectCollection.Entry.cs

@@ -19,6 +19,11 @@ public Entry(object item)
             }
 
             public object Item { get; set; }
+
+            public override string? ToString()
+            {
+                return Item.ToString();
+            }
         }
     }
 }

src/System.Windows.Forms/tests/IntegrationTests/DesignSurface/DemoConsole/DemoConsole.csproj

@@ -15,6 +15,7 @@
     <PackageLicenseExpression>CPOL</PackageLicenseExpression>
     <PackageProjectUrl>https://www.codeproject.com/Articles/24385/Have-a-Great-DesignTime-Experience-with-a-Powerful</PackageProjectUrl>
     <SuppressLicenseValidation>true</SuppressLicenseValidation>
+    <EnableUnsafeBinaryFormatterSerialization>true</EnableUnsafeBinaryFormatterSerialization>
   </PropertyGroup>
 
   <ItemGroup>

src/System.Windows.Forms/tests/IntegrationTests/DesignSurface/DesignSurfaceExt/DesignSurfaceExt.cs

@@ -371,29 +371,30 @@ private void InitServices()
     public void DoAction(string command)
     {
         if (string.IsNullOrEmpty(command))
+        {
             return;
+        }
 
         IMenuCommandService ims = GetService(typeof(IMenuCommandService)) as IMenuCommandService;
 
         try
         {
-            switch (command.ToUpper())
+            CommandID commandId = command.ToUpper() switch
+            {
+                "CUT" => StandardCommands.Cut,
+                "COPY" => StandardCommands.Copy,
+                "PASTE" => StandardCommands.Paste,
+                "DELETE" => StandardCommands.Delete,
+                _ => null,
+            };
+
+            if (commandId is not null)
             {
-                case "CUT":
-                    ims.GlobalInvoke(StandardCommands.Cut);
-                    break;
-                case "COPY":
-                    ims.GlobalInvoke(StandardCommands.Copy);
-                    break;
-                case "PASTE":
-                    ims.GlobalInvoke(StandardCommands.Paste);
-                    break;
-                case "DELETE":
-                    ims.GlobalInvoke(StandardCommands.Delete);
-                    break;
-                default:
-                    // do nothing;
-                    break;
+                MenuCommand menuCommand = ims?.FindCommand(commandId);
+                if (menuCommand?.Enabled is true)
+                {
+                    menuCommand.Invoke();
+                }
             }
         }
         catch (Exception ex)