Fix: Hide 'Edit Items...' when ListBox DataSource is set

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

@@ -72,6 +72,12 @@ protected override void PreFilterProperties(IDictionary properties)
             properties[nameof(Dock)] = TypeDescriptor.CreateProperty(typeof(ListBoxDesigner), dockProp, []);
         }
 
+        // Wrap Items property to control editor availability based on DataSource
+        if (properties["Items"] is PropertyDescriptor itemsProp && Component is ListBox listBox)
+        {
+            properties["Items"] = new ItemsPropertyDescriptor(itemsProp, listBox);
+        }
+
         base.PreFilterProperties(properties);
     }
 
@@ -218,4 +224,47 @@ public override DesignerActionListCollection ActionLists
             return _actionLists;
         }
     }
+
+    /// <summary>
+    ///  Custom property descriptor that removes the editor when DataSource is set.
+    /// </summary>
+    private sealed class ItemsPropertyDescriptor : PropertyDescriptor
+    {
+        private readonly PropertyDescriptor _baseDescriptor;
+        private readonly ListBox _listBox;
+
+        public ItemsPropertyDescriptor(PropertyDescriptor baseDescriptor, ListBox listBox)
+            : base(baseDescriptor)
+        {
+            _baseDescriptor = baseDescriptor;
+            _listBox = listBox;
+        }
+
+        public override Type ComponentType => _baseDescriptor.ComponentType;
+
+        public override bool IsReadOnly => _baseDescriptor.IsReadOnly;
+
+        public override Type PropertyType => _baseDescriptor.PropertyType;
+
+        public override bool CanResetValue(object component) => _baseDescriptor.CanResetValue(component);
+
+        public override object? GetValue(object? component) => _baseDescriptor.GetValue(component);
+
+        public override void ResetValue(object component) => _baseDescriptor.ResetValue(component);
+
+        public override void SetValue(object? component, object? value) => _baseDescriptor.SetValue(component, value);
+
+        public override bool ShouldSerializeValue(object component) => _baseDescriptor.ShouldSerializeValue(component);
+
+        public override object? GetEditor(Type editorBaseType)
+        {
+            // Don't provide an editor if DataSource is set
+            if (_listBox.DataSource is not null)
+            {
+                return null;
+            }
+
+            return _baseDescriptor.GetEditor(editorBaseType);
+        }
+    }
 }

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

@@ -22,13 +22,17 @@ public void InvokeItemsDialog()
 
     public override DesignerActionItemCollection GetSortedActionItems()
     {
-        DesignerActionItemCollection returnItems =
-        [
-            new DesignerActionMethodItem(this, "InvokeItemsDialog",
+        DesignerActionItemCollection returnItems = [];
+
+        // Only show "Edit Items..." if DataSource is not set
+        if (Component is ListControl control && control.DataSource is null)
+        {
+            returnItems.Add(new DesignerActionMethodItem(this, "InvokeItemsDialog",
                 SR.ListControlUnboundActionListEditItemsDisplayName,
                 SR.ItemsCategoryName,
-                SR.ListControlUnboundActionListEditItemsDescription, true),
-        ];
+                SR.ListControlUnboundActionListEditItemsDescription, true));
+        }
+
         return returnItems;
     }
 }

src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/ListControlUnboundActionListTests.cs

@@ -10,24 +10,28 @@ namespace System.Windows.Forms.Design.Tests;
 public sealed class ListControlUnboundActionListTests : IDisposable
 {
     private readonly ComponentDesigner _designer;
-    private readonly Mock<IComponent> _componentMock;
+    private readonly ListBox _listBox;
     private readonly ListControlUnboundActionList _actionList;
 
     public ListControlUnboundActionListTests()
     {
         _designer = new();
-        _componentMock = new();
-        _designer.Initialize(_componentMock.Object);
+        _listBox = new();
+        _designer.Initialize(_listBox);
         _actionList = new(_designer);
     }
 
-    public void Dispose() => _designer.Dispose();
+    public void Dispose()
+    {
+        _designer.Dispose();
+        _listBox.Dispose();
+    }
 
     [Fact]
     public void Constructor_ShouldInitializeDesigner() => _actionList.Should().NotBeNull();
 
     [Fact]
-    public void GetSortedActionItems_ShouldReturnCorrectItems()
+    public void GetSortedActionItems_ShouldReturnCorrectItems_WhenDataSourceIsNull()
     {
         DesignerActionItemCollection items = _actionList.GetSortedActionItems();
 
@@ -39,4 +43,15 @@ public void GetSortedActionItems_ShouldReturnCorrectItems()
         methodItem.Category.Should().Be(SR.ItemsCategoryName);
         methodItem.Description.Should().Be(SR.ListControlUnboundActionListEditItemsDescription);
     }
+
+    [Fact]
+    public void GetSortedActionItems_ShouldReturnEmpty_WhenDataSourceIsSet()
+    {
+        _listBox.DataSource = new List<string> { "Item1", "Item2" };
+
+        DesignerActionItemCollection items = _actionList.GetSortedActionItems();
+
+        items.Should().NotBeNull();
+        items.Count.Should().Be(0);
+    }
 }