Makes PropertyGrid for CoboBox show "(none)" when DataSource is null …

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

@@ -0,0 +1,24 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.ComponentModel;
+using System.Globalization;
+
+namespace System.Windows.Forms.Design;
+
+internal class DataSourceConverter : ReferenceConverter
+{
+    public DataSourceConverter() : base(typeof(IListSource))
+    {
+    }
+
+    public override object? ConvertTo(ITypeDescriptorContext? context, CultureInfo? culture, object? value, Type destinationType)
+    {
+        if (destinationType == typeof(string) && value is null)
+        {
+            return SR.None_lc.ToString();
+        }
+
+        return base.ConvertTo(context, culture, value, destinationType);
+    }
+}

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

@@ -0,0 +1,32 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Windows.Forms.Design.Tests;
+
+public class DataSourceConverterTests
+{
+    private readonly DataSourceConverter _converter = new();
+
+    [Fact]
+    public void ConvertTo_NullValueToString_ReturnsNoneLowercase() =>
+        (_converter.ConvertTo(context: null, culture: null, value: null, destinationType: typeof(string)) as string).Should().Be("(none)");
+
+    [Fact]
+    public void ConvertTo_NonNullValueToString_CallsBase() =>
+        _converter.ConvertTo(context: null, culture: null, value: new(), destinationType: typeof(string)).Should().NotBe("(none)");
+
+    [Fact]
+    public void ConvertTo_NullValueToNonString_ThrowsNotSupportedException()
+    {
+        Action act = () => _converter.ConvertTo(context: null, culture: null, value: null, destinationType: typeof(int));
+
+        act.Should().Throw<NotSupportedException>();
+    }
+
+    [Theory]
+    [InlineData("en-US")]
+    [InlineData("fr-FR")]
+    [InlineData("de-DE")]
+    public void ConvertTo_NullValueWithDifferentCultures_ReturnsNoneLowercase(string cultureName) =>
+        (_converter.ConvertTo(context: null, culture: new(cultureName), value: null, destinationType: typeof(string)) as string).Should().Be("(none)");
+}

src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.cs

@@ -405,6 +405,7 @@ protected override Size DefaultSize
     [RefreshProperties(RefreshProperties.Repaint),
     AttributeProvider(typeof(IListSource))]
     [SRDescription(nameof(SR.ListControlDataSourceDescr))]
+    [TypeConverter("System.Windows.Forms.Design.DataSourceConverter, System.Windows.Forms.Design")]
     public new object? DataSource
     {
         get => base.DataSource;