From 58589e966bfa29e8199395a5a3517e54a3f7811b Mon Sep 17 00:00:00 2001
From: "Ricardo Bossan (BEYONDSOFT CONSULTING INC) (from Dev Box)"
 <v-rbossan@microsoft.com>
Date: Thu, 11 Sep 2025 02:24:44 -0300
Subject: [PATCH 1/2] Makes PropertyGrid for CoboBox show "(none)" when
 DataSource is null for ComboBox

---
 .../Forms/Design/DataSourceConverter.cs       | 24 ++++++++++++++
 .../Forms/Design/DataSourceConverterTests.cs  | 32 +++++++++++++++++++
 .../Forms/Controls/ComboBox/ComboBox.cs       |  1 +
 3 files changed, 57 insertions(+)
 create mode 100644 src/System.Windows.Forms.Design/src/System/Windows/Forms/Design/DataSourceConverter.cs
 create mode 100644 src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/DataSourceConverterTests.cs

diff --git a/src/System.Windows.Forms.Design/src/System/Windows/Forms/Design/DataSourceConverter.cs b/src/System.Windows.Forms.Design/src/System/Windows/Forms/Design/DataSourceConverter.cs
new file mode 100644
index 00000000000..092e6b6cdae
--- /dev/null
+++ b/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;
+        }
+
+        return base.ConvertTo(context, culture, value, destinationType);
+    }
+}
diff --git a/src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/DataSourceConverterTests.cs b/src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/DataSourceConverterTests.cs
new file mode 100644
index 00000000000..9b6f68f1c09
--- /dev/null
+++ b/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)");
+}
diff --git a/src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.cs b/src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.cs
index cf37b868689..2491fb6af40 100644
--- a/src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.cs
+++ b/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;

From 9120a2bd88dc3b85140299abc90ac66b04f923e2 Mon Sep 17 00:00:00 2001
From: "Ricardo Bossan (BEYONDSOFT CONSULTING INC) (from Dev Box)"
 <v-rbossan@microsoft.com>
Date: Mon, 15 Sep 2025 13:01:31 -0300
Subject: [PATCH 2/2] Converts DataSourceConverter value to string, for parity
 with NetFx

---
 .../src/System/Windows/Forms/Design/DataSourceConverter.cs      | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/System.Windows.Forms.Design/src/System/Windows/Forms/Design/DataSourceConverter.cs b/src/System.Windows.Forms.Design/src/System/Windows/Forms/Design/DataSourceConverter.cs
index 092e6b6cdae..7698bec6c70 100644
--- a/src/System.Windows.Forms.Design/src/System/Windows/Forms/Design/DataSourceConverter.cs
+++ b/src/System.Windows.Forms.Design/src/System/Windows/Forms/Design/DataSourceConverter.cs
@@ -16,7 +16,7 @@ public DataSourceConverter() : base(typeof(IListSource))
     {
         if (destinationType == typeof(string) && value is null)
         {
-            return SR.None_lc;
+            return SR.None_lc.ToString();
         }
 
         return base.ConvertTo(context, culture, value, destinationType);