Add ComboBoxDesigner tests (#10772)

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

@@ -0,0 +1,49 @@
+// 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.System.Windows.Forms.Design;
+
+public sealed class ComboBoxDesignerTests
+{
+    [Fact]
+    public void AutoResizeHandles_WithInitialize_ShouldBeTrue()
+    {
+        using ComboBoxDesigner comboBoxDesigner = new();
+        using ComboBox comboBox = new();
+        comboBoxDesigner.Initialize(comboBox);
+
+        comboBoxDesigner.AutoResizeHandles.Should().BeTrue();
+    }
+
+    [Fact]
+    public void SnapLines_WithDefaultComboBox_ShouldReturnExpectedCount()
+    {
+        using ComboBoxDesigner comboBoxDesigner = new();
+        using ComboBox comboBox = new();
+        comboBoxDesigner.Initialize(comboBox);
+
+        comboBoxDesigner.SnapLines.Count.Should().Be(9);
+    }
+
+    [Fact]
+    public void SelectionRules_WithDefaultComboBox_ShouldThrowNullReferenceException()
+    {
+        using ComboBoxDesigner comboBoxDesigner = new();
+        using ComboBox comboBox = new();
+        comboBoxDesigner.Initialize(comboBox);
+
+        Action action = () => _ = comboBoxDesigner.SelectionRules;
+
+        action.Should().ThrowExactly<NullReferenceException>();
+    }
+
+    [Fact]
+    public void ActionLists_WithDefaultComboBox_ShouldReturnExpectedCount()
+    {
+        using ComboBoxDesigner comboBoxDesigner = new();
+        using ComboBox comboBox = new();
+        comboBoxDesigner.Initialize(comboBox);
+
+        comboBoxDesigner.ActionLists.Count.Should().Be(1);
+    }
+}