Add code coverage for DataGridViewColumnConverter (#12766)

related #10453

Proposed changes
Add unit tests for DataGridViewColumnConverter.cs to test its methods

src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/DataGridViewColumnConverterTests.cs

@@ -0,0 +1,88 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable enable
+
+using System.ComponentModel.Design.Serialization;
+using System.Globalization;
+
+namespace System.Windows.Forms.Tests;
+
+public class DataGridViewColumnConverterTests : IDisposable
+{
+    private readonly DataGridViewColumnConverter _converter = new();
+    private readonly DataGridViewColumn _column = new();
+
+    public void Dispose() => _column.Dispose();
+
+    [Theory]
+    [InlineData(typeof(InstanceDescriptor))]
+    [InlineData(typeof(string))]
+    public void CanConvertTo_ReturnsExpectedResult(Type destinationType) =>
+        _converter.CanConvertTo(context: null, destinationType).Should().BeTrue();
+
+    [Fact]
+    public void ConvertTo_InstanceDescriptorWithoutCellType_ReturnsInstanceDescriptor() =>
+        AssertInstanceDescriptor(_column);
+
+    [Fact]
+    public void ConvertTo_InstanceDescriptorWithHeaderText_ReturnsInstanceDescriptor()
+    {
+        _column.HeaderText = "Test Header";
+        AssertInstanceDescriptor(_column);
+    }
+
+    [Fact]
+    public void ConvertTo_InstanceDescriptorWithToolTipText_ReturnsInstanceDescriptor()
+    {
+        _column.ToolTipText = "Test ToolTip";
+        AssertInstanceDescriptor(_column);
+    }
+
+    [Fact]
+    public void ConvertTo_NonInstanceDescriptorWithDataPropertyName_ReturnsBaseResult()
+    {
+        _column.DataPropertyName = "TestDataProperty";
+        AssertBaseResult(_column);
+    }
+
+    [Fact]
+    public void ConvertTo_NonInstanceDescriptor_ReturnsBaseResult() =>
+        AssertBaseResult(_column);
+
+    [Fact]
+    public void ConvertTo_NullDestinationType_ThrowsArgumentNullException()
+    {
+        Action action = () => _converter.ConvertTo(context: null, culture: CultureInfo.InvariantCulture, value: _column, destinationType: null!);
+        action.Should().Throw<ArgumentNullException>();
+    }
+
+    [Fact]
+    public void ConvertTo_InvalidDestinationType_ThrowsNotSupportedException()
+    {
+        Action action = () => _converter.ConvertTo(context: null, culture: CultureInfo.InvariantCulture, value: _column, destinationType: typeof(int));
+        action.Should().Throw<NotSupportedException>();
+    }
+
+    [Fact]
+    public void ConvertTo_WithCellTypeNotNull_ReturnsInstanceDescriptor()
+    {
+        using DataGridViewTextBoxCell cellTemplate = new();
+        using DataGridViewColumn column = new(cellTemplate);
+
+        var descriptor = _converter.ConvertTo(context: null, culture: CultureInfo.InvariantCulture, value: column, destinationType: typeof(InstanceDescriptor)) as InstanceDescriptor;
+
+        descriptor.Should().NotBeNull();
+    }
+
+    private void AssertInstanceDescriptor(DataGridViewColumn column)
+    {
+        var descriptor = _converter.ConvertTo(context: null, culture: CultureInfo.InvariantCulture, value: column, destinationType: typeof(InstanceDescriptor)) as InstanceDescriptor;
+        descriptor.Should().NotBeNull();
+    }
+
+    private void AssertBaseResult(DataGridViewColumn column)
+    {
+        _converter.ConvertTo(context: null, culture: CultureInfo.InvariantCulture, value: column, destinationType: typeof(string)).Should().Be(column.ToString());
+    }
+}