Add code coverage for OleDragDropHandler.cs file (#13022)

Related #10773


## Proposed changes

- Add unit test file: OleDragDropHandlerTests.cs for public methods of
the OleDragDropHandler.cs.

Author: Olina-Zhang

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

@@ -0,0 +1,185 @@
+// 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.ComponentModel.Design;
+using System.Drawing.Design;
+using Moq;
+
+namespace System.Windows.Forms.Design.Tests;
+
+public class OleDragDropHandlerTests : IDisposable
+{
+    private readonly Mock<SelectionUIHandler> _selectionHandlerMock;
+    private readonly Mock<IServiceProvider> _serviceProviderMock;
+    private readonly Mock<IOleDragClient> _clientMock;
+    private readonly OleDragDropHandler _oleDragDropHandler;
+    private readonly IComponent _component;
+
+    public OleDragDropHandlerTests()
+    {
+        _selectionHandlerMock = new() { CallBase = true };
+        _serviceProviderMock = new();
+        _clientMock = new();
+        _component = new Component();
+        _oleDragDropHandler = new(_selectionHandlerMock.Object, _serviceProviderMock.Object, _clientMock.Object);
+    }
+
+    public void Dispose()
+    {
+        _component.Dispose();
+    }
+
+    [Fact]
+    public void DataFormat_ShouldReturnCF_CODE()
+    {
+        OleDragDropHandler.DataFormat.Should().Be(OleDragDropHandler.CF_CODE);
+    }
+
+    [Fact]
+    public void ExtraInfoFormat_ShouldReturnCF_COMPONENTTYPES()
+    {
+        OleDragDropHandler.ExtraInfoFormat.Should().Be(OleDragDropHandler.CF_COMPONENTTYPES);
+    }
+
+    [Fact]
+    public void NestedToolboxItemFormat_ShouldReturnCF_TOOLBOXITEM()
+    {
+        OleDragDropHandler.NestedToolboxItemFormat.Should().Be(OleDragDropHandler.CF_TOOLBOXITEM);
+    }
+
+    [Fact]
+    public void Dragging_ShouldReturnFalseInitially()
+    {
+        _oleDragDropHandler.Dragging.Should().BeFalse();
+    }
+
+    [Fact]
+    public void FreezePainting_ShouldReturnFalseInitially()
+    {
+        OleDragDropHandler.FreezePainting.Should().BeFalse();
+    }
+
+    [Fact]
+    public void CreateTool_ShouldReturnEmptyArray_WhenCheckoutExceptionIsCanceled()
+    {
+        ToolboxItem toolboxItem = new();
+        _serviceProviderMock.Setup(sp => sp.GetService(typeof(IDesignerHost))).Returns((IDesignerHost?)null);
+        _serviceProviderMock.Setup(sp => sp.GetService(typeof(IToolboxService))).Returns((IToolboxService?)null);
+
+        var result = _oleDragDropHandler.CreateTool(toolboxItem, null, 0, 0, 0, 0, true, true);
+
+        result.Should().BeEmpty();
+    }
+
+    [Fact]
+    public void DoBeginDrag_ShouldReturnTrue_WhenRulesAreNotSizeable()
+    {
+        object[] components = [_component];
+        bool result = _oleDragDropHandler.DoBeginDrag(components, SelectionRules.Moveable, 0, 0);
+
+        result.Should().BeTrue();
+    }
+
+    [Fact]
+    public void DoBeginDrag_ShouldReturnTrue_WhenRulesAreSizeable()
+    {
+        object[] components = [_component];
+        bool result = _oleDragDropHandler.DoBeginDrag(components, SelectionRules.AllSizeable, 0, 0);
+
+        result.Should().BeTrue();
+    }
+
+    [Fact]
+    public void DoEndDrag_ShouldSetDraggingToFalse()
+    {
+        _oleDragDropHandler.DoEndDrag();
+
+        _oleDragDropHandler.Dragging.Should().BeFalse();
+    }
+
+    [Fact]
+    public void DoOleDragDrop_ShouldSetEffectToNone_WhenSelectionHandlerIsNotNull()
+    {
+        OleDragDropHandler handler = new(_selectionHandlerMock.Object, _serviceProviderMock.Object, _clientMock.Object);
+        DragEventArgs dragEventArgs = new(new DataObject(), 0, 0, 0, DragDropEffects.None, DragDropEffects.None);
+
+        handler.DoOleDragDrop(dragEventArgs);
+
+        dragEventArgs.Effect.Should().Be(DragDropEffects.None);
+    }
+
+    [Fact]
+    public void DoOleDragEnter_ShouldSetEffectToNone_WhenDraggingIsFalseAndCannotDropDataObject()
+    {
+        DragEventArgs dragEventArgs = new(new DataObject(), 0, 0, 0, DragDropEffects.None, DragDropEffects.None);
+
+        _oleDragDropHandler.DoOleDragEnter(dragEventArgs);
+
+        dragEventArgs.Effect.Should().Be(DragDropEffects.None);
+    }
+
+    [Fact]
+    public void DoOleDragEnter_ShouldSetEffectToNone_WhenCannotModifyComponents()
+    {
+        DragEventArgs dragEventArgs = new(new DataObject(), 0, 0, 0, DragDropEffects.Move, DragDropEffects.None);
+        _clientMock.Setup(client => client.CanModifyComponents).Returns(false);
+
+        _oleDragDropHandler.DoOleDragEnter(dragEventArgs);
+
+        dragEventArgs.Effect.Should().Be(DragDropEffects.None);
+    }
+
+    [Fact]
+    public void DoOleDragLeave_ShouldSetDraggingToFalse()
+    {
+        _oleDragDropHandler.DoOleDragLeave();
+
+        _oleDragDropHandler.Dragging.Should().BeFalse();
+    }
+
+    [Fact]
+    public void DoOleDragOver_ShouldSetEffectToNone_WhenDraggingIsFalseAndDragOkIsFalse()
+    {
+        DragEventArgs dragEventArgs = new(new DataObject(), 0, 0, 0, DragDropEffects.None, DragDropEffects.None);
+
+        _oleDragDropHandler.DoOleDragOver(dragEventArgs);
+
+        dragEventArgs.Effect.Should().Be(DragDropEffects.None);
+    }
+
+    [Fact]
+    public void DoOleDragOver_ShouldSetEffectToNone_WhenNotDraggingAndAllowedEffectIsNone()
+    {
+        DragEventArgs dragEventArgs = new(new DataObject(), 0, 0, 0, DragDropEffects.None, DragDropEffects.None);
+
+        _oleDragDropHandler.DoOleDragOver(dragEventArgs);
+
+        dragEventArgs.Effect.Should().Be(DragDropEffects.None);
+    }
+
+    [Fact]
+    public void DoOleGiveFeedback_ShouldSetUseDefaultCursorsToTrue_WhenEffectIsNotNoneAndSelectionHandlerIsNotNull()
+    {
+        GiveFeedbackEventArgs giveFeedbackEventArgs = new(DragDropEffects.Move, false);
+
+        _oleDragDropHandler.DoOleGiveFeedback(giveFeedbackEventArgs);
+
+        giveFeedbackEventArgs.UseDefaultCursors.Should().BeTrue();
+        _selectionHandlerMock.Verify(handler => handler.SetCursor(), Times.Never);
+    }
+
+    [Fact]
+    public void GetDraggingObjects_ShouldReturnNull_WhenDataObjectIsNull()
+    {
+        OleDragDropHandler.GetDraggingObjects((IDataObject?)null).Should().BeNull();
+    }
+
+    [Fact]
+    public void GetDraggingObjects_ShouldReturnNull_WhenDragEventArgsDataIsNull()
+    {
+        DragEventArgs dragEventArgs = new(null, 0, 0, 0, DragDropEffects.None, DragDropEffects.None);
+
+        OleDragDropHandler.GetDraggingObjects(dragEventArgs).Should().BeNull();
+    }
+}