diff --git a/src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/ToolStripSplitButtonTests.cs b/src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/ToolStripSplitButtonTests.cs
index 9c681053735..df1b54e8d89 100644
--- a/src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/ToolStripSplitButtonTests.cs
+++ b/src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/ToolStripSplitButtonTests.cs
@@ -1,72 +1,436 @@
 // 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.Drawing;
+
 namespace System.Windows.Forms.Tests;
 
-public class ToolStripSplitButtonTests
+public class ToolStripSplitButtonTests : IDisposable
 {
-    public static IEnumerable<object[]> ToolStripItem_Set_TestData()
+    private readonly ToolStripSplitButton _toolStripSplitButton;
+
+    public ToolStripSplitButtonTests()
     {
-        yield return new object[] { null };
-        yield return new object[] { new SubToolStripItem() };
+        _toolStripSplitButton = new();
     }
 
+    public void Dispose() => _toolStripSplitButton.Dispose();
+
+    public static TheoryData<ToolStripItem?> ToolStripItem_Set_TestData() => new()
+     {
+        null,
+        new SubToolStripItem()
+     };
+
     [WinFormsTheory]
     [MemberData(nameof(ToolStripItem_Set_TestData))]
-    public void ToolStripSplitButton_DefaultItem_Set_GetReturnsExpected(ToolStripItem value)
+    public void ToolStripSplitButton_DefaultItem_Set_GetReturnsExpected(ToolStripItem? value)
     {
-        using ToolStripSplitButton button = new()
-        {
-            DefaultItem = value
-        };
-        Assert.Same(value, button.DefaultItem);
-
-        // Set same.
-        button.DefaultItem = value;
-        Assert.Same(value, button.DefaultItem);
+        _toolStripSplitButton.DefaultItem = value;
+        _toolStripSplitButton.DefaultItem.Should().Be(value);
     }
 
     [WinFormsFact]
     public void ToolStripSplitButton_DefaultItem_SetWithHandler_CallsOnDefaultItemChanged()
     {
-        using ToolStripSplitButton button = new();
-
         int callCount = 0;
         EventHandler handler = (sender, e) =>
         {
-            Assert.Same(button, sender);
-            Assert.Same(EventArgs.Empty, e);
+            sender.Should().Be(_toolStripSplitButton);
+            e.Should().Be(EventArgs.Empty);
             callCount++;
         };
-        button.DefaultItemChanged += handler;
+        _toolStripSplitButton.DefaultItemChanged += handler;
 
         // Set non-null.
         using SubToolStripItem item1 = new();
-        button.DefaultItem = item1;
-        Assert.Same(item1, button.DefaultItem);
-        Assert.Equal(1, callCount);
+        _toolStripSplitButton.DefaultItem = item1;
+        _toolStripSplitButton.DefaultItem.Should().Be(item1);
+        callCount.Should().Be(1);
 
         // Set same.
-        button.DefaultItem = item1;
-        Assert.Same(item1, button.DefaultItem);
-        Assert.Equal(1, callCount);
+        _toolStripSplitButton.DefaultItem = item1;
+        _toolStripSplitButton.DefaultItem.Should().Be(item1);
+        callCount.Should().Be(1);
 
         // Set different.
         using SubToolStripItem item2 = new();
-        button.DefaultItem = item2;
-        Assert.Same(item2, button.DefaultItem);
-        Assert.Equal(2, callCount);
+        _toolStripSplitButton.DefaultItem = item2;
+        _toolStripSplitButton.DefaultItem.Should().Be(item2);
+        callCount.Should().Be(2);
 
         // Set null.
-        button.DefaultItem = null;
-        Assert.Null(button.DefaultItem);
-        Assert.Equal(3, callCount);
+        _toolStripSplitButton.DefaultItem = null;
+        _toolStripSplitButton.DefaultItem.Should().BeNull();
+        callCount.Should().Be(3);
 
         // Remove handler.
-        button.DefaultItemChanged -= handler;
-        button.DefaultItem = item1;
-        Assert.Equal(item1, button.DefaultItem);
-        Assert.Equal(3, callCount);
+        _toolStripSplitButton.DefaultItemChanged -= handler;
+        _toolStripSplitButton.DefaultItem = item1;
+        _toolStripSplitButton.DefaultItem.Should().Be(item1);
+        callCount.Should().Be(3);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_Ctor_Default()
+    {
+        using ToolStripSplitButton toolStripSplitButton = new();
+
+        _toolStripSplitButton.Text.Should().BeEmpty();
+        _toolStripSplitButton.Image.Should().BeNull();
+        toolStripSplitButton.DropDownButtonWidth.Should().Be(toolStripSplitButton.TestAccessor().Dynamic.DefaultDropDownButtonWidth);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_Ctor_String()
+    {
+        using ToolStripSplitButton toolStripSplitButton = new("Test");
+
+        toolStripSplitButton.Text.Should().Be("Test");
+        _toolStripSplitButton.Image.Should().BeNull();
+        toolStripSplitButton.DropDownButtonWidth.Should().Be(toolStripSplitButton.TestAccessor().Dynamic.DefaultDropDownButtonWidth);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_Ctor_Image()
+    {
+        using Bitmap image = new(10, 10);
+        using ToolStripSplitButton toolStripSplitButton = new(image);
+
+        toolStripSplitButton.Text.Should().BeNull();
+        toolStripSplitButton.Image.Should().Be(image);
+        toolStripSplitButton.DropDownButtonWidth.Should().Be(toolStripSplitButton.TestAccessor().Dynamic.DefaultDropDownButtonWidth);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_Ctor_String_Image()
+    {
+        using Bitmap image = new(10, 10);
+        using ToolStripSplitButton toolStripSplitButton = new("Test", image);
+
+        toolStripSplitButton.Text.Should().Be("Test");
+        toolStripSplitButton.Image.Should().Be(image);
+        toolStripSplitButton.DropDownButtonWidth.Should().Be(toolStripSplitButton.TestAccessor().Dynamic.DefaultDropDownButtonWidth);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_Ctor_String_Image_EventHandler()
+    {
+        bool clickInvoked = false;
+        EventHandler onClick = (sender, e) => clickInvoked = true;
+
+        using Bitmap image = new(10, 10);
+        using ToolStripSplitButton toolStripSplitButton = new("Test", image, onClick);
+
+        toolStripSplitButton.Text.Should().Be("Test");
+        toolStripSplitButton.Image.Should().Be(image);
+        toolStripSplitButton.DropDownButtonWidth.Should().Be(toolStripSplitButton.TestAccessor().Dynamic.DefaultDropDownButtonWidth);
+
+        toolStripSplitButton.PerformClick();
+        clickInvoked.Should().BeTrue();
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_Ctor_String_Image_EventHandler_Name()
+    {
+        bool clickInvoked = false;
+        EventHandler onClick = (sender, e) => clickInvoked = true;
+
+        using Bitmap image = new(10, 10);
+        using ToolStripSplitButton toolStripSplitButton = new("Test", image, onClick, "TestButton");
+
+        toolStripSplitButton.Text.Should().Be("Test");
+        toolStripSplitButton.Image.Should().Be(image);
+        toolStripSplitButton.Name.Should().Be("TestButton");
+        toolStripSplitButton.DropDownButtonWidth.Should().Be(toolStripSplitButton.TestAccessor().Dynamic.DefaultDropDownButtonWidth);
+
+        toolStripSplitButton.PerformClick();
+        clickInvoked.Should().BeTrue();
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_Ctor_String_Image_DropDownItems()
+    {
+        using Bitmap image = new(10, 10);
+        using ToolStripMenuItem item1 = new("Item1");
+        using ToolStripMenuItem item2 = new("Item2");
+
+        using ToolStripSplitButton toolStripSplitButton = new("Test", image, item1, item2);
+
+        toolStripSplitButton.Text.Should().Be("Test");
+        toolStripSplitButton.Image.Should().Be(image);
+        toolStripSplitButton.DropDownItems.Cast<ToolStripItem>().Should().ContainInOrder(new[] { item1, item2 });
+        toolStripSplitButton.DropDownButtonWidth.Should().Be(toolStripSplitButton.TestAccessor().Dynamic.DefaultDropDownButtonWidth);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_AutoToolTip_DefaultValue()
+    {
+        _toolStripSplitButton.AutoToolTip.Should().BeTrue();
+    }
+
+    [WinFormsTheory]
+    [BoolData]
+    public void ToolStripSplitButton_AutoToolTip_Set_GetReturnsExpected(bool value)
+    {
+        _toolStripSplitButton.AutoToolTip.Should().BeTrue();
+
+        _toolStripSplitButton.AutoToolTip = value;
+        _toolStripSplitButton.AutoToolTip.Should().Be(value);
+
+        _toolStripSplitButton.AutoToolTip = !value;
+        _toolStripSplitButton.AutoToolTip.Should().Be(!value);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_ButtonBounds_ReturnsExpected()
+    {
+        _toolStripSplitButton.ButtonBounds.Should().Be(_toolStripSplitButton.TestAccessor().Dynamic.SplitButtonButton.Bounds);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_ButtonPressed_ReturnsExpected()
+    {
+        _toolStripSplitButton.ButtonPressed.Should().Be(_toolStripSplitButton.TestAccessor().Dynamic.SplitButtonButton.Pressed);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_ButtonSelected_ReturnsExpected()
+    {
+        _toolStripSplitButton.ButtonSelected.Should().Be(_toolStripSplitButton.TestAccessor().Dynamic.SplitButtonButton.Selected);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_ButtonClick_EventTriggered()
+    {
+        bool eventTriggered = false;
+
+        EventHandler handler = (sender, e) =>
+        {
+            eventTriggered = true;
+        };
+
+        _toolStripSplitButton.ButtonClick += handler;
+
+        _toolStripSplitButton.PerformButtonClick();
+
+        eventTriggered.Should().BeTrue();
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_ButtonClick_EventHandlerCalledExpectedTimes()
+    {
+        int callCount = 0;
+
+        EventHandler handler = (sender, e) =>
+        {
+            callCount++;
+        };
+
+        _toolStripSplitButton.ButtonClick += handler;
+
+        _toolStripSplitButton.PerformButtonClick();
+        _toolStripSplitButton.PerformButtonClick();
+
+        callCount.Should().Be(2);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_ButtonClick_EventHandlerRemoved()
+    {
+        int callCount = 0;
+
+        EventHandler handler = (sender, e) =>
+        {
+            callCount++;
+        };
+
+        _toolStripSplitButton.ButtonClick += handler;
+        _toolStripSplitButton.PerformButtonClick();
+        _toolStripSplitButton.ButtonClick -= handler;
+        _toolStripSplitButton.PerformButtonClick();
+
+        callCount.Should().Be(1);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_ButtonDoubleClick_EventTriggered()
+    {
+        bool eventTriggered = false;
+        _toolStripSplitButton.ButtonDoubleClick += (sender, e) => eventTriggered = true;
+
+        _toolStripSplitButton.OnButtonDoubleClick(EventArgs.Empty);
+
+        eventTriggered.Should().BeTrue();
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_ButtonDoubleClick_EventHandlerCalledExpectedTimes()
+    {
+        int callCount = 0;
+        _toolStripSplitButton.ButtonDoubleClick += (sender, e) => callCount++;
+
+        _toolStripSplitButton.OnButtonDoubleClick(EventArgs.Empty);
+        _toolStripSplitButton.OnButtonDoubleClick(EventArgs.Empty);
+
+        callCount.Should().Be(2);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_ButtonDoubleClick_EventHandlerRemoved()
+    {
+        int callCount = 0;
+        EventHandler handler = (sender, e) => callCount++;
+        _toolStripSplitButton.ButtonDoubleClick += handler;
+        _toolStripSplitButton.ButtonDoubleClick -= handler;
+
+        _toolStripSplitButton.OnButtonDoubleClick(EventArgs.Empty);
+
+        callCount.Should().Be(0);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_DropDownButtonArea_ReturnsExpected()
+    {
+        _toolStripSplitButton.DropDownButtonArea.Should().Be( _toolStripSplitButton.DropDownButtonBounds);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_DropDownButtonSelected_DefaultValue()
+    {
+        _toolStripSplitButton.DropDownButtonSelected.Should().BeFalse();
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_DropDownButtonSelected_Selected()
+    {
+        _toolStripSplitButton.Select();
+        _toolStripSplitButton.DropDownButtonSelected.Should().BeTrue();
+    }
+
+    [WinFormsTheory]
+    [InlineData(0)]
+    [InlineData(5)]
+    [InlineData(10)]
+    public void ToolStripSplitButton_DropDownButtonWidth_Set_GetReturnsExpected(int value)
+    {
+        _toolStripSplitButton.DropDownButtonWidth = value;
+        _toolStripSplitButton.DropDownButtonWidth.Should().Be(value);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_DropDownButtonWidth_SetInvalidValue_ThrowsArgumentOutOfRangeException()
+    {
+        Action action = () => _toolStripSplitButton.DropDownButtonWidth = -1;
+        action.Should().Throw<ArgumentOutOfRangeException>();
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_SplitterBounds_DefaultValue()
+    {
+        _toolStripSplitButton.SplitterBounds.Should().Be(Rectangle.Empty);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_OnButtonClick_TriggersButtonClickEvent()
+    {
+        bool eventTriggered = false;
+        _toolStripSplitButton.ButtonClick += (sender, e) => eventTriggered = true;
+
+        _toolStripSplitButton.TestAccessor().Dynamic.OnButtonClick(EventArgs.Empty);
+
+        eventTriggered.Should().BeTrue();
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_OnButtonClick_RoutesClickToDefaultItem()
+    {
+        using ToolStripButton defaultItem = new();
+        bool eventTriggered = false;
+        defaultItem.Click += (sender, e) => eventTriggered = true;
+        _toolStripSplitButton.DefaultItem = defaultItem;
+
+        _toolStripSplitButton.TestAccessor().Dynamic.OnButtonClick(EventArgs.Empty);
+
+        eventTriggered.Should().BeTrue();
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_OnButtonDoubleClick_TriggersButtonDoubleClickEvent()
+    {
+        bool eventTriggered = false;
+        _toolStripSplitButton.ButtonDoubleClick += (sender, e) => eventTriggered = true;
+
+        _toolStripSplitButton.OnButtonDoubleClick(EventArgs.Empty);
+
+        eventTriggered.Should().BeTrue();
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_OnButtonDoubleClick_RoutesDoubleClickToDefaultItem()
+    {
+        using ToolStripButton defaultItem = new();
+        bool eventTriggered = false;
+        defaultItem.DoubleClick += (sender, e) => eventTriggered = true;
+        _toolStripSplitButton.DefaultItem = defaultItem;
+
+        _toolStripSplitButton.OnButtonDoubleClick(EventArgs.Empty);
+
+        eventTriggered.Should().BeTrue();
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_PerformButtonClick_TriggersButtonClickEvent()
+    {
+        bool eventTriggered = false;
+        _toolStripSplitButton.ButtonClick += (sender, e) => eventTriggered = true;
+
+        _toolStripSplitButton.PerformButtonClick();
+
+        eventTriggered.Should().BeTrue();
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_PerformButtonClick_RoutesClickToDefaultItem()
+    {
+        using ToolStripButton defaultItem = new();
+        bool eventTriggered = false;
+        defaultItem.Click += (sender, e) => eventTriggered = true;
+        _toolStripSplitButton.DefaultItem = defaultItem;
+
+        _toolStripSplitButton.PerformButtonClick();
+
+        eventTriggered.Should().BeTrue();
+    }
+
+    [WinFormsTheory]
+    [InlineData(20)]
+    [InlineData(11)]
+    public void ToolStripSplitButton_ResetDropDownButtonWidth_SetsToDefault(int initialWidth)
+    {
+        _toolStripSplitButton.DropDownButtonWidth = initialWidth;
+
+        _toolStripSplitButton.ResetDropDownButtonWidth();
+
+        _toolStripSplitButton.DropDownButtonWidth.Should().Be(_toolStripSplitButton.TestAccessor().Dynamic.DefaultDropDownWidth);
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_ShouldSerializeDropDownButtonWidth_NotDefault_ReturnsTrue()
+    {
+        _toolStripSplitButton.DropDownButtonWidth = 20;
+
+        _toolStripSplitButton.ShouldSerializeDropDownButtonWidth().Should().BeTrue();
+    }
+
+    [WinFormsFact]
+    public void ToolStripSplitButton_ShouldSerializeDropDownButtonWidth_Default_ReturnsFalse()
+    {
+        _toolStripSplitButton.DropDownButtonWidth = 11;
+
+        _toolStripSplitButton.ShouldSerializeDropDownButtonWidth().Should().BeFalse();
     }
 
     private class SubToolStripItem : ToolStripItem