Add test coverage for DateTimePicker (#11033)

* Add unit test cases for DateTimePicker

* Updated test cases

Author: John-Qiao

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

@@ -161,6 +161,143 @@ public void DateTimePicker_Format_SetInvalid_ThrowsInvalidEnumArgumentException(
         Assert.Throws<InvalidEnumArgumentException>("value", () => control.Format = value);
     }
 
+    [WinFormsFact]
+    public void DateTimePicker_CalendarTitleBackColor_GetSet_ReturnsExpected()
+    {
+        using DateTimePicker control = new();
+        var expectedColor = Color.Red;
+
+        control.CalendarTitleBackColor = expectedColor;
+
+        control.CalendarTitleBackColor.Should().Be(expectedColor);
+    }
+
+    [WinFormsTheory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("invalid color")]
+    public void DateTimePicker_CalendarTitleBackColor_SetInvalid_ThrowsArgumentException(string value)
+    {
+        using DateTimePicker control = new();
+        Action act = () => control.CalendarTitleBackColor = ColorTranslator.FromHtml(value);
+        act.Should().Throw<ArgumentException>();
+    }
+
+    [WinFormsFact]
+    public void DateTimePicker_CalendarForeColor_GetSet_ReturnsExpected()
+    {
+        using DateTimePicker control = new();
+        var expectedColor = Color.Red;
+
+        control.CalendarForeColor = expectedColor;
+
+        control.CalendarForeColor.Should().Be(expectedColor);
+    }
+
+    [WinFormsTheory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("invalid color")]
+    public void DateTimePicker_CalendarForeColor_SetInvalid_ThrowsArgumentException(string value)
+    {
+        using DateTimePicker control = new();
+        Action act = () => control.CalendarForeColor = ColorTranslator.FromHtml(value);
+        act.Should().Throw<ArgumentException>();
+    }
+
+    [WinFormsFact]
+    public void DateTimePicker_CalendarTitleForeColor_GetSet_ReturnsExpected()
+    {
+        using DateTimePicker control = new();
+        var expectedColor = Color.Red;
+
+        control.CalendarTitleForeColor = expectedColor;
+
+        control.CalendarTitleForeColor.Should().Be(expectedColor);
+    }
+
+    [WinFormsTheory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("invalid color")]
+    public void DateTimePicker_CalendarTitleForeColor_SetInvalid_ThrowsArgumentException(string value)
+    {
+        using DateTimePicker control = new();
+        Action act = () => control.CalendarTitleForeColor = ColorTranslator.FromHtml(value);
+        act.Should().Throw<ArgumentException>();
+    }
+
+    [WinFormsFact]
+    public void DateTimePicker_CalendarTrailingForeColor_GetSet_ReturnsExpected()
+    {
+        using DateTimePicker control = new();
+        var expectedColor = Color.Red;
+
+        control.CalendarTrailingForeColor = expectedColor;
+
+        control.CalendarTrailingForeColor.Should().Be(expectedColor);
+    }
+
+    [WinFormsTheory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("invalid color")]
+    public void DateTimePicker_CalendarTrailingForeColor_SetInvalid_ThrowsArgumentException(string value)
+    {
+        using DateTimePicker control = new();
+        Action act = () => control.CalendarTrailingForeColor = ColorTranslator.FromHtml(value);
+        act.Should().Throw<ArgumentException>();
+    }
+
+    [WinFormsFact]
+    public void DateTimePicker_CalendarMonthBackground_GetSet_ReturnsExpected()
+    {
+        using DateTimePicker control = new();
+        var expectedColor = Color.Red;
+
+        control.CalendarMonthBackground = expectedColor;
+
+        control.CalendarMonthBackground.Should().Be(expectedColor);
+    }
+
+    [WinFormsTheory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("invalid color")]
+    public void DateTimePicker_CalendarMonthBackground_SetInvalid_ThrowsArgumentException(string value)
+    {
+        using DateTimePicker control = new();
+        Action act = () => control.CalendarMonthBackground = ColorTranslator.FromHtml(value);
+        act.Should().Throw<ArgumentException>();
+    }
+
+    [WinFormsFact]
+    public void DateTimePicker_BackColorChangedEvent_Raised_Success()
+    {
+        using DateTimePicker control = new();
+        bool eventWasRaised = false;
+
+        control.BackColorChanged += (sender, args) => eventWasRaised = true;
+        control.BackColor = Color.Red;
+
+        eventWasRaised.Should().BeTrue();
+    }
+
+    [WinFormsFact]
+    public void DateTimePicker_BackgroundImageChangedEvent_Raised_Success()
+    {
+        using DateTimePicker control = new();
+        bool eventWasRaised = false;
+
+        control.BackgroundImageChanged += (sender, args) => eventWasRaised = true;
+        using (Bitmap bmp = new Bitmap(10, 10))
+        {
+            control.BackgroundImage = bmp;
+        }
+
+        eventWasRaised.Should().BeTrue();
+    }
+
     [WinFormsTheory]
     [InlineData(ControlStyles.ContainerControl, false)]
     [InlineData(ControlStyles.UserPaint, false)]