具有不同文化的日期时间值不正确格式化

我在线文化遇到一个小问题，并得到日期显示正确。
我正在重载DateTime类的ToString（）方法。

I'm having a slight issue with Thread culture and getting a date to display properly. I am overloading the ToString() method of the DateTime class.

使用文化en-CA，我的日期以正确的格式出现 yyyy / MM / dd
但与文化fr-CA，我的日期正在出来yyyy-MM-dd

With culture "en-CA", my date is coming out in the right format "yyyy/MM/dd" but with culture "fr-CA", my date is coming out "yyyy-MM-dd"

一些单元测试来显示问题。
英语测试工作，但法语总是失败。

I've made some unit test to display the issue. The english test works but the french always fails.

即使我更改了GetDateInStringMethod来做.ToShortDateString。我仍然遇到同样的问题。

Even if I change the GetDateInStringMethod to do .ToShortDateString. I still get the same issue.

[Test()]
public void ValidInEnglish()
{
    Thread.CurrentThread.CurrentCulture = new CultureInfo("en-CA");
    Thread.CurrentThread.CurrentCulture.DateTimeFormat.ShortDatePattern = Utility.DatePattern;
    DateTime? currentDate = new DateTime(2009,02,7);
    string expected = "2009/02/07";
    string actual = DateUtils.GetDateInString(currentDate);

//This works
    Assert.AreEqual(expected, actual);
}

[Test()]
public void ValidInFrench()
{
    Thread.CurrentThread.CurrentCulture = new CultureInfo("fr-CA");
    Thread.CurrentThread.CurrentCulture.DateTimeFormat.ShortDatePattern = Utility.DatePattern;
    DateTime? currentDate = new DateTime(2009, 02, 7);
    string expected = "2009/02/07";
    string actual = DateUtils.GetDateInString(currentDate);
// This doesn't work
    Assert.AreEqual(expected, actual);
}

public static string GetDateInString(DateTime? obj)
{
    if (obj == null || !obj.HasValue)
    {
        return string.Empty;
    }

    return obj.Value.ToString(Utility.DatePattern);
}

public const string DatePattern = "yyyy/MM/dd";