小数点后的千位分隔符

Question

如何获取数字格式，以便在小数点后也使用（特定于文化的）千位分隔符？

检查此代码（NUnit测试用例，按原样通过）：

    [Test]
    [SetCulture("en-US")]
    public void FormattingOnly()
    {
        // a large number
        double result = 1E9 + 0.212324;
        string printout = result.ToString("N9");
        // Would like to have this as "1,000,000,000.212,320,000"
        Assert.AreEqual("1,000,000,000.212320000", printout); 

        result = 0.212324;
        printout = result.ToString("N9");
        // Hard to read, should better be "0.212,324,000"
        Assert.AreEqual("0.212324000", printout);

        result = 0.212324;
        printout = result.ToString("E9");
        // Even harder to read, needs users to have a PhD ;)
        Assert.AreEqual("2.123240000E-001", printout);

        result = 1E9 + 0.212324;
        printout = result.ToString("E");
        // Completely worthless for many users
        Assert.AreEqual("1.000000E+009", printout);
    }

我正在尝试找到最易读的格式，即使没有数学背景的用户也可以理解。在我的应用程序中，用户可以根据自己的需要设置输出格式中的位数。

注意：这不是.NET String.Format() to add commas in thousands place for a number的重复项。一般而言，大约有数千个分隔符。