double string format problem

I'm not aware of any culture settings that display a decimal point after a number that does not have any decimal places (I've been through the entire CultureInfo.GetCultures that are installed here). The closest you can get is with Dave Kreskoviak's solution of

String.Format("{0:0.0##}", number))

but that doesn't seem to be what you're after given your example above.

You will have to override the normal formatting process e.g.

private string myDoubleToString(double d)
{
    if (d % 1 == 0)
        return d.ToString() + CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator;
    else
        return d.ToString();
}

or reconsider why you want the trailing decimal point in these circumstances - it's rather strange.