Convert.ToString custom class

It appears from the documentation that your custom class would need to implement either IFormattable or IConvertible for the IFormatProvider to be used. See the remarks in this link:

http://msdn.microsoft.com/en-us/library/ms131014(VS.100).aspx

That makes sense as there will be no way of knowing whether the custom class has an override of ToString() which takes an IFormatProvider parameter.

If you're going to always use the invariant culture, then you could add an additional overload of ToString() to your custom class which takes no parameters:

public override string ToString()
{
    return ToString(CultureInfo.InvariantCulture);
}

public string ToString(IFormatProvider fp)
{
    // existing code
}