Formatting Conversion from Double to String...

I don't know how to do it with an IFormatProvider, but I do know how to do it with String::Format (http://msdn2.microsoft.com/en-us/library/fht0f5be.aspx):

textBox1->Text = String::Format("{0:N}", x);

Note that the above line will also append thousand separators for large numbers (eg. 1,000,000). Please take a look at the link for further information about the String::Format method.

The format string is just a string just concatenate the format string you want.

Something like this:
MyFormat = "{0:F" + int(myPrec).ToString() + "}";