Converting double to string with N decimals, dot as decimal separator, and no thousand separator

For a decimal, use the ToString method, and specify the Invariant culture to get a period as decimal separator:

value.ToString("0.00", System.Globalization.CultureInfo.InvariantCulture)

The long type is an integer, so there is no fraction part. You can just format it into a string and add some zeros afterwards:

value.ToString() + ".00"