There is another solution.
Implement IFormatProvider to return a ICustomFormatter that will be passed to string.Format :
public class StringPadder : ICustomFormatter
{
public string Format(string format, object arg,
IFormatProvider formatProvider)
{
// do padding for string arguments
// use default for others
}
}
public class StringPadderFormatProvider : IFormatProvider
{
public object GetFormat(Type formatType)
{
if (formatType == typeof(ICustomFormatter))
return new StringPadder();
return null;
}
public static readonly IFormatProvider Default =
new StringPadderFormatProvider();
}
Then you can use it like this :
string.Format(StringPadderFormatProvider.Default, "->{0:x20}<-", "Hello");