创建自定义格式提供程序

public class CustomFormat : IFormatProvider, ICustomFormatter
{
    public string Format(string format, object arg, IFormatProvider formatProvider)
    {
        if (!this.Equals(formatProvider))
        {
            return null;
        }

        if (format == "Reverse")
        {
            return String.Join("", arg.ToString().Reverse());
        }

        return arg.ToString();
    }

    public object GetFormat(Type formatType)
    {
        return formatType==typeof(ICustomFormatter) ? this:null;
    }
}

用法:

String.Format(new CustomFormat(), "-> {0:Reverse} <-", "Hello World");

输出:

-> dlroW olleH <-