Custom string formatter in C#

Question

String formatting in C#;

Can I use it? Yes.

Can I implement custom formatting? No.

I need to write something where I can pass a set of custom formatting options to string.Format, which will have some effect on the particular item.

at the moment I have something like this:

string.Format("{0}", item);

but I want to be able to do things with that item:

string.Format("{0:lcase}", item); // lowercases the item
string.Format("{0:ucase}", item); // uppercases the item
string.Format("{0:nospace}", item); // removes spaces

I know I can do things like .ToUpper(), .ToLower() etc. but I need to do it with string formatting.

I've been looking into things like IFormatProvider and IFormattable but I don't really know if they are the things I should be using, or, how to implement them.

Can anyone explain how I can solve this problem?

Rationale (just in case you want to know...)

I have a small program, where I can enter a comma delimited set of values, and a template. The items are passed into string.Format, along with the template which creates an output. I want to provide template formatting options, so that the user can control how they want items to be output.

Answer 1

You can make a custom formatter, something like:

public class MyFormatter : IFormatProvider, ICustomFormatter
{
   public object GetFormat(Type formatType)
   {
      if (formatType == typeof(ICustomFormatter))
         return this;
      else
         return null;
   }

   public string Format(string fmt, object arg, IFormatProvider formatProvider) 
   {
       if(arg == null) return string.Empty;

       if(fmt == "lcase")
           return arg.ToString().ToLower();
       else if(fmt == "ucase")
           return arg.ToString().ToUpper();
       else if(fmt == "nospace")
           return arg.ToString().Replace(" ", "");
       // Use this if you want to handle default formatting also
       else if (arg is IFormattable) 
           return ((IFormattable)arg).ToString(fmt, CultureInfo.CurrentCulture);
       return arg.ToString();
   }
}

Then use it like:

 string.Format(new MyFormatter(),
            "{0:lcase}, {0:ucase}, {0:nospace}", 
            "My Test String")

This should return:

my test string, MY TEST STRING, MyTestString