C# Tip: IFormattable interface, to define different string formats for the same object

Even when the internal data is the same, sometimes you can represent it in different ways. Think of the DateTime structure: by using different modifiers, you can represent the same date in different formats.

DateTime dt = new DateTime(2024, 1, 1, 8, 53, 14);

Console.WriteLine(dt.ToString("yyyy-MM-dddd")); //2024-01-Monday
Console.WriteLine(dt.ToString("Y")); //January 2024

Same datetime, different formats.

You can further customise it by adding the CultureInfo:

System.Globalization.CultureInfo italianCulture = new System.Globalization.CultureInfo("it-IT");

Console.WriteLine(dt.ToString("yyyy-MM-dddd", italianCulture)); //2024-01-lunedì
Console.WriteLine(dt.ToString("Y", italianCulture)); //gennaio 2024

Now, how can we use this behaviour in our custom classes?

IFormattable interface for custom ToString definition

Take this simple POCO class:

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public DateTime BirthDate { get; set; }
}

We can make this class implement the IFormattable interface so that we can define and use the advanced ToString:

public class Person : IFormattable
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public DateTime BirthDate { get; set; }

    public string ToString(string? format, IFormatProvider? formatProvider)
    {
        // Here, you define how to work with different formats
    }
}

Now, we can define the different formats. Since I like to keep the available formats close to the main class, I added a nested class that only exposes the names of the formats.

public class Person : IFormattable
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public DateTime BirthDate { get; set; }

    public string ToString(string? format, IFormatProvider? formatProvider)
    {
        // Here, you define how to work with different formats
    }

    public static class StringFormats
    {
        public const string FirstAndLastName = "FL";
        public const string Mini = "Mini";
        public const string Full = "Full";
    }
}

Finally, we can implement the ToString(string? format, IFormatProvider? formatProvider) method, taking care of all the different formats we support (remember to handle the case when the format is not recognised!)

public string ToString(string? format, IFormatProvider? formatProvider)
{
    switch (format)
    {
        case StringFormats.FirstAndLastName:
            return string.Format("{0} {1}", FirstName, LastName);
        case StringFormats.Full:
        {
            FormattableString fs = $"{FirstName} {LastName} ({BirthDate:D})";
            return fs.ToString(formatProvider);
        }
        case StringFormats.Mini:
            return $"{FirstName.Substring(0, 1)}.{LastName.Substring(0, 1)}";
        default:
            return this.ToString();
    }
}

A few things to notice:

1. I use a switch statement based on the values defined in the StringFormats subclass. If the format is empty or unrecognised, this method returns the default implementation of ToString.
2. You can use whichever way to generate a string, like string interpolation, or more complex ways;
3. In the StringFormats.Full branch, I stored the string format in a FormattableString instance to apply the input formatProvider to the final result.

Getting a custom string representation of an object

We can try the different formatting options now that we have implemented them all.

Look at how the behaviour changes based on the formatting and input culture (Hint: venerdí is the Italian for Friday.).

Person person = new Person
{
    FirstName = "Albert",
    LastName = "Einstein",
    BirthDate = new DateTime(1879, 3, 14)
};

System.Globalization.CultureInfo italianCulture = new System.Globalization.CultureInfo("it-IT");

Console.WriteLine(person.ToString(Person.StringFormats.FirstAndLastName, italianCulture)); //Albert Einstein

Console.WriteLine(person.ToString(Person.StringFormats.Mini, italianCulture)); //A.E

Console.WriteLine(person.ToString(Person.StringFormats.Full, italianCulture)); //Albert Einstein (venerdì 14 marzo 1879)

Console.WriteLine(person.ToString(Person.StringFormats.Full, null)); //Albert Einstein (Friday, March 14, 1879)

Console.WriteLine(person.ToString(Person.StringFormats.Full, CultureInfo.InvariantCulture)); //Albert Einstein (Friday, 14 March 1879)

Console.WriteLine(person.ToString("INVALID FORMAT", CultureInfo.InvariantCulture)); //Scripts.General.IFormattableTest+Person

Console.WriteLine(string.Format("I am {0:Mini}", person)); //I am A.E

Console.WriteLine($"I am not {person:Full}"); //I am not Albert Einstein (Friday, March 14, 1879)

Not only that, but now the result can also depend on the Culture related to the current thread:

using (new TemporaryThreadCulture(italianCulture))
{
    Console.WriteLine(person.ToString(Person.StringFormats.Full, CultureInfo.CurrentCulture)); // Albert Einstein (venerdì 14 marzo 1879)
}

using (new TemporaryThreadCulture(germanCulture))
{
    Console.WriteLine(person.ToString(Person.StringFormats.Full, CultureInfo.CurrentCulture)); //Albert Einstein (Freitag, 14. März 1879)
}

(note: TemporaryThreadCulture is a custom class that I explained in a previous article - see below)

Further readings

You might be thinking «wow, somebody still uses String.Format? Weird!»

Well, even though it seems an old-style method to generate strings, it's still valid, as I explain here:

🔗How to use String.Format - and why you should care about it | Code4IT

Also, how did I temporarily change the culture of the thread? Here's how:
🔗 C# Tip: How to temporarily change the CurrentCulture | Code4IT

This article first appeared on Code4IT 🐧

Wrapping up

I hope you enjoyed this article! Let's keep in touch on Twitter or LinkedIn! 🤜🤛

Happy coding!

🐧