Two C# idioms: On defaults and switch

This post is part of the series "C# idioms"

1. Two C# idioms
2. Another two C# idioms
3. Two C# idioms: On Dictionaries
4. Two C# idioms: On defaults and switch This post

In this part of the C# idioms series, we have one idiom to write more intention-revealing defaults and another idiom to convert mapping code using a switch to a more compact alternative using a dictionary.

Use intention-revealing defaults

When initializing variables to default values, use intention-revealing alternatives.

Are you initializing a string variable to later assign it? Use "". Do you want to return an empty string from a method? Use string.Empty.

The same is true for collections. If you’re initializing a collection to later add some elements, use the normal constructors like new string[length]; or new List<string>();.

But, if you want to return an empty collection. Use Array.Empty<string>() or Enumerable.Empty<string>().

Replace switch with a dictionary

Replace switch mapping two types with a dictionary.

Turn every value in the case statements into a key in the dictionary. And, turn the returned value in every case into the value of the matching key in the dictionary.

To replace the default case, take advantage of the TryGetValue() or GetValueOrDefault() methods.

Before, to map from a credit card brand name in strings to a CardType enum, we did this,

public static CardType MapToCardType(string cardBrand)
{
    CardType cardType;

    switch (cardBrand)
    {
        case "Visa":
            cardType = CardType.Visa;
            break;

        case "Mastercard":
            cardType = CardType.MasterCard;
            break;

        case "American Express":
            cardType = CardType.AmericanExpress;
            break;

        default:
            cardType = CardType.Unknown;
            break;
    }
    return cardType;
}

After, replacing the switch with a Dictionary,

public static CardType MapToCardType(string cardBrand)
{
    var cardTypeMappings = new Dictionary<string, CardType>
    {
        { "Visa", CardType.Visa },
        { "Mastercard", CardType.Mastercard },
        { "American Express", CardType.AmericanExpress }
    };
    return cardTypeMappings.TryGetValue(cardBrand, out var cardType)
                ? cardType
                : CardType.Unknown;
}

C# 8.0 and Dictionaries

Also, we can use the newer switch syntaxt from C# version 8.0 to write more compact switch.

Starting from C# version 8.0, switch are expressions. It means we can assign switch to variables and use switch as returned values.

Notice how we use a discard _ in the default case to throw an exception.

public static CardType MapToCardType(string cardBrand)
{
    return cardBrand switch
    {
      "Visa" => CardType.Visa,
      "MasterCard" => CardType.MasterCard,
      "American Express" => CardType.AmericanExpress,
      _ => CardType.Unknown
    };
}

Voilà! These are the C# idioms for today. Remember to use intention-revealing defaults and take advantage of the new C# features for switch.

If you want to check more C# recent features, check my Top 16 newest C# features. To get rid of exceptions when working with dictionaries, check Idioms on Dictionaries.

And, don’t miss the other C# Idioms.

Happy coding!