#1,030 – Requiring Generic Type Parameters to Derive from a Specified Class

By default, when you specify a type parameter in a generic class, the type can be any .NET type.  There are times, however, when you’d like to constrain the type in some ways, allowing only certain types.

You can constrain a type parameter to be of a type that derives from (or is identical to) a particular type, using the where keyword.  In the example below, the PileOf class has a type parameter that must derive from Animal–because we look for a Habitat property.

    public class PileOf<T> where T : Animal
    {
        private List<T> thePile;
        private List<string> habitats;

        public PileOf()
        {
            thePile = new List<T>();
            habitats = new List<string>();
        }

        public void AddThing(T thing)
        {
            thePile.Add(thing);
            habitats.Add(thing.Habitat);
        }
    }

We can construct a PileOf<Dog>, but not a PileOf<int>.

            // Works
            PileOf<Dog> dogPile = new PileOf<Dog>();
            dogPile.AddThing(new Dog("Fido"));

            // Compile-time error: int can't be converted to Animal
            PileOf<int> intPile = new PileOf<int>();
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About Sean
Software developer in the Twin Cities area, passionate about .NET technologies. Equally passionate about my own personal projects related to family history and preservation of family stories and photos.

3 Responses to #1,030 – Requiring Generic Type Parameters to Derive from a Specified Class

  1. ud says:

    what’s the difference between?
    public class PileOf where T : Animal
    public class PileOf
    you can still pileof

    • Sean says:

      If you use: class PileOf where T : Animal
      The type T used to construct the type must derive from animal.
      If you use: class PileOf without the where clause,
      you can use any type for T

  2. Pingback: #1,035 – Summary of Type Parameter Constraints | 2,000 Things You Should Know About C#

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