CODEWITHSUNDEEP
c#
user978605
user978605

Reputation: 53

C# base class as parameter type

I have two classes as follows, and Class A inherits from Class B.

public class A
{
     public string Title { get; set; }
}

public class B: A
{

}

Then I have a function as follows which takes in a list of items of class A:

public static void Get(List<A> values)
{
    // Do something
}

Now when I try to pass a list of items of Class B, into the function as follows,

private function void Test()
{
     var k = new List<B>();
     Get(k);
}

I get the following error message

cannot convert from 'System.Collections.Generic.List<B>' to 'System.Collections.Generic.List<A>'

Should this not work as B just inherits from A and will have the properties that A has?

I have already spent hours trying to look this up, but I have not been to figure out what I an doing wrong. How can I fix it?

Upvotes: 0

Views: 1408

Answers (3)

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726479

Although B can be used everywhere an A can be, the List<B> and List<A> are not covariant. However, starting with .NET 4.0 you can use a list of Derived class in place of IEnumerable<Base>. So if you declare your function to take IEnumerable<A> rather than List<A>, your code will compile.

Upvotes: 1

Mehrdad Afshari
Mehrdad Afshari

Reputation: 421978

No, it should not. It is potentially unsafe:

class C : A {

}

public static void Get(List<A> values)
{
    values.Add(new C());
}

If you were to call Get with new List<B>(), it would be unsafe, as you'd be inserting instances of C in List<B>.

It would be safe, however, if List<T> was a "read-only" interface, in which case covariance, which is what you are looking for, would be safe. You can change List<A> to IEnumerable<A> instead and it'll work.

Upvotes: 5

Charles Bretana
Charles Bretana

Reputation: 146429

B does inherit from A, but that doesn't mean that List<B> Inherits from List<A> You should just put the B objects into a List<A> and pass that to the method. This is what polymorphism is all about. 

  private function void Test()
  {
      var k = new List<A>();
      k.Add(new B());
      k.Add(new B());
      k.Add(new B());
      k.Add(new B());
      k.Add(new B());
      k.Add(new B());
      Get(k);
  }

Upvotes: 0

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