Difference between Foo where T : BaseObject and Foo

Question

What is the difference between

Foo<T> where T : BaseObject

and

Foo<BaseObject>

Isn't this statement the same?

possible duplicate of [C# generic "where constraint" with "any generic type" definition?](http://stackoverflow.com/questions/1541152/c-sharp-generic-where-constraint-with-any-generic-type-definition) — Travis J, May 13 '12 at 18:56

score 7 · Accepted Answer · answered May 13 '12 at 18:55

7

No, it is not the same.

With:

Foo<T> where T : BaseObject

T can be any BaseObject type and its inheritors.

With:

Foo<BaseObject>

T must be BaseObject exactly (assuming no variance modifiers were declared in Foo for the generic type parameter).

answered May 13 '12 at 18:55

Oded

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If BaseObject is an abstract class, then wont `InheritedObject` work for `Foo` if `InheritedObject : BaseObject`? – Travis J May 13 '12 at 18:57
2

@TravisJ - No, not unless covariance was declared on the generic type parameter. – Oded May 13 '12 at 18:59

score 0 · Answer 2 · answered May 13 '12 at 19:19

Consider this:

var list = new List<object>();
list.Add("Hello");
Console.WriteLine(list[0].Length); // doesn't compile

Similarly, with Foo<BaseObject>, consumers of Foo will only have access to the BaseObject members from Foo's T members. With Foo<T> where T : BaseObject, consumers of Foo will have access to all members of whatever derived type is actually passed for the type argument.

Difference between Foo where T : BaseObject and Foo

2 Answers2