cannot convert from 'out T' to 'out Component'

Question

here is some C# code that does not compile, giving the following message :

cannot convert from 'out T' to 'out Component'

    public void Get<T>(out T c) where T : Component
    {
        m_components.TryGetValue(typeof(T), out c);
    }

Here is code that does compile :

    public void Get<T>(out T c) where T : Component
    {
        Component temp;
        m_components.TryGetValue(typeof(T), out temp);
        c = (T)temp;
    }

I wonder why the first code is not valid since the "where T : Component" explicitly states that T is of type Component.

Thanks

Why would you need to make "Get" a generic method if T is defined to be of type Component? May be you are haven;t mentioned the exact scenario here. — Manish Basantani, Dec 03 '09 at 06:43
*twiddles thumbs waiting for Mr. Skeet to enlighten us on this intriguing issue* — tster, Dec 03 '09 at 06:44
This is a duplicate. http://stackoverflow.com/questions/1207144/c-why-doesnt-ref-and-out-support-polymorphism/1207302#1207302 — Eric Lippert, Dec 03 '09 at 15:52
@Amby : m_components is a Dictionary of Components Dictionary. It may contains any derivated object based on the abstract class Component. The usage is for example : PhysicComponent physicComponent; ModelComponent modelComponent; entity.Get(out physicComponent); entity.Get(out modelComponent); — noon, Dec 04 '09 at 05:14

score 13 · Answer 1 · answered Dec 03 '09 at 06:53

It's because out parameter types cannot be covariant/contravariant. The type of the variable must exactly match the parameter type.

See:

class Super { }
class Sub : Super { }

void Test(out Super s)
{
    s = new Super();
}

void Main()
{
    Sub mySub = new Sub();
    Test(out mySub); //doesn't work
}

score 4 · Accepted Answer · answered Dec 03 '09 at 07:10

I'm going to take a shot at a more elaborate version of @Rex's answer and slightly corrected version of @Courtney's signature in that the type is Component, not object. Courtney's answer is basically right, just the types are a bit off.

bool TryGetValue(Type key, out Component result)
{
       if (this.Contains(key))
       {
           result = this[key]; // the type of result is Component!
           return true;
       }

      return false;
}

by passing T as the object type, you are trying to implicitly cast the base type Component to a subtype T. That is why your second example works. TryGetValue doesn't know about your Generic type T, it thinks everything in m_Components is a Component object.

This is a pretty common thing to get stuck thinking backwards. Because it's a parameter, not the return type, you get sucked into thinking it should work like any other parameter. However, because it's an out, it actually is better thought of as a return type for this purpose... it is going to try to assign the value of it's internal work to the parameter you provide.

score 3 · Answer 3 · answered Dec 03 '09 at 06:43

3

I think the problem is in the out and it does not relate to Generics

we can produce the same error as follows

class A
{
}

void fn(out object x)
{
 x= new object();
}
void main()
{
 A x;
fn(out x); //error
}

answered Dec 03 '09 at 06:43

Ahmed

7,148
12
57
96

score 0 · Answer 4 · answered Dec 03 '09 at 06:54

OK, I've figured it out:

What if you have the following:

 public class BaseClass { ... }
 public class SubClass : BaseClass { ... }

then I have the code:

 Dictionary<int, BaseClass> comps;

 public void Get<T>(int num, out T c) where T : BaseClass
 {
     comps.TryGetValue(num, out c);
 }

And I try to call it as thus:

 SubClass sub;
 Get<SubClass>(1, out sub);

Sub assume that the BaseClass with a key of 1 was actually a BaseClass and not a SubClass. Or Perhaps it is OtherSubClass where OtherSubClass : BaseClass

score -2 · Answer 5 · answered Dec 03 '09 at 06:44

This is interesting.

I don't have an answer, but it's worth noting (to me, anyway), that the following works:

    public void M<T> (out T c) where T : Test
    {
        // Test m;
        // A(out m);
        // c = (T) m;

        A(out c);
    }

    public void A<T> (out T t) where T : Test
    {
        t = null;
    }

-- Edit:

(It's interesting because even if it's out object t it's still not convertible between out T and out object)

cannot convert from 'out T' to 'out Component'

5 Answers5