cannot convert from 'out T' to 'out Component'
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;
}
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I wonder why the first code is not valid since the "where T : Component" explicitly states that T is of type Component.
Thanks
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
}
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.
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
}
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
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
)
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