The type of the conditional expression can not be determined? [duplicate]

Answer 1

Because of interfaces they can have multiple different common ancestors.

One could add a requirement that it only auto-converts if the ancestor is unambiguous. But then adding additional interfaces a class implements suddenly becomes a breaking change. And that might not be desirable.

For example suppose you make these types implement ISerializeable. This shouldn't change the behavior of your code, but if you supported that casting to common interface it would.

edit: Thought a bit more about it and noticed that this function already has exactly the same problem:

T MyFunc<T>(T left,T right)

And this code doesn't compile:

ICollection<string> r=MyFunc(new List<string>() , new LinkedList<string>());

because it can't decide which type to use as the type-parameter T. So the behavior of the ?: operator is consistent with overload resolution.

Answer 2

It simply is the definition of ?: to require equal types.
You could of course use

? (ICollection<string>) new List<string>() 
: (ICollection<string>) new LinkedList<string>();

Or just use an if/else.

According to the C# reference, § 14.13,

[Given] A conditional expression of the form b ? x : y

• If X and Y are the same type, then this is the type of the conditional expression.
• Otherwise, if an implicit conversion (§13.1) exists from X to Y, but not from Y to X, then Y is the type of the conditional expression.
• Otherwise, if an implicit conversion (§13.1) exists from Y to X, but not from X to Y, then X is the type of the conditional expression.
• Otherwise, no expression type can be determined, and a compile-time error occurs.

In your case, both X and Y have a conversion to Z, but that doesn't help. It is a general principle in the language that the compiler does not even look at the target variable when applying the rules. Simple example: double a = 7 / 2; // a becomes 3.0

So after reading this I thing it would suffice to cast just 1 of the results to ICollection<string>. I didn't test that.

Answer 3

The left-hand side isn't taken into consideration at all when determining the type of the right-hand side.

It's only when the compiler has independently determined the right-hand side's type that it checks for assignment compatibility with the left-hand side.

As for your claim that both types "have one common ancestor": would that be ICollection, IEnumerable, ICollection<T>, IEnumerable<T> or Object? What heuristic should the compiler use to unambiguously determine which type you want? The compiler is simply asking you to specify, rather than trying to guess your intention.

Answer 4

The expression (a ? b : c) has to resolve to a type. The type will be either the type of b or c. If these are different (and there isn't an implicit conversion from one to the other) the compiler doesn't know which type this is at compile time.

You may say it should deduce that there is common root type, but there is always a common root type (e.g. Object).

In general the C# compiler will not attempt to guess what you mean. If you want to use the common root type then cast b and c to that type.

This kind of logic runs throughout the design of C#, it is occasionally a bit annoying, but far more often it stops you making mistakes.