Question

I have following classes:

abstract class AClass { }
class Foo : AClass { }
class Bar : AClass { }

And when I am trying to use them:

AClass myInstance;
myInstance = true ? new Foo() : new Bar();

This code won’t compiling because of the “Type of conditional expression cannot be determined because there is no implicit conversion between ‘CSharpTest.Class1.Foo’ and ‘CSharpTest.Class1.Bar'”

But following samples compiling ok:

if (true)
{
    myInstance = new Foo();
}
else
{
    myInstance = new Bar();
}

This is ok too:

myInstance = true ? (AClass) new Foo() : new Bar();

or

myInstance = true ? new Foo() : (AClass) new Bar();

Why there is so big difference in behavior of the conditional operator and if clause?

Answer 1

This is the expected behavior.

Since no implicit conversion exists between X and Y (even if they share a common base, there is no implicit conversion between them), you need to explicitly cast (at least) one of them to the base class so that an implicit conversion exists.

A detailed explanation from the C# specification:

The second and third operands of the ?: operator control the type of the conditional expression. Let X and Y be the types of the second and third operands. Then,

If X and Y are the same type, then this is the type of the conditional expression.

Otherwise, if an implicit conversion (Section 6.1) 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 (Section 6.1) 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.