Difference between C# and Java’s ternary operator (? :)

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Looking through the C# 5 Language Specification section 7.14: Conditional Operator we can see the following:

  • If x has type X and y has type Y then

    • If an implicit conversion (§6.1) exists from X to Y, but not from Y to X, then Y is the type of the
      conditional expression.

    • If an implicit conversion (§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

In other words: it tries to find whether or not x and y can be converted to eachother and if not, a compilation error occurs. In our case int and string have no explicit or implicit conversion so it won’t compile.

Contrast this with the Java 7 Language Specification section 15.25: Conditional Operator:

  • If the second and third operands have the same type (which may be the null type), then that is the type of the conditional expression. (NO)
  • If one of the second and third operands is of primitive type T, and the type of the other is the result of applying boxing conversion (§5.1.7) to T, then the type of the conditional expression is T. (NO)
  • If one of the second and third operands is of the null type and the type of the other is a reference type, then the type of the conditional expression is that reference type. (NO)
  • Otherwise, if the second and third operands have types that are convertible (§5.1.8) to numeric types, then there are several cases: (NO)
  • Otherwise, the second and third operands are of types S1 and S2 respectively. Let T1 be the type that results from applying boxing conversion to S1, and let T2 be the type that results from applying boxing conversion to S2.
    The type of the conditional expression is the result of applying capture conversion (§5.1.10) to lub(T1, T2) (§15.12.2.7). (YES)

And, looking at section 15.12.2.7. Inferring Type Arguments Based on Actual Arguments we can see it tries to find a common ancestor that will serve as the type used for the call which lands it with Object. Object is an acceptable argument so the call will work.

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