The conditional operator ?: as used in the form condition ? x : y has a few rules about type conversion.

If both types are the same no problem or if one side supports an implicit conversion to the other but not vice-versa then that is used.

If neither of these cases were true you had to specify the desired type such as by casting the second argument.

C# 9.0 relaxes this rule by considering the "target type" - in the case of a variable assignment the declared type (so var won't work) and in the case of a return statement the return type of the method or function.

Code

Now

// Inferring from declared variable type
IList<int> list = capacity > 0 ? new List<int>(capacity) : Array.Empty();

// Inferring from return type
IList<int> GetList(int capacity) {
    return capacity > 0 ? new List<int>(capacity) : Array.Empty();
}

Before

// Inferring from declared variable type
IList<int> list = capacity > 0 ? new List<int>(capacity) : (IList<int>)Array.Empty();

// Inferring from return type
IList<int> GetList(int capacity) {
    return capacity > 0 ? new List<int>(capacity) : (IList<int>)Array.Empty();
}

More information

• Specification