三元运算符中的意外行为

Question

I came across a weird behavior when I changed an if-else to a ternary operator for a return statement.

I've simplified the code here:

class Foo
{
    private bool condition;
    private int intValue = 1;
    private decimal decimalValue = 1M;

    public object TernaryGet
    {
        get
        {
            return condition ? decimalValue : intValue;
        }
    }

    public object IfElseGet
    {
        get
        {
            if (condition)
                return decimalValue;
            return intValue;
        }
    }

    public Foo(bool condition)
    {
        this.condition = condition;
    }
}

class Program
    {
        static void Main(string[] args)
        {
            var fooTrue = new Foo(true);
            var fooFalse = new Foo(false);

            Console.WriteLine("{0}, {1}", fooTrue.TernaryGet.GetType(), fooTrue.IfElseGet.GetType());
            Console.WriteLine("{0}, {1}", fooFalse.TernaryGet.GetType(), fooFalse.IfElseGet.GetType());
        }        
    }

The output from this is:

System.Decimal, System.Decimal
System.Decimal, System.Int32

I'd expect the second row to output Int32 on both getters, but for the ternary I'm getting the incorrect CLR type back for the int.

Never mind the code and what it's trying to do - I'm curious to why this is happening, so if anyone can explain it, I'd appreciate it.

Answer 1

Result of ternary (conditional) operator is always of single type - one/both of the options is casted to common type:

var result = condition ? decimalValue : intValue;

Type of result must be known statically at compile time. Since there is cast from int to decimal than decimal type is selected as type of whole ? : ? : operator.

So you whole function can be written as (showing automatic casts):

public object TurnaryGet
{
    get
    {
        /*decimal*/ var result = condition ? decimalValue : (decimal)intValue;
        return (object)result;
    }
}

Answer 2

condition ? decimalValue : intValue;

means

condition ? decimalValue : (decimal) intValue;

try if this work: (I'm stranger to C#, but this work in Java)

condition ? (object) decimalValue : (object) intValue;