如何在C＃中测试条件运算符的右关联性

Question

I wrote the following code.

static void Main(string[] args)
{
    var xyz = 10 < 11 ? 12 : 5 < 21 ? getValue() : 15;
    Console.WriteLine(xyz);
    Console.ReadLine();
}

static int getValue()
{
    Console.WriteLine("hello");
    return 100;
}

Since the first condition is always true, the value that xyz will get is 12 but since the conditional operator is itself right associative, I was expecting that the getValue() function would be executed first and "hello" would be printed. When I run this code, it doesn't run that way.

Can someone please enlighten me on this behavior.

Answer 1

This:

var xyz = 10 < 11 ? 12 : 5 < 21 ? getValue() : 15;

is treated as:

var xyz = 10 < 11 ? 12 : (5 < 21 ? getValue() : 15);

So we have a conditional expression with:

• Condition: 10 < 11
• First branch: 12
• Second branch: 5 < 21 ? getValue() : 15 5 < 21 ? getValue() : 15

As 10 < 11 is true, the first branch is evaluated but the second branch is not.

The associativity isn't as easily shown with your code as with code which uses a bool expression for every operand - as that could compile with either associativity.

Consider this code:

using System;

class Test
{
  static void Main()
  {            
    var xyz = Log(1, true) ? Log(2, true) : Log(3, true) ? Log(4, false) : Log(5, true);
  }

  static bool Log(int x, bool result)
  {
    Console.WriteLine(x);
    return result;
  }
}

Renaming each Log(x, ...) to Lx for simplicity, we have:

var xyz = L1 ? L2 : L3 ? L4 : L5;

That could be handled as:

// Right associativity
var xyz = L1 ? L2 : (L3 ? L4 : L5);

// Left associativity
var xyz = (L1 ? L2 : L3) ? L4 : L5;

Given the results we're returning, with right-associativity we'd expect to see output of 1, 2. With left-associativity, we'd see 1, 2, 4. The actual output is 1, 2, showing right-associativity.

Answer 2

If you translate the code into in..else statement, it would be -

        int xyz = 0;
        if (10 < 11)
        {
            xyz = 12;
        }
        else
        {
            if (5 < 21)
            {
                xyz = getValue();
            }
            else
            {
                xyz = 15;
            }
        }

And since the first if condition evaluates to true, the else will not execute and you will get 12 printed on console.