为什么C＃中的这段代码不会引发错误

Question

Ideally the variable returnString being local variable and not initialised wouldn't have been able to be used and there would have been compile time error:

Use of unassigned local variable 'returnString'

In fact, that's what I got in another context. Still it works. Why?

 using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;

 namespace ConsoleApplication1
 {
     class a {
         public static string testMethod(){
             string returnString;
             try
             {
                 int a = 100;
                 int b = 0;
                 b = a / b;
                 returnString= Convert.ToString(a - b);
                 // return returnString;
             }
             catch (Exception ex)
             {
                  return  returnString = Convert.ToString(ex);
                  //throw ex;
             }
             return returnString;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            // a A = new a();
            Console.WriteLine(a.testMethod());
            Console.ReadKey();
        }
    }
}

Answer 1

Decompiling the code (use release mode) often helps in such situations. Here's the decompiled source generated by dotPeek:

public static string testMethod()
{
  try
  {
    int num1 = 100;
    int num2 = 0;
    int num3 = num1 / num2;
    return Convert.ToString(num1 - num3);
  }
  catch (Exception ex)
  {
    return Convert.ToString((object) ex);
  }
}

As you can see, for the compiler there's no uninitialized variable. So why should it complain?

Answer 2

Because the compiler understands that it will be assigned later.

This does not compile, because it's possible that the variable is not assigned before returning it (if a == 2 for example):

    private string method(int a)
    {
        string result;

        if (a == 1)
        {
            result = "2334234";           
        }
        return result;
    }

Answer 3

Looks to me as if every path through the function returns an initialized variable. Either you successfully convert the division by zero, or as you will have an exception, you assign the variable the value of the exception. You are always going to have a properly assigned value and the compiler sees that.

Answer 4

Your code either sets returnString if:

1. Code doesn't throw an exception before setting the returnString variable inside the try/catch block.

2. If the code in the try block throws an exception, returnString will be assigned.

Conclusion: there's no case where the returnString variable could be unassigned.