循环内的异常处理。 为什么此代码给出的不是所有代码路径都返回值错误？

Question

public static Boolean CanParseStringToInt(String s)
    {
        Boolean retry = false;
        int n = 0;

        do
        {
            try
            {
                n = int.Parse(s);
                return true;
            }
            catch (Exception ex)
            {
                if (!retry)
                {
                    retry = true;
                }
                else
                {
                    return false;
                }
            }
        }
        while (retry);
    }

I'm targeting .Net 4.0 and using VS2012. The code is just an example.

Answer 1

The compiler can't absolutely tell if a piece of code will eventually return a value. Look at the Halting problem for an example of the difficulty.

If you restructure the code like this, it should behave the same way but will fix the error:

public static Boolean CanParseStringToInt(String s)
{
    Boolean retry = false;
    int n = 0;

    do
    {
        try
        {
            n = int.Parse(s);
            return true;
        }
        catch
        {
            if (!retry)
            {
                retry = true;
            }
            else
            {
                break;
            }
        }
    }
    while (retry);

    return false;
}

Answer 2

The compiler is not smart enough to know that you will eventually return inside the loop.

I would add this line at the end of the method:

throw new InvalidOperationException("Should not have reached here.");

Answer 3

You are not returning anything after your while loop.

Not sure what this method is used for, but I would suggest Int32.TryParse() instead of this implementation.

Answer 4

There is no return statement in the if block inside the catch block. Hence the body of the loop is assumed to execute. This means the while condition also executes and hence the compiler assumes it can return false. This means there must also be a return following the while statement

Answer 5

In your catch the else condition returns but the if does not, it instead does assignment. There's also no return outside of your while.

       catch (Exception ex)
        {
            if (!retry)
            {
                retry = true;
            }
            else
            {
                return false;
            }
        }

In general I prefer to write retry logic like below;

 for (int i = 0; i < retries; i++)
 {
     try
     {
         //do stuff that could throw
     }
     catch (Exception e)
     {
         Thread.Sleep(500);
         if (i == retries - 1)
         {
             throw new Exception(e.Message);
         }
     }
  }

Also, after a second look at your code it seems like you're re writing int.TryParse(string input, out int output)