Question

I met a very odd problem. The following code doesn’t run anticipated.

static IEnumerable<int> YieldFun()
{
    int[] numbers = new int[3] { 1, 2, 3 };

    if(numbers.Count()==3)
        throw new Exception("Test...");

    //This code continues even an exception was thrown above.
    foreach(int i in numbers)
    {
        if(i%2==1)
            yield return numbers[i];
    }
}

static void Main(string[] args)
{
    IEnumerable<int> result = null;
    try
    {
        result = YieldFun();
    }
    catch (System.Exception ex) //Cannot catch the exception
    {
        Console.WriteLine(ex.Message);
    }


    foreach (int i in result)
    {
        Console.Write(" " + i);
    }
}

two questions.
First, YieldFun seems continue to work even an exception has been thrown.
Second, the caller’s try-catch block cannot catch the thrown exception.

Why this? and how to solve this?

Answer 1

This is caused by delayed execution of iterator. Your exception is thrown later than you think: foreach (int i in result) tries to iterate and throws, but you don’t catch exception there.

Body of the function is not executed till one tries to iterate items. So just calling this function does not actually reach “throw …” statement. You can manually iterate result to see at what exact point the exception is thrown.