Reusing of survived local variable in each call. UB?

I'm new in C#. I encountered such code-example:

namespace App1
{
    delegate int Sum(int number);

    class TestAnonymusMethod
    {
        static Sum m()
        {
            int result = 0;  // is not zeroed between calls

            Sum del = delegate (int number)
            {
                for (int i = 0;  i <= number;  i++)
                    result += i;
                return result;
            };
            return del;
        }

        static void Main()
        {
            Sum del1 = m();

            for (int i = 1;  i <= 5;  i++)
                Console.WriteLine("Sum of {0} == {1}", i, del1(i));

            Console.ReadKey();
        }
    }
}

Output is:

Sum of 1 == 1
Sum of 2 == 4
Sum of 3 == 10
Sum of 4 == 20
Sum of 5 == 35

As you see, local variable result is not zeroed between calls. Is it "undefined behaviour"? Looks like it happens because of when scope of result is closed, its life time is undefined.

But what are the rules of reusing alive entities in C#? Is it the rule - "to reuse always", or there are some cases, when new one will be created instead of reusing survived old one?