out variable assignment

Thanks Dave,

 

Question answered.

 

regards,

George

That's correct.
out basically allows you to pass a variable, which has been declared but not yet initialised, to a function . In my example, the variable is initialised and populated in the NewPerson function. No need to return a value, as the object is initialised in the memory location of the out variable. So the return value is void.

Thanks Dave,

 

I like your sample, which shows there is no need to fill in values for out variable -- it is the responsibility of the called function to fill the values. Correct? :-)

 

regards,

George

I think you are probably right George. Unlike C++ (which I am currently teaching myself), in C#, we don't have visibility of how the DateTime structure overloads the assignment operator. That being the case, we need to resort to the intermediate language to see what is really going on. So, there is probably a temporary DateTime instance created, which is then memberwise copied to the memory location of abcd.

The little program that I wrote a couple of years back to remember how the out keyword works follows:

class Program
    {
        static void Main(string[] args)
        {
            Person[] pArray = new Person[3];                    

            Person aGuy;
            NewPerson(out aGuy);
            pArray[0] = aGuy;
            NewPerson(out aGuy);
            pArray[1] = aGuy;
            NewPerson(out aGuy);
            pArray[2] = aGuy;

            foreach (Person p in pArray)
                Console.WriteLine(p);
        }

        private static void NewPerson(out Person aGuy)
        {
            int age;
            string name;

            Console.Write("Enter the guy's age: ");
            age = int.Parse(Console.ReadLine());

            Console.Write("Enter the guy's name: ");
            name = Console.ReadLine();

            aGuy = new Person(age, name);         
        }

    class Person
    {
        private int age;
        private string name;

        public Person()
        {        }

        public Person(int age, string name)
        {
            this.age = age;
            this.name = name;
        }

        public int Age
        { get { return age; } set { age = value; } }

        public string Name
        { get { return name; } set { name = value; } }

        public override string ToString()
        {
            return String.Format("Name: {0}\nAge: {1}", age, name);
        }
    }

Perhaps you will find it useful.

Thanks Fernando,

 

After some further study, I do not agree with you. :-)

Here is the IL code, you can see DateTime.Now will create a new instance and put it on evaluation stack, then stobj [mscorlib]System.DateTime statement will create another new instance and load the instance to variable abcd. Any comments? Please feel free to correct me if I am wrong.

.method private hidebysig static void Test2([out] valuetype [mscorlib]System.DateTime& abcd) cil managed
{
    .maxstack 8
    L_0000: nop
    L_0001: ldarg.0
    L_0002: call valuetype [mscorlib]System.DateTime [mscorlib]System.DateTime::get_Now()
    L_0007: stobj [mscorlib]System.DateTime
    L_000c: ret
}

 

regards,

George