In c# , when sending a parameter to a method, when should we use "ref" and when "out" and when without any of them?

Question

In c# , when sending a parameter to a method, when should we use "ref" and when "out" and when without any of them?

Answer 1

In general, you should avoid using ref and out, if possible.

That being said, use ref when the method might need to modify the value. Use out when the method always should assign something to the value.

The difference between ref and out, is that when using out, the compiler enforces the rule, that you need to assign something to the out paramter before returning. When using ref, you must assign a value to the variable before using it as a ref parameter.

Obviously, the above applies, when you are writing your own methods. If you need to call methods that was declared with the ref or out modifiers on their parameters, you should use the same modifier before your parameter, when calling the method.

Also remember, that C# passes reference types (classes) by reference (as in, the reference is passed by value). So if you provide some method with a reference type as a parameter, the method can modify the data of the object; even without ref or out. But it cannot modify the reference itself (as in, it cannot modify which object is being referenced).

Answer 2

They are used mainly to obtain multiple return values from a method call. Personally, I tend to not use them. If I want multiple return values from a method then I'll create a small class to hold them.

ref and out are used when you want something back from the method in that parameter. As I recall, they both actually compile down to the same IL, but C# puts in place some extra stuff so you have to be specific.

Here are some examples:

static void Main(string[] args)
{
    string myString;
    MyMethod0(myString);
    Console.WriteLine(myString);

    Console.ReadLine();
}

public static void MyMethod0(string param1)
{
    param1 = "Hello";
}

The above won't compile because myString is never initialised. If myString is initialised to string.Empty then the output of the program will be a empty line because all MyMethod0 does is assign a new string to a local reference to param1.

static void Main(string[] args)
{
    string myString;
    MyMethod1(out myString);
    Console.WriteLine(myString);

    Console.ReadLine();
}


public static void MyMethod1(out string param1)
{
    param1 = "Hello";
}

myString is not initialised in the Main method, yet, the program outputs "Hello". This is because the myString reference in the Main method is being updated from MyMethod1. MyMethod1 does not expect param1 to already contain anything, so it can be left uninitialised. However, the method should be assigning something.

static void Main(string[] args)
{
    string myString;
    MyMethod2(ref myString);
    Console.WriteLine(myString);

    Console.ReadLine();
}

public static void MyMethod2(ref string param1)
{
    param1 = "Hello";
}

This, again, will not compile. This is because ref demands that myString in the Main method is initialised to something first. But, if the Main method is changed so that myString is initialised to string.Empty then the code will compile and the output will be Hello.

So, the difference is out can be used with an uninitialised object, ref must be passed an initialised object. And if you pass an object without either the reference to it cannot be replaced.

Just to be clear: If the object being passed is a reference type already then the method can update the object and the updates are reflected in the calling code, however the reference to the object cannot be changed. So if I write code like this:

static void Main(string[] args)
{
    string myString = "Hello";
    MyMethod0(myString);
    Console.WriteLine(myString);

    Console.ReadLine();
}

public static void MyMethod0(string param1)
{
    param1 = "World";
}

The output from the program will be Hello, and not World because the method only changed its local copy of the reference, not the reference that was passed in.

I hope this makes sense. My general rule of thumb is simply not to use them. I feel it is a throw back to pre-OO days. (But, that's just my opinion)