How many times I read: "In C# reference types are passed by reference, primitive types by value"? The essential point is that reference types has nothing to do with passing by reference!

But let us start with some basics. There are two types of objects, the value types (structs) where the variable is the object and the reference types which only points to the object. When you pass in a variable to a method, its value gets copied to the method by default. For values types, that means the object itself gets copied. For reference types, that means only the thing that points at the object gets copied!

It is a little bit confusing, because only the reference to the object is copied, not the entire structure. It is a way to save performance, otherwise as larger as the reference type would be as more performance it would cost. This strategy is also know as call by sharing (which makes it a less confusing than calling it also call by value with reference types). So in a call by value scenario, if the data of a reference type is changed inside the method, the caller variable is also affected. If you change a the value of a value type, which is passed to a method, it will not affect the caller variable.

public void Sample()
{
    Point point = new Point(20, 30);
    Person person = new Person() { Name = "Meier" };

    CallByValue(point, person); 

    Console.WriteLine("Point X: {0}", point.X);
    Console.WriteLine("Name: {0}", person.Name); 
}

public void CallByValue(Point point, Person person)
{
    point.X = 10;
    person.Name = "Müller"; 
}

The ref keyword indicates in C# a call by reference. When pass a value type by reference to a method, the changes to the value which are done in the method scope, will also affect the variable in the code of the caller. But what happens if you pass reference type to a method by ref?

public void Sample()
{
    Point point = new Point(20, 30);
    Person person = new Person() { Name = "Meier" };

    CallByValue(ref point, ref person); 

    Console.WriteLine("Point X: {0}", point.X);
    Console.WriteLine("Name: {0}", person.Name); 
}

public void CallByValue(ref Point point, ref Person person)
{
    point.X = 10;
    person = new Person() { Name = "Müller" };
}

It allows to change the instance which the variable points to. So you can assign a new object to the variable and also the caller variable will point to the new object. By the way the behaviour in VB.NET is similar to the behaviour of C#, the only difference is that in VB.NET you always have to define if you want to pass a value by value or reference.

.net, c#