C#•12mo ago

❔ Why can't I reinitialize an object inside a function?

Hi, I created an object in Program.cs file, and passed it as a parameter in a function. If I change the value of a property in the object, it reflects back in Program.cs as well, but if I reinitialize the object, the object reverts to its original values when back in Program.cs. Why is this happening? Program.cs

var person = new Person
{
    Name = "ABC",
    Age = 10
};

Console.WriteLine($"Program.cs : Name: {person.Name}, Age: {person.Age}");

PersonMethods personMethods = new PersonMethods();
personMethods.Evaluate(person);

Console.WriteLine($"Program.cs : Name: {person.Name}, Age: {person.Age}");

var person = new Person
{
    Name = "ABC",
    Age = 10
};

Console.WriteLine($"Program.cs : Name: {person.Name}, Age: {person.Age}");

PersonMethods personMethods = new PersonMethods();
personMethods.Evaluate(person);

Console.WriteLine($"Program.cs : Name: {person.Name}, Age: {person.Age}");

PersonMethods.cs

class PersonMethods
{
    public void Evaluate(Person person)
    {
        person = new Person();
        person.Name = "XYZ";
        person.Age = 0;
        Console.WriteLine($"PersonMethods.cs : Name: {person.Name}, Age: {person.Age}");
    }
}

class PersonMethods
{
    public void Evaluate(Person person)
    {
        person = new Person();
        person.Name = "XYZ";
        person.Age = 0;
        Console.WriteLine($"PersonMethods.cs : Name: {person.Name}, Age: {person.Age}");
    }
}

Output

Program.cs : Name: ABC, Age: 10
PersonMethods.cs : Name: XYZ, Age: 0
Program.cs : Name: ABC, Age: 10

Program.cs : Name: ABC, Age: 10
PersonMethods.cs : Name: XYZ, Age: 0
Program.cs : Name: ABC, Age: 10

8 Replies

cap5lut•12mo ago

its because how passing the parameters happens, this is always by value var person = new Person(): new Person() creates a new object somewhere in the memory and returns a reference to that object which u then store in the variable person public void Evaluate(Person person) is a method that has its own person variable when u call personMethods.Evaluate(person) the following happens: the reference stored in person of the main method (ur Program.cs), is copied to the variable person of Evaluate inside there u only store a new reference of the person inEvaluate the person of ur main method/Program.cs has not been affected by that if u would have the following:

    public void Evaluate(Person person)
    {
        person.Name = "XYZ";
        person.Age = 0;
        Console.WriteLine($"PersonMethods.cs : Name: {person.Name}, Age: {person.Age}");
    }

    public void Evaluate(Person person)
    {
        person.Name = "XYZ";
        person.Age = 0;
        Console.WriteLine($"PersonMethods.cs : Name: {person.Name}, Age: {person.Age}");
    }

(note that i left the person = new Person();) out its a bit different:

Niekon•12mo ago

But objects are passed by reference, right? And didn't I change the reference I passed it to?

cap5lut•12mo ago

u passed a copy of the reference to the person object to the Evaluate method u basically have 2 different person variables that have a reference stored: one in main method/Program.cs and one in Evaluate passed by reference means that not the object itself is passed, but a reference to it, the object here is the Person object BUT, its not the local person variable of the Program.cs

Niekon•12mo ago

Oh! That makes sense

cap5lut•12mo ago

so by doing person = new Person(); in Evaluate, where person is its own variable, u only overwrite that local one with that code u would still work on the same Person object, because the Evaluate's person variable still stores the same reference to an object as the one in Program.cs

Niekon•12mo ago

Okk, I think I understand

cap5lut•12mo ago

basically everything is passed by value, its just that for reference types not the whole object is copied/passed, but a reference to it oh, i totally forget to mention it, there is a way to manipulate the contents of the person variable of the main/Program.cs from within the Evaluate method: refs these are a bit special, they are basically a pointer to the actual variable (with a ref Person person u basically have a reference to the variable that holds a reference to the Person object)

public void Evaluate(ref Person person)
{
  person = new Person();
}

public void Evaluate(ref Person person)
{
  person = new Person();
}

// Program.cs
Person person = ...;
someObject.Evaluate(ref person);

// Program.cs
Person person = ...;
someObject.Evaluate(ref person);

in this case the Program.cs' person will be overwritten by Evaluate's person = new Person(); but note that this has some more details to think (performance impact due to one more indirection - reference of reference of object, can not be used in async contexts, to name two)

Accord•12mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Why can't I reinitialize an object inside a function?