Passing Objects By Reference or Value in C

Question

In C   I have always thought that non-primitive variables were passed by reference and primitive values passed by value   So when passing to a method any non-primitive object  anything done to the object in the method would effect the object being passed   C  101 stuff   However  I have noticed that when I pass a System Drawing Image object  that this does not seem to be the case  If I pass a system drawing image object to another method  and load an image onto that object  then let that method go out of scope and go back to the calling method  that image is not loaded on the original object   Why is this

User · Answer

One more code sample to showcase this   void Main           int k   0      TestPlain k       Console WriteLine  TestPlain     k        TestRef ref k       Console WriteLine  TestRef     k        string t    test        TestObjPlain t       Console WriteLine  TestObjPlain    t        TestObjRef ref t       Console WriteLine  TestObjRef     t      public static void TestPlain int i        i   5     public static void TestRef ref int i        i   5     public static void TestObjPlain string s        s    TestObjPlain      public static void TestObjRef ref string s        s    TestObjRef       And the output      TestPlain 0      TestRef 5      TestObjPlain test      TestObjRef TestObjRef

User · Answer

Objects aren t passed at all  By default  the argument is evaluated and its value is passed  by value  as the initial value of the parameter of the method you re calling  Now the important point is that the value is a reference for reference types - a way of getting to an object  or null   Changes to that object will be visible from the caller  However  changing the value of the parameter to refer to a different object will not be visible when you re using pass by value  which is the default for all types   If you want to use pass-by-reference  you must use out or ref  whether the parameter type is a value type or a reference type  In that case  effectively the variable itself is passed by reference  so the parameter uses the same storage location as the argument - and changes to the parameter itself are seen by the caller   So   public void Foo Image image           This change won t be seen by the caller  it s changing the value        of the parameter      image   Image FromStream          public void Foo ref Image image           This change  will  be seen by the caller  it s changing the value        of the parameter  but we re using pass by reference     image   Image FromStream          public void Foo Image image           This change  will  be seen by the caller  it s changing the data        within the object that the parameter value refers to      image RotateFlip           I have an article which goes into a lot more detail in this  Basically   pass by reference  doesn t mean what you think it means

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In Pass By Reference You only add  ref  in the function parameters and one  more thing you should be declaring function  static  because of main is static  public void main String   args       namespace preparation     public  class Program             public static void swap ref int lhs ref int rhs                    int temp   lhs            lhs   rhs            rhs   temp                    static void Main string   args                        int a   10              int b   80     Console WriteLine  a is before sort     a               Console WriteLine  b is before sort     b               swap ref a  ref b               Console WriteLine                  Console WriteLine  a is after sort     a               Console WriteLine  b is after sort     b

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I guess its clearer when you do it like this  I recommend downloading LinqPad to test things like this   void Main         var Person   new Person   FirstName    Egli   LastName    Becerra           Will update egli     WontUpdate Person       Console WriteLine  WontUpdate        Console WriteLine   First name   Person FirstName   Last name   Person LastName  n         UpdateImplicitly Person       Console WriteLine  UpdateImplicitly        Console WriteLine   First name   Person FirstName   Last name   Person LastName  n         UpdateExplicitly ref Person       Console WriteLine  UpdateExplicitly        Console WriteLine   First name   Person FirstName   Last name   Person LastName  n         Class to test public class Person      public string FirstName  get  set       public string LastName  get  set        public string printName            return   First name   FirstName  Last name  LastName             public static void WontUpdate Person p          New instance does jack        var newP   new Person   FirstName   p FirstName  LastName   p LastName       newP FirstName    Favio       newP LastName    Becerra      public static void UpdateImplicitly Person p          Passing by reference implicitly     p FirstName    Favio       p LastName    Becerra      public static void UpdateExplicitly ref Person p          Again passing by reference explicitly  reduntant      p FirstName    Favio       p LastName    Becerra       And that should output  WontUpdate  First name  Egli  Last name  Becerra  UpdateImplicitly  First name  Favio  Last name  Becerra  UpdateExplicitly  First name  Favio  Last name  Becerra

User · Answer

Lots of good answers had been added  I still want to contribute  might be it will clarify slightly more    When you pass an instance as an argument to the method it passes the copy of the instance  Now  if the instance you pass is a value type resides in the stack  you pass the copy of that value  so if you modify it  it won t be reflected in the caller  If the instance is a reference type you pass the copy of the reference again resides in the stack  to the object  So you got two references to the same object  Both of them can modify the object  But if within the method body  you instantiate new object your copy of the reference will no longer refer to the original object  it will refer to the new object you just created  So you will end up having 2 references and 2 objects

User · Answer

When you pass the the System Drawing Image type object to a method you are actually passing a copy of reference to that object   So if inside that method you are loading a new image you are loading using new copied reference  You are not making change in original   YourMethod System Drawing Image image          now this image is a new reference       if you load a new image      image   new Image           you are not changing the original reference you are just changing the copy of original reference

User · Answer

How did you pass object to method   Are you doing new inside that method for object  If so  you have to use ref in method   Following link give you better idea   http   dotnetstep blogspot com 2008 09 passing-reference-type-byval-or-byref html

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