C string reference type

Question

I know that  string  in C  is a reference type   This is on MSDN   However  this code doesn t work as it should then   class Test       public static void Main                 string test    before passing           Console WriteLine test           TestI test           Console WriteLine test              public static void TestI string test                test    after passing             The output should be  before passing   after passing  since I m passing the string as a parameter and it being a reference type  the second output statement should recognize that the text changed in the TestI method   However  I get  before passing   before passing  making it seem that it is passed by value not by ref  I understand that strings are immutable  but I don t see how that would explain what is going on here   What am I missing   Thanks

User · Answer

Try    public static void TestI ref string test                test    after passing

User · Answer

Here s a good way to think about the difference between value-types  passing-by-value  reference-types  and passing-by-reference   A variable is a container   A value-type variable contains an instance  A reference-type variable contains a pointer to an instance stored elsewhere   Modifying a value-type variable mutates the instance that it contains  Modifying a reference-type variable mutates the instance that it points to   Separate reference-type variables can point to the same instance  Therefore  the same instance can be mutated via any variable that points to it   A passed-by-value argument is a new container with a new copy of the content  A passed-by-reference argument is the original container with its original content   When a value-type argument is passed-by-value  Reassigning the argument s content has no effect outside scope  because the container is unique  Modifying the argument has no effect outside scope  because the instance is an independent copy   When a reference-type argument is passed-by-value  Reassigning the argument s content has no effect outside scope  because the container is unique  Modifying the argument s content affects the external scope  because the copied pointer points to a shared instance   When any argument is passed-by-reference  Reassigning the argument s content affects the external scope  because the container is shared  Modifying the argument s content affects the external scope  because the content is shared   In conclusion   A string variable is a reference-type variable   Therefore  it contains a pointer to an instance stored elsewhere  When passed-by-value  its pointer is copied  so modifying a string argument should affect the shared instance  However  a string instance has no mutable properties  so a string argument cannot be modified anyway  When passed-by-reference  the pointer s container is shared  so reassignment will still affect the external scope

User · Answer

Actually it would have been the same for any object for that matter i e  being a reference type and passing by reference are 2 different things in c    This would work  but that applies regardless of the type   public static void TestI ref string test    Also about string being a reference type  its also a special one  Its designed to be immutable  so all of its methods won t modify the instance  they return a new one   It also has some extra things in it for performance

User · Answer

If we have to answer the question  String is a reference type and it behaves as a reference  We pass a parameter that holds a reference to  not the actual string  The problem is in the function   public static void TestI string test        test    after passing      The parameter test holds a reference to the string but it is a copy  We have two variables pointing to the string  And because any operations with strings actually create a new object  we make our local copy to point to the new string  But the original test variable is not changed   The suggested solutions to put ref in the function declaration and in the invocation work because we will not pass the value of the test variable but will pass just a reference to it  Thus any changes inside the function will reflect the original variable   I want to repeat at the end  String is a reference type but since its immutable the line test    after passing   actually creates a new object and our copy of the variable test is changed to point to the new string

User · Answer

Above answers are helpful  I d just like to add an example that I think is demonstrating  clearly what happens when we pass parameter without the ref keyword  even when that parameter is a reference type   MyClass c   new MyClass    c MyProperty    foo    CNull c      only a copy of the reference is sent  Console WriteLine c MyProperty      still foo  we only made the copy null CPropertyChange c    Console WriteLine c MyProperty      bar   private void CNull MyClass c2                                  c2   null            private void CPropertyChange MyClass c2                         c2 MyProperty    bar      c2 is a copy  but it refers to the same object that c does  on heap  and modified property would appear on c MyProperty as well

User · Answer

As others have stated  the String type in  NET is immutable and it s reference is passed by value    In the original code  as soon as this line executes   test    after passing     then test is no longer referring to the original object  We ve created a new String object and assigned test to reference that object on the managed heap    I feel that many people get tripped up here since there s no visible formal constructor to remind them  In this case  it s happening behind the scenes since the String type has language support in how it is constructed   Hence  this is why the change to test is not visible outside the scope of the TestI string  method - we ve passed the reference by value and now that value has changed  But if the String reference were passed by reference  then when the reference changed we will  see it outside the scope of the TestI string  method   Either the ref or out keyword are needed in this case  I feel the out keyword might be slightly better suited for this particular situation   class Program       static void Main string   args                string test    before passing           Console WriteLine test           TestI out test           Console WriteLine test           Console ReadLine               public static void TestI out string test                test    after passing

User · Answer

For curious minds and to complete the conversation  Yes  String is a reference type    unsafe        string a    Test        string b   a       fixed  char  p   a                   p 0     B               Console WriteLine a      output   Best       Console WriteLine b      output   Best      But note that this change only works in an unsafe block  because Strings are immutable   From MSDN       The contents of a string object cannot be changed after the object is   created  although the syntax makes it appear as if you can do this    For example  when you write this code  the compiler actually creates a   new string object to hold the new sequence of characters  and that new   object is assigned to b  The string  h  is then eligible for garbage   collection    string b    h     b     ello       And keep in mind that      Although the string is a reference type  the equality operators     and       are defined to compare the values of string objects  not   references

User · Answer

I believe your code is analogous to the following  and you should not have expected the value to have changed for the same reason it wouldn t here    public static void Main           StringWrapper testVariable   new StringWrapper  before passing         Console WriteLine testVariable        TestI testVariable        Console WriteLine testVariable        public static void TestI StringWrapper testParameter          testParameter   new StringWrapper  after passing             this will change the object that testParameter is pointing referring         to but it doesn t change testVariable unless you use a reference         parameter as indicated in other answers

User · Answer

The reference to the string is passed by value  There s a big difference between passing a reference by value and passing an object by reference  It s unfortunate that the word  reference  is used in both cases   If you do pass the string reference by reference  it will work as you expect   using System   class Test       public static void Main                 string test    before passing           Console WriteLine test           TestI ref test           Console WriteLine test              public static void TestI ref string test                test    after passing             Now you need to distinguish between making changes to the object which a reference refers to  and making a change to a variable  such as a parameter  to let it refer to a different object  We can t make changes to a string because strings are immutable  but we can demonstrate it with a StringBuilder instead   using System  using System Text   class Test       public static void Main                 StringBuilder test   new StringBuilder            Console WriteLine test           TestI test           Console WriteLine test              public static void TestI StringBuilder test                   Note that we re not changing the value            of the  test  parameter - we re changing            the data in the object it s referring to         test Append  changing              See my article on parameter passing for more details

User · Answer

A picture is worth a thousand words    I have a simple example here  it s similar to your case   string s1    abc   string s2   s1  s1    def   Console WriteLine s2      Output  abc   This is what happened      Line 1 and 2  s1 and s2 variables reference to the same  abc  string object  Line 3  Because strings are immutable  so the  abc  string object do not modify itself  to  def    but a new  def  string object is created instead  and then s1 references to it  Line 4  s2 still references to  abc  string object  so that s the output

[c#] C# string reference type?

Examples related to c#

Examples related to string

Examples related to reference

Examples related to types