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Capoeirista

C# Reference Types

6 posts in this topic

Hi there,

 

I've got a query about C# and returning reference types... if I have a class like :

class TestClass
{
  private int aNumber                 = 3;
  private string aString              = "hello";

  public int Number
  {
     get { return aNumber; }
     set { aNumber = value; }
  }

  public String TheString
  {
    get { return aString; }
  }
}

And I do the following :

TestClass testClass = new TestClass();

String aString = testClass.TheString;
System.Console.WriteLine( "String Value " + aString );
aString = "there";
System.Console.WriteLine( "Class String Value " + testClass.TheString );

The string value stored in the TestClass instance (testClass) isn't modified by :

 

   aString = "there"; 

 

I thought that strings were a reference type... must be missing something here smile.png

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They are a reference type, a reference to the (shared) value - not a reference to the variable.

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They are a reference type, a reference to the (shared) value - not a reference to the variable.

Ah gotcha - so the local variable initially references the data in the class's variable, but the assignment operation changes the local variable's reference to another bit of data...

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In c#, a String is immutable.

 

Regardless, in this case he is assigning a completely new value to a local variable - at this point it has nothing to do with TestClass.aString (i.e. this would be true even if he were using a class other than string - it has nothing to do with the fact that strings are immutable).

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In c#, a String is immutable.

 
Regardless, in this case he is assigning a completely new value to a local variable - at this point it has nothing to do with TestClass.aString (i.e. this would be true even if he were using a class other than string - it has nothing to do with the fact that strings are immutable).

 
True, but the fact that Strings are immutable is important here.
 
Consider the following code:

public class RefClass
{
    public int IntProperty { get; private set; }

    public void DoStuff()
    {
        IntProperty++;
    }
}

public class ContainingClass
{
    private readonly RefClass _ref = new RefClass();

    public RefClass Ref
    {
        get { return _ref; }
    }
}

It's possible to do something like

ContainingClass c = new ContainingClass();
Debug.Assert(c.Ref.IntProperty == 0);
RefClass r = c.Ref;
r.DoStuff();
Debug.Assert(c.Ref.IntProperty == 1);

With a string, that wouldn't be possible because there are no operations that mutate a string.

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You're right, but I was specifically talking about assignment. In the original case, if you substitute string with StringBuilder (and have TestClass put "hello" in it by default), then by reassigning:

 

TestClass testClass = new TestClass();
StringBuilder aString = testClass.TheString;
System.Console.WriteLine( "String Value " + aString ); // "hello"
aString = new StringBuilder("there");
System.Console.WriteLine( "Class String Value " + testClass.TheString ); // Still "hello"

 

 

but, as in your "DoStuff" example:

 


TestClass testClass = new TestClass();
StringBuilder aString = testClass.TheString;
System.Console.WriteLine( "String Value " + aString ); // "hello"
aString.Clear();
aString.Append("there");
System.Console.WriteLine( "Class String Value " + testClass.TheString ); // "there"
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