C# String reference type passed as copy?

Question

I have this doubt related to C# "String" reference types .

The following code:

string s = "lana del rey" 
string d = s;
s = "elvis presley";
Console.Writeline(d);

Why the Output is not "elvis presley" ? If d is pointing to the same memory location of s ?

Could you please explain this?

A more detailed explanation of my initial question:

All your answers were very useful. That question came to me with this common code sample frecuently used to explain difference between value types and reference types:

class Rectangle
{
    public double Length { get; set; }
}

struct Point 
{
    public double X, Y;
}

Point p1 = new Point();
p1.X = 10;
p1.Y = 20;
Point p2 = p1;
p2.X = 100;
Console.WriteLine(“p1.X = {0}”, p1.X);

Rectangle rect1 = new Rectangle
{ Length = 10.0, Width = 20.0 };
Rectangle rect2 = rect1;
rect2.Length = 100.0;
Console.WriteLine(“rect1.Length = {0}”,rect1.Length);

In this case, the second Console.WriteLine statement will output: “rect1.Length = 100”

In this case class is reference type, struct is value type. How can I demostrate the same reference type behaviour using a string ?

Thanks in advance.

Answer 1

It has nothing to do with mutability

string s = "lana del rey" 
string d = s;

Here 2 variables s and d refer to the same object in memory.

s = "elvis presley";

here in the right part of the statement the new object is allocated and initialized with "elvis presley" and assigned to s. So now s refers to another object. And while we haven't change the d reference value - it continues referring to the "lana del rey" as it originally did.

Now the real life analogy:

There are 2 people (A and B) pointing using their fingers to a building far away. They are independent to each other, and don't even see what another is pointing to. Then A decides to start pointing to another building. As long as they aren't connected to each other - now A points to another building, and B continues pointing to the original building (since no one asked them to stop doing that)

PS: what you probably are confusing is the concept behind a pointer and a reference. Not sure if it makes sense to explain it here since you might be confused even more. But now at least you might google for the corresponding keywords.

Answer 2

Strings in C# are immutable; that means they cannot change. When you say s = "elvis presley" you are creating a new string and assigning its reference to s; this does not affect the reference saved to d which still points to the original string.

Answer 3

Strings are immutable. s = "elvis presley" is actually creating a new string and assigning it's reference to the variable s. While the variable d still references the first string "lana del rey".

Answer 4

Let's look at your code line by line

string s = "lana del rey";

With this line, you created a string object lana del rey referenced by s

string d = s;

with this line, you created a reference called d which referenced to same object in memory (which is lana del rey in this case) with s

s = "elvis presley";

With this line, you created a new string object elvis presley and referenced by s (s doesn't reference anymore lana del rey)

Console.Writeline(d);

Since d still reference to lana del rey, it prints lana del rey.

Answer 5

Strings are handled differently than regular reference types, this is essentially what the compiler does:

string s = new String("lana del rey");
string d = new String(s);
s = new String("elvis presley");
Console.Writeline(d);

The point is a "string reference" points to the string VALUE of s. Any time a NEW string is created and referenced that is a new string and any "references" to the original value are still intact.

Answer 6

From Microsoft .NET Framework 2.0 Application Development Foundation 70 536 book:

"Because reference types represent the address of data rather than the data itself, assigning one reference variable to another doesn’t copy the data. Instead, assigning a reference variable to another instance merely creates a second copy of the reference, which refers to the same memory location on the heap as the original variable."

Then I think:

string s = "lana del rey"; // (creates a reference to the memory location X)
string d = s; // (creates a copy of the reference to the same memory location X)
d = "elvis presley"; // (creates a new reference to the new memory location Y)