What is the main difference between user define class and String class.

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2.15/5 (4 votes)

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I have one student class which has properties,methods and variables which any class in object orientation concept contains. When I gone through metadata of String class in dot net, It also contains properties,methods,variables. So when we assign value to the String we does this normally :

C#

String s = "Code Project"; // No compile error (1st line)

String test = new String(); // Compile error  (2rd line)

Error in 2nd line:
'string' does not contain a constructor that takes 0 arguments

but as I know every class in dot net should have at least one constructor

Secondly,

If String class contains properties,methods,variables. Then how the 1st line code works.
and the if same thing I have tried to do with my Student class it shows me compile error.

C#

Student s= "Code Project"; // Compile error  (3rd line)

Is there any extra things they did with the predefined classes?
which we cannot do in user define classes.

Can I make a another String class similar to this?

Thanks,
Suraj

Posted 6-Sep-14 6:41am

surajsurve

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Philippe Mori 6-Sep-14 16:52pm

By the way string class does not have a constructor without parameters because a string object is immutable and cannot be modified once created. String functions that modify content returns a new object.

Raul Iloc 7-Sep-14 2:42am

You agreed a solution that is only partially correct! See my comments in the 1st solution!
For details about string class and string literals you can search on MSDN or you can read the next article: http://www.dotnetperls.com/string-literal

2 solutions

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Raul Iloc · Accepted Answer · 2014-09-06T07:20:00

1.Your first line declare a variable of type string and inits it with the "string literal"-the string expression, like a temp constant, given in the second part.

2.The second line return error, because String class does not have an constructor with no parameters.
For details about string class, and its constructors, see details in MSDN : http://msdn.microsoft.com/en-us/library/System.String(v=vs.110).aspx[^]

3.To create your own class that could init your object like in your 3rd example your should implement your implicit operator converter. See details in the next article: Understanding Implicit Operator Overloading in C#[^]

George Jonsson · Accepted Answer · 2014-09-06T07:03:00

Solution 1

If you look at the documentation for the
String Class[^]
you will see that there is no constructor that takes 0 arguments, hence the first compiler error.

The first line works because the literal string "Code Project" is treated as a constant by the compiler and stored on the heap in table.
Then the reference of that constant is assigned to the variable s.
See this link for more information about string storage.
String.Intern Method[^]

To add assignment functionality to your own class, you need to add an implicit operator.

C#

public static implicit operator Student(string s)
{
    Student stud = new Student(s); //Internally call Student constructor
    return stud;
}

= Operator (C# Reference)[^]
implicit (C# Reference)[^]

Of course you need to have a constructor in your Student class that takes a string as an argument.

If you have other data types that you want to assign like this, you just add one operator per type.

Posted 6-Sep-14 7:03am

George Jonsson

Updated 7-Sep-14 0:27am

v4

Comments

surajsurve 6-Sep-14 13:25pm

Wow......!!!! what an answer!! Thank a lot. 5 stars also less for this one!!! Really appreciated.

George Jonsson 6-Sep-14 13:29pm

You are welcome. It was a good question.

Raul Iloc 7-Sep-14 3:10am

There are 3 errors, regarding String class, in the solution above, see details in my comment bellow!

George Jonsson 7-Sep-14 3:19am

I have corrected the faulty part of the explanation.

Thanks7872 6-Sep-14 15:14pm

I have always appreciated descriptive answers. Great explanation. +5.

I request you to add at least one such answer every day! :-)

George Jonsson 6-Sep-14 23:43pm

Thanks Rohan.
I will if I can.

Raul Iloc 7-Sep-14 3:09am

There are 3 errors, regarding String class, in the solution above, see details in my comment bellow!

George Jonsson 7-Sep-14 3:19am

I have corrected the faulty part of the explanation.

Raul Iloc 7-Sep-14 2:39am

Your solution is only partially correct.
You have 3 errors in your solution:
1.The String class does not have any constructor like this: String(string);
2.There is no "implicit operator" in String class.
3.Also there is no assignment operator "=" in String class, see details in MSDN!

So the expression: string s = "Hello World"; create a string literal (constant that will be store in the application) and then assign the reference to this constant to the variable "s" and are not any constructor or operators involved.

George Jonsson 7-Sep-14 3:16am

Thanks for pointing that out. I have revised my answer and stricken out the incorrect explanation.

Raul Iloc 7-Sep-14 5:18am

No problem! Now is better.

surajsurve 7-Sep-14 9:24am

@Raul Iloc thanks a lot!!

Raul Iloc 7-Sep-14 12:04pm

Welcome!