Constant class variable in c# [duplicate]

Question

I want to declare a variable in my class, that cannot be changed later like this:

obj myobj=new obj()
myobj.CONSTANT_VAR="Changed value" //ERROR!!

...but whose value can be accessed like:

Console.WriteLine(myobj.CONSTANT_VAR)

I tried the following:

public class obj{
    public int a, b;
    public const string CONSTANT_VAR;
    public obj(int x,int y){
        a=x;
        b=y;
        CONSTANT_VAR=1/(a*((""+a).Length)+3/(b*((""+b).Length)).ToString();
    }
    public int do(){
        return this.a+this.b-(CONSTANT_VAR).Length;
    }
}
class DriverClass(){
    static void Main(){
        obj myObj=new obj(2,3);
        myObj.a=34;
        myObj.b=35;
        myObj.CONSTANT_VAR="changed ur string lol"; //i want it to print error
      
        Console.WriteLine(CONSTANT_VAR); //no error
        Console.WriteLine(myObj.add());
    }
}

But i instead get the following error message:

constants must have a value assigned

But i dont want to assign it a value beforehand..... What do i do?

Answer 1

You're looking for read-only fields or properties, not const which is for genuine global constants.

I'd recommend avoiding public fields entirely, and instead using properties - so in this case you'd want a get-only property. Following .NET naming conventions, you'd have something like:

public class Obj
{
    public int A { get; set; }
    public int B { get; set; }

    public string ConstantVar { get; }

    public Obj(int x, int y)
    {
        A = x;
        B = y;
        ConstantVar = /* complex expression */
    }

    public int Do() => A + B - ConstantVar.Length;
}

Answer 2

You can use Readonly, it gives you option to set the value once and can not be changed later.

public class obj(){
    public int a, b;
    public readonly string CONSTANT_VAR;
    public obj(int x,int y){
        a=x;
        b=y;
        CONSTANT_VAR=1/(a*((""+a).Length)+3/(b*((""+b).Length)).ToString();
    }
    public int do(){
        return this.a+this.b-(CONSTANT_VAR).Length;
    }
}
class DriverClass(){
    static void Main(){
        obj myObj=new obj(2,3);
        myObj.a=34;
        myObj.b=35;
        myObj.CONSTANT_VAR="changed ur string lol"; //i want it to print error
      
        Console.WriteLine(CONSTANT_VAR); //no error
        Console.WriteLine(myObj.add());
    }
}