Need some explaination with beginner C# code

Question

I have started to learn c#. I am trying to declare a class and some variables and trying to do a simple concatenation of strings. But i am getting some error - the code is below

namespace ConsoleApplication1
{
    class Class1
    {
        string s1 = "hi";
        string s2 = "hi";
        string s3 = s1 + s2;
    }
}

The error i am getting is - a field initializer cannot reference the non-static field, method, property 'ConsoleApplication1.Class1.s1

Can someone explain what is happening here.

Thanks.

Answer 1

Can someone explain what is happening here.

Well, the compiler error message says it all, really, once you've got past the terminology. This line is invalid:

string s3 = s1 + s2;

You're declaring instance variables, and instance variable initializers (s1 + s2 here) aren't allowed to refer to other fields within the instance that's being created - or indeed the instance itself. Bear in mind that the above declaration is equivalent to:

string s3 = this.s1 + this.s2;

From section 10.5.5.2 of the C# 4 specification:

A variable initializer for an instance field cannot reference the instance being created. Thus it is a compile-time error to reference this in a variable initializer, because it is a compile-time error for a variable initializer to reference any instance member through a simple-name.

(Admittedly that's one of the more odder bits of wording in the spec...)

You have to put the logic into the constructor body instead:

class Class1
{
    string s1 = "hi";
    string s2 = "hi";
    string s3;

    public Class1()
    {
        s3 = s1 + s2;
    }
}

Answer 2

Variables are not [logically] initialized in a particular order. It is best to design your programs such that they will work regardless of the order that the variables are initialized.

For non-trivial assignments of local variables you use a constructor for that type:

class Class1
{
    string s1 = "hi";
    string s2 = "hi";
    string s3;

    public Class1()
    {
         s3 = s1 + s2;
    }
}

Answer 3

Try initializing s3 in a method, preferably a constructor

class Class1
{
    string s1 = "hi";
    string s2 = "hi";
    string s3;


    public Class1()
    {
        s3 = s1 + s2;
    }
}