why assign null value or another default value firstly?

Answer 1

This is not neccessary and should not result in a warning.
Section 10.4.4 of the C# (3.0) spec says:

The initial value of a field, whether it be a static field or an instance field, is the default value (Section 5.2) of the field's type. It is not possible to observe the value of a field before this default initialization has occurred, and a field is thus never "uninitialized". The following categories of variables are automatically initialized to their default values:

Section 5.2 of the C# (3.0) spec says:

The default value of a variable depends on the type of the variable and is determined as follows:

• For a variable of a value-type, the default value is the same as the value computed by the value-type's default constructor (Section 4.1.1).
• For a variable of a reference-type, the default value is null.

So public Hesap Calculator; is initialized automatically to its default value (null).

Answer 2

Your class HesapMak is internal. The compiler can therefore see every use of the field Calculator immediately.

If nowhere in your code you assign to Calculator, the compiler will give you a warning. This is because the field is then unnecessary.

The warning will go away as soon as you start using the field somewhere in your code. So just ignore it for now and write on.