data fields or local variables whose value cannot be modified?

Question

Hey guys I am brand new to c#. I was reading this book and this fill in the blanks says __ are the data fields or local variables whose values cannot be modifed? Can someone tell me the answer? thanks. ps: total newbie I searched youtube but I was messed up in code. thanks.

`readonly` is also the correct answer. Since the question is stated as "are the data fields **or** local variables". Readonly can't be used for local variables, but can be used for fields, so *or* condition isn't violated. The difference between const and readonly is that you can assign readonly fields in ctor. They are not compile time constants, while const are. — Ilya Ivanov, Sep 23 '13 at 16:19

score 3 · Answer 1 · edited May 23 '17 at 12:11

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The complete answer is twofold:

1) For primitive types (such as int, double, string, etc.) a const modifier can be used, which indeed means that the value of the variable of this type cannot be changed by any means

2) For non-primitive types a readonly modifier exists. Note, that you cannot apply const to non-primitive types. It means the same thing for reference and value types: the reference to this variable cannot be changed after the construction of this object. However, it has different consequences:

2.1) The readonly object itself can be changed by means of it's public API. For example:

class Foo{
  private readonly List<int> list;
  public Foo(){ list = new List<int>();}

  public Test()
  {
    list = new List<int>(); // invalid; your reference is readonly
    list.add(5);//works, you are changing the object, but not touching it's reference
  }
}

2.2) For value types readonly one must be careful while using readonly modifier - it may lead to subtle errors with mutable structs. See C#: Why do mutations on readonly structs not break? for example. However, if your struct is immutable you effectively get the semantics of const keyword

edited May 23 '17 at 12:11

Community

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answered Sep 23 '13 at 16:22

undefined

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`readonly` can be used for non-primitive types. Both keywords work the same way for values and reference types and their semantics doesn't change depending of the type of a variable. – Ilya Ivanov Sep 23 '13 at 16:23
the consequenses for programmer do change. You don't have problems with mutable classes, but you do have them with mutable structs – undefined Sep 23 '13 at 16:25
`You don't have problems with mutable classes` depends on your intention. If I mark reference variable with readonly I may suppose it won't be changed, but if an object is mutable of that type - I might be very wrong. – Ilya Ivanov Sep 23 '13 at 16:27
@IlyaIvanov I am mentioning this in my answer. The thing is that readonly != const by any means, which I tried to emphasise – undefined Sep 23 '13 at 16:30
totally agree. From my side I just wanted to note, that `readonly` behaves exactly the same for value and reference types. As well as `const` behaviour doesn't change for value and reference types. – Ilya Ivanov Sep 23 '13 at 16:32

score 2 · Accepted Answer · answered Sep 23 '13 at 16:14

2

Answer is Const. here is the link Read this

answered Sep 23 '13 at 16:14

designerNProgrammer

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data fields or local variables whose value cannot be modified?

2 Answers2