How to keep field readonly?

Question

Let's suppose, that we have the following class:

class C
{
    private readonly DataType data;
    private readonly AdditionalType additional;

    C(DataType newData)
    {
        data = newData;
        // LONG code evaluating additional
    }

    C(OtherType newData)
    {
        // eval data from newData
        // the same LONG code evaluating additional
    }
}

Both data and additional remain unchanged through the whole life of C. However, there is a code smell: the section evaluating additional is doubled in both constructors. The natural choice is then to extract it to another method:

class C
{
    private readonly DataType data;
    private readonly AdditionalType additional;

    private void EvalAdditional()
    {
        // LONG code evaluating additional
    }

    C(DataType newData)
    {
        data = newData;
        EvalAdditional();
    }

    C(OtherType newData)
    {
        // eval data from newData
        EvalAdditional();
    }
}

But then additional no longer can be readonly (beacuse it is not initialized in ctor).

How to solve this problem in an elegant way?

Does the evaluation of `additional` depend on the constructor arguments? If not you can just move the evaluation to another private constructor and call that from your public constructors. — Lee, Jun 22 '13 at 12:33
*...// LONG code...* evaluating anything in a constructor is a code smell in itself. From MSDN: Constructors should not do much work other than capture the constructor parameters. The cost of any other processing should be delayed until required. — inquisitive, Jun 22 '13 at 12:43

score 4 · Accepted Answer · answered Jun 22 '13 at 12:33

Just let the method EvalAdditional return the additional object (instead of void).

readonly DataType data;
readonly AdditionalType additional;

AdditionalType EvalAdditional()
{
    // LONG code evaluating additional
    return additional;
}

C(DataType newData)
{
    data = newData;
    additional = EvalAdditional();
}

C(OtherType newData)
{
    // eval data from newData
    additional = EvalAdditional();
}

score 2 · Answer 2 · answered Jun 22 '13 at 12:35

If the evaluation of additional does not depend on the arguments given to the other constructors, you can just move it to a private constructor and call that from your public constructors:

private C()
{
    //long code evaluating additional
}

C(DataType newData)
    : this()
{
    data = newData;
}

score 0 · Answer 3 · answered Jun 22 '13 at 12:35

0

You could do something like this:

C(DataType newData)
    :this ()
{
    data = newData;
}

C(OtherType newData)
    :this ()
{
    // eval data from newData
}

private C()
{
    EvalAdditional();
}

Although that depends heavily on what EvalAdditional is doing, you could pass parameters if required.

answered Jun 22 '13 at 12:35

Chris

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The question was how to keep the `additional` field `readonly`. – Jeppe Stig Nielsen Jun 22 '13 at 14:13

How to keep field readonly?

3 Answers3