Loading base class data throught parent class

Question

I got a class Fruit that has a lot of variables: mass, taste... etc. I got a class Apple that has a few more variables added to it: size, texture... etc.

I wrote a simple function that loads the Fruit variables, and don't want to copy all of it to fill the Apple variables too.

public void ReadFruitData(string Name, ref Fruit newFruit);

public void ReadAppleData(string Name, ref Apple newApple);

I wish to call ReadFruitData from ReadAppleData, but not quite sure how to do it, since I can't pass newApple as newFruit

class Apple : Fruit

Any ideas how I can achieve that?

why the `ref` and why can`t you pass the apple to `ReadFruitData`? — Random Dev, May 04 '15 at 09:56

score 5 · Answer 1 · answered May 04 '15 at 09:57

Well, in fact you can.

If your Apple class inherits Fruit, you are able to pass in an instance of a deriving type into that method. The actual problem is the use of the ref keyword.

Try to write out ref. Don't use it if you don't really need to, or use the return value to give back a newly created instance.

If you just update values in newApple, you can omit ref and it works:

public void ReadFruitData(string Name, Fruit newFruit)
{ }

public void ReadAppleData(string Name, Apple newApple)
{
    ReadFruitData(Name, newApple);
}

score 4 · Accepted Answer · edited May 23 '17 at 11:59

4

Inside ReadAppleData use a temporary variable to do it:

Fruit TempFruit = (Fruit)newApple;
ReadFruitData(Name, ref TempFruit);
// carry on with the rest of your code

Note: This is a workaround to that pesky compiler complaining that The best overloaded method match for... has some invalid arguments if you try to send the newApple directly.
It will not fit any situation.

As Patrick Hofman mentioned on the comments, if the ReadFruitData assigns a new instance of Fruit to the ref parameter, this is not the solution for you. (and in that case you should probably use out instead of ref).

Here is an example of when not to use this and an example of when you can use it.

edited May 23 '17 at 11:59

Community

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answered May 04 '15 at 09:58

Zohar Peled

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That breaks the `ref`. – Patrick Hofman May 04 '15 at 09:59
It references `TempFruit`, not `newApple`. – Patrick Hofman May 04 '15 at 10:00
doesn't matter, they are both referencing the same object – Zohar Peled May 04 '15 at 10:00
But if the method inside assigns a new instance, the reference is broken! – Patrick Hofman May 04 '15 at 10:01
@PatrickHofman If the method inside assign a new instance it should be out, not ref. in that case, this will obviously not work. – Zohar Peled May 04 '15 at 10:01
@ZoharPeled: Not if it 'updates or creates a new one' depending on the logic. (I do agree `ref` should be omitted) – Patrick Hofman May 04 '15 at 10:02
If `newApple` is of type `Apple` and `Apple` derives from `Fruit` then `Fruit TempFruit = (Fruit)newApple;` does nothing, and afterwards `ReferenceEquals(TempFruit, newApple)` will return true. – Matthew Watson May 04 '15 at 10:04
@PatrickHofman but the OP never wrote 'updates or creates a new one', he wrote 'loads'. I agree that this is not a solution that will fit all scenarios, but I think in this one it fits perfectly, as the OP doesn't need to change the code he is already using. – Zohar Peled May 04 '15 at 10:05
@MatthewWatson: Correct. this is exactly why this solution will work. It's a simple work-around to prevent the compiler from complaining that there is no overload for `ReadFruitData` that accepts `ref Apple`. – Zohar Peled May 04 '15 at 10:07
@PatrickHofman I've edited my answer to include a "disclaimer", I hope we can agree now that it's a good answer. – Zohar Peled May 04 '15 at 10:24
@ZoharPeled: We can. +1 – Patrick Hofman May 04 '15 at 10:28

score 2 · Answer 3 · edited May 23 '17 at 12:15

2

You don't need the ref keyword.

You are using an reference type. If you modify the Fruit or Apple inside those two methods, you modify the original Fruit or Apple.

If you would have a method where you pass in an int or an bool without defining them as ref parameters and you modify the value inside the method, the value of the original variables will not change.

See:

edited May 23 '17 at 12:15

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answered May 04 '15 at 10:55

whymatter

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Still, you can set the reference to the reference type using `ref`. But in general, you are right. +1 – Patrick Hofman May 06 '15 at 07:46

score -3 · Answer 4 · answered May 04 '15 at 09:57

-3

Modify your function definition of ReadFruitData(string Name, ref Fruit newFruit) to ReadFruitData(string Name, ref Object newFruit)

And inside your method, do a cast.

e.g. ReadFruitData(string Name, ref Object newFruit) { Fruit myFruit = newFruit as Fruit; OR Apple myApple = newFruit as Apple; }

Hope this helps. :)

answered May 04 '15 at 09:57

Harshit Gandhi

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no please don't cast when you don't need it - casts are almost as bad as `null` – Random Dev May 04 '15 at 09:58
Your answer is really bad. You shouldn't add dependencies to calling methods just to fulfill your own... (I would suggest to delete it) – Patrick Hofman May 04 '15 at 13:22

Loading base class data throught parent class

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