在抽象基类中使用具体实现的类型

Question

Is there any way to do what I want here?

The base class is defined as follows:

public abstract class BaseClass<TExists>
    where TExists : BaseExists
{
    // needs to be overridden by child
    protected abstract bool Exists(TExists existsData, out /*typeof(this)*/ existingElement); // <- how to have the concrete type here?

    // static method to be invoked without any need of an instance
    public static bool Exists(TExists existsData, out /*typeof(this)*/ existingElement)
    {
        var temp; // <-- how to set the type here?

        // create a concrete instance
        var instance = Activator.CreateInstance(???);

        // call the concrete implementation
        if(instance.Exists(existsData, out temp))
        {
            return true;
        }

        return false;
    }
}

And here we have some concrete implementation:

public class ChildClass : BaseClass<ChildClassExists>
{
    protected override bool Exists(ChildClassExists exists, out ChildClass existingElement)
    {
        // do child-related things here
    }
}

At the end I want to use it like

ChildClass existing;    
if(ChildClass.Exists(new ChildClassExists(), out existing)){
    // do things here with the existing element of type 'ChildClass'
}

because I don't need an instance here (this is hidden inside the base class implementation of Exists).

Update #1

As implemented like in InBetweens first answer I now have:

public static bool Exists<T>(TExists existsModel, out T existingEntityModel)
    where T : BaseClass<TExists>
{
    var instance = Activator.CreateInstance<T>();
    return instance.ExistsInternal(existsModel, out existingEntityModel);
}

protected abstract bool ExistsInternal<T>(TExists createModel, out T existingEntityModel)
    where T : BaseClass<TExists>;

But this will throw an error inside a concrete implementation of the ExistsInternal method

Cannot convert source type 'ChildClass' to target type 'T'

in override

protected override bool ExistsInternal<T>(ChildClassExists existsData, out T existingElement)
{
    existingElement = new ChildClass(); // <-- here the error is thrown
    return true;
}

Answer 1

Just add a new generic parameter in the Exists method. This type will be inferred by the compiler so there is no real impact in usability:

public abstract class BaseClass<TExists> where TExists : BaseExists
{
    // needs to be overridden by child
    protected abstract bool InternalExistsCheck<T>(TExists existsData, out T existingElement) where T : BaseClass<TExists>, new();

    // static method to be invoked without any need of an instance
    public static bool Exists<T>(TExists existsData, out T existingElement) where T : BaseClass<TExists>, new()
    {
        T temp; // <-- how to set the type here?
        existingElement = null;

        // create a concrete instance
        var instance = new T();

        // call the concrete implementation
        if (instance.InternalExistsCheck(existsData, out temp))
        {
            existingElement = temp;
            return true;
        }

        return false;
    }
}

Note that if you don't change the protected Exists method, you'll get an ambigous call compile time error (VS 2013).

Now, its perfectly fine to do:

public class ChildClass : BaseClass<ChildClassExists>
{
    protected override bool InternalExistsCheck<T>(ChildClassExists exists, out T existingElement)
    {
        ....
    }
}

and

ChildClass existing;

if (ChildClass.Exists(new ChildClassExists(), out existing))
{
     // do things here with the existing element of type 'ChildClass'
}

UPDATE

Adressing your concern about not being able to assign a ChildInstance to existing in the overriden ChildClass.InternalExistsCheck(,) , yes you can by simply doing:

existing = new T();

If T is ChildClass (inferred by the compiler) then you will be creating a ChildClass instance. Bear in mind though that you are getting a BaseClass<ChildExists> typed reference, not a ChildClass one.

If you absolutely need a ChildClass typed reference then there is a workaround (if you need to do this, its probably because generics is not the right tool for you):

var childClassTypedReference = (object)existing as ChildClass.

Do realize that the whole solution is not as type safe as you'd maybe wish; you must consider the possibility of existing not being a ChildClass typed reference (and therefore childClassTypedReference being null ). There is nothing preventing existing from being any type extending BaseClass<ChildExists> .

I don't have the whole picture of your code but I really think you are misusing generics here. I think a non generic approach with an IExists interface dependency would be a much cleaner approach.

Answer 2

Inspired by the comments/answers I came up with the following solution which perfectly fits my needs.

1 Added a new (non-generic) base-class BaseClass

2 Let generic base inherit from non-generic

public abstract class BaseClass<TExists> : BaseClass
{
}

3 Setup abstract and static methods in BaseClass<TExists>

// abstract to be overriden by child-classes
protected abstract BaseClass ExistsInternal(TExists existsModel);

public static bool Exists<T>(TExists existsModel, out T existing) where T : BaseClass<TExists>
{
    // create a new instance of the concrete child
    var instance = (T) Activator.CreateInstance<T>;

    // set the out parameter (can be null)
    existing = instance.ExistsInternal(existsModel) as T;

    // return if already existing
    return existing!= null;
}

4 Implement override in child-classes

protected override BaseClass ExistsInternal(ChildClassExists existsModel)
{
    // check for existing
    var existing = ...; // do child-class dependend stuff

    // return concrete instance or null
    return existing != null ? new ChildClass(existing) : null;
}

5 Call it (with types inferred from usage through ChildClass )

ChildClass childClass;
if (ChildClass.Exists(childClassExists, out childClass))
{
    // do things with existing childClass
}

6 Say thanks to InBetween and haim7770

@InBetween, @haim7770 -> Thank you! ;)