Question

I have a parent type and there are some children types inheriting from it.

I want to make sure there is only one instance of the parent and also for all the children types. Parent type:

private static int _instanceCount = 0;

public ParentClass()
{
    protected ParentClass() // constructor
    {
        _instanceCount++;

    if (_instanceCount > 1)
        throw new exception("Only one instance is allowed.");
    }
}

Sample child class:

private static int _instanceCount = 0;

public ChildClass() : ParentClass
{
    public ChildClass() : base() // constructor
    {
        _instanceCount++;

        if (_instanceCount > 1)
            throw new exception("Only one instance is allowed.");
    }
}

The solution works for children types but when they call the base class constructor I cannot distinguish if the base constructor is called from other types or not so the solution fails.

How can I achieve this?

Answer 1

You should be able to tell if you’re being called from a subclass like this:

if( this.GetType().Equals(typeof(ParentClass)) )
{
    //we know we're not being called by a sub-class.
}

Of course, you could just skip the step of incrementing the count in child classes and only do it in the parent as well…and there are threading issues.