problem adding object to hashtable

Question

I am trying to call a class method dynamically depending on a condition. This is how I am doing it

I have three classes implement a single interface

interface IReadFile
{
    string DoStuff();
}

The three classes A,B,C implement the interface above.

I am trying to add them to a hashtable with the code below

        _HashT.Add("a", new classA());
        _HashT.Add("b", new classB());
        _HashT.Add("c", new classC());

This compiles fine, but gives a runtime error.{Object reference not set to an instance of an object.}

I was planning to return the correct class to the interface type depending on a parameter that matches the key value. say if I send in a. ClassA is returned to the interface type and the method is called.

        IReadFile Obj = (IReadFile )_HashT["a"].GetType();
        obj.DoStuff();

How do I correct the part above where the objects need to be added to the hashtable? Or do I need to use a different approach? All the classes are in the same assembly and namespace.

Thanks for your time.

Answer 1

As a guess, you have not instantiated your _HashT object.

You need somewhere in your code (declaration or constructor probably) to instantiate it:

HashTable _HashT = new HashTable();

If you do not do this, _HashT will be null and an attempt to add to it will fail with a NullReferenceException as you have been getting.

Answer 2

Following your approach, you do not need to call GetType() on the value you pull out of _HashT. The value should already be an object of type IReadFile.

Answer 3

Why are you calling GetType? The IReadFile object is the thing you are putting in the hash. Casting a Type object into a IReadFile is not going to cast correctly.

Answer 4

It appears you are seeing a NullReferenceException. Based on the limited code you provided I would say it is likely that the _HashT variable is not assigned. It could be possible that the exception is being generated from one of your class constructors as well.

If you use Dictionary<> you can use the following code to add and extract objects from the hashtable.

var hashtable = new Dictionary<IReadFile>();
hashtable.Add("a", new ClassA());
hashtable.Add("b", new ClassB());
hashtable.Add("c", new ClassC());
IReadFile obj = hashtable["a"];
obj.DoStuff();