NullReferenceException in C# when the container is initialized

Question

In the C# code listed below, I get a "NullReferenceException" with the error:

"Object reference not set to an instance of an object"

I guess the error is related to the inheritance and/or the template definitions. The list gets initialized, and when debugging I can confirm that the list does not point to NULL. I can't figure out how to do this in another way. (Sorry about the confusing class names / structure). The exception happens here: this.localSMT.doSomething(base.list);

public class VTEST<V>
{
    public List<V> list;
    public LocalSMT<V> localSMT;
    public VTEST()
    {
        list = new List<V>();
    }
}
public class VTEST_FSUB<V> : VTEST<V>
{
    public VTEST_FSUB()
    {
        do_virtual();
    }
    public void do_virtual()
    {
        this.localSMT.doSomething(base.list);
    }
}
public class VTEST_RUN : VTEST_FSUB<int>
{
    public VTEST_RUN()
    {
        localSMT = new VTEST_SUB();
    }
}

public class LocalSMT<V>
{
    public LocalSMT() { }
    public virtual void doSomething(List<V> value) { }
}
public class VTEST_SUB : LocalSMT<int>
{
    public VTEST_SUB(){}
    public override void doSomething(List<int> value) { 
            System.Console.WriteLine("VTEST_SUB VIRTUAL");
    }
}
class Program 
{

    Program() {}

    static void Main(string[] args)
    {
        VTEST_RUN run = new VTEST_RUN();
    }
}

Possible duplicate of [What is a NullReferenceException, and how do I fix it?](https://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — ekad, Jan 07 '19 at 12:36

Jon Skeet · Accepted Answer · 2013-10-02T20:14:21.327

The problem is that the VTEST_FSUB<V> constructor body is executing before the VTEST_RUN constructor body. So when do_virtual is called, localSMT is still null. Then do_virtual tries to call a method on localSMT, hence the exception.

Basically the initialization order for any class in the hierarchy is:

Initialize variables which have been declared within an initializer at the point of declaration (any other variables just have the variable type's default value)
Chain up to the base class initialization
Execute the constructor body

See my article on constructor chaining for more details.

Lessons to learn:

Avoid public fields. If you use private fields, it's easy to find every piece of code that reads them and writes to them
Ideally, use readonly fields: if you'd passed the value up the constructor chain and set it in the VTEST<V> constructor, you wouldn't have had a problem. (Admittedly readonly fields can still be a pain because of the next point...)
Avoid virtual method calls in constructors. In this case that wasn't the problem, but you could easily have had the same issue if do_virtual had been abstract in VTEST_FSUB<V> and overridden to call localSMT.doSomething in VTEST_RUN. It would still have executed before the constructor body had run, which would be surprising. Anything you call within a constructor is operating on a partially-initialized object, which is a precarious situation.
Avoid large inheritance hierarchies. They're a pain to work with and reason about.
Follow .NET naming conventions! Your code is partly hard to read because it's so unidiomatic. Even when you're just giving sample code, at least follow the capitalization conventions.

score 3 · Answer 2 · answered Oct 02 '13 at 20:01

3

try:

public void do_virtual()
{
    localSMT=new LocalSMT<V>();
    localSMT.doSomething(list);
}

in public class VTEST_FSUB<V> : VTEST<V>

You are not instatianing localSMT before using, so it's not working.

EDIT: OR

public class VTEST<V>
{
    public List<V> list;
    public LocalSMT<V> localSMT;
    public VTEST()
    {
        list = new List<V>();
        localSMT = new LocalSMT<V>();
    }
}

initialize it in constructor, preferable.

Second solution is cleaner.

answered Oct 02 '13 at 20:01

Kamil Budziewski

22,699
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ok, so I cannot instantiate it like this : localSMT = new VTEST_SUB(); ? – user1511956 Oct 02 '13 at 20:05
2

`new VTEST_SUB()` returns a `LocalSMT`, where `localSMT` is of type `LocalSMT`. That'll move you from a run time error to a compiler error. – Preston Guillot Oct 02 '13 at 20:08

score 1 · Answer 3 · edited May 23 '17 at 10:25

public class VTEST_RUN : VTEST_FSUB<int>
{
   public VTEST_RUN()
   {
    localSMT = new VTEST_SUB();  // BAD!  localSMT isn't initialized yet!
    }
}

I believe that you have failed to new up one of your objects:

public void do_virtual()
{
   localSMT = new LocalSMT<V>();
   localSMT.doSomething(list);
}

Make sure that when you are trying to use an object that you initialize them! And don't worry too much, this is a very common problem in coding.

NullReferenceException in C# when the container is initialized

3 Answers3