# V3019. It is possible that an incorrect variable is compared with null after type conversion using 'as' keyword.

The analyzer has detected a potential error that may lead to memory access by a null reference.

The situation that the analyzer detected deals with the following algorithm. An object of the base class is first cast to a derived class by using the 'as' operator. Then the same object is checked for a null value, though it is the object of the derived class that this check should have been applied to.

Here's an example. In this code, the baseObj object may not be an instance of the Derived class, in which case, when calling the Func function, the program will crash, raising the NullReferenceException. The analyzer will output a warning pointing out two lines. The first line is the spot where the object of the base class is checked for null; the second is where it is cast to an object of the derived class.

    Base baseObj;
    Derived derivedObj = baseObj as Derived;
    if (baseObj != null)
    {
        derivedObj.Func();
    }

It is most likely the object of the derived class that the programmer intended to check for null before using it. This is the fixed version of the code:

    Base baseObj;
    Derived derivedObj = baseObj as Derived;
    if (derivedObj != null)
    {
        derivedObj.Func();
    }

This diagnostic is classified as:

CWE-697

You can look at examples of errors detected by the V3019 diagnostic.