V3149. Dereferencing the result of 'as' operator can lead to NullReferenceException.

The analyzer has detected an unsafe dereference of the value resulting from type conversion using the 'as' operator.

Consider the following contrived example:

void Foo()
{
  BaseItem a = GetItem();
  var b = a as SpecificItem;
  b.Bar();
}

The type of the value returned from the method may be different from the type we want to cast to. In that case, casting between the types using the 'as' operator will result in writing the value null to the variable 'b'. Even though no error will occur at the moment of the cast itself, further use of this variable without a prior null check will lead to raising a 'NullReferenceException'. The fixed code:

void Foo()
{
  BaseItem a = GetItem();
  var b = a as SpecificItem;
  b?.Bar();
}

If you are sure that the variable to which the 'as' operator is applied will always be successfully cast from the runtime type to the specified type, use the explicit cast operator:

void Foo()
{
  BaseItem a = GetItem();
  var b = (SpecificItem)a;
  b.Bar();
}

If the program's behavior changes later and the 'GetItem' method is no longer guaranteed to return a value convertible to the specified type, an invalid cast will raise an 'InvalidCastException', allowing you to quickly identify the problem spot. In contrast, using the 'as' operator will lead to raising a 'NullReferenceException' further in the code when attempting to dereference the variable resulting from an invalid cast and having the value null, and this may not happen until execution gets far from the failed cast, say, in some other method, thus making it difficult to find and fix the bug.

This diagnostic also points out possible typos in type checks:

void Foo()
{
  IDisposable a = GetItem();
  if(a is NonSpecificItem)
  {
    var b = a as SpecificItem;
    b.Bar();
  }
}

In this example, the types SpecificItem and NonSpecificItem are not related, so the cast will return a null pointer. To prevent typos like that from breaking the program, you can implement the check using the Is Type Pattern syntax provided by C# 7.0:

void Foo()
{
  IDisposable a = GetItem();
  if(a is NonSpecificItem item)
  {
    item.Bar();
  }
}

The following snippet is taken from a real open-source project:

....
FuelDefinition = MyDefinitionManager.Static.GetPhysicalItemDefinition(FuelId);
MyDebug.AssertDebug(FuelDefinition != null);
....
String constraintTooltip = FuelDefinition.DisplayNameText;

The 'GetPhysicalItemDefinition' method returns an object of type MyPhysicalItemDefinition retrieved from an array of objects of the basic type 'MyDefinitionBase':

public MyPhysicalItemDefinition GetPhysicalItemDefinition(MyDefinitionId id)
{
  ....
  return m_definitions.m_definitionsById[id] as MyPhysicalItemDefinition;
}

The call of the 'GetPhysicalItemDefinition' method is followed by a null check (MyDebug.AssertDebug) of the value resulting from the cast, which suggests that the method may return an object of an incompatible type. This check, however, will work only in the Debug version. In the Release version, the failed cast will result in null dereference further in the code (FuelDefinition.DisplayNameText).

This diagnostic is classified as:

CWE-476