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Constructor should not call a virtual method - Object Oriented Design


This rule matches constructors of a non-sealed class that call one or several virtual methods.

During the construction of a .NET object at runtime, constructors execute sequentially, starting from the base class and progressing to the most derived class.

Objects maintain their type throughout construction; they begin as the most derived type with the method table corresponding to that type. Consequently, virtual method calls always execute on the most derived type, even when called from within the constructor.

Combining these two observations leads to the issue that when you invoke a virtual method within a constructor and the current class is not the most derived type in its inheritance hierarchy, the method will execute on a class whose constructor hasn't been executed yet. This may result in an unsuitable state for the method to be called.

Therefore, this situation renders the class error-prone when deriving from it.

How to Fix:

Issues reported can be solved by re-designing object initialisation or by declaring the parent class as sealed, if possible.

A constructor should primarily focus on initializing the state of its own class. Additional setup can be handled in constructors of derived classes or in dedicated methods called after object construction.