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Do not call overridable methods in constructors

TypeName

DoNotCallOverridableMethodsInConstructors

CheckId

CA2214

Category

Microsoft.Usage

Breaking Change

Non Breaking

The constructor of an unsealed type calls a virtual method defined in its class.

When a virtual method is called, the actual type that executes the method is not selected until run time. When a constructor calls a virtual method, it is possible that the constructor for the instance that invokes the method has not executed.

To fix a violation of this rule, do not call a type's virtual methods from within the type's constructors.

Do not suppress a warning from this rule. The constructor should be redesigned to eliminate the call to the virtual method.

The following example demonstrates the effect of violating this rule. The test application creates an instance of DerivedType, which causes its base class (BadlyConstructedType) constructor to execute. BadlyConstructedType's constructor incorrectly calls the virtual method DoSomething. As the output shows, DerivedType.DoSomething() executes, and does so before DerivedType's constructor executes.

Imports System

Namespace UsageLibrary

Public Class BadlyConstructedType
    Protected initialized As String = "No" 


    Public Sub New()
        Console.WriteLine("Calling base ctor.")
        ' Violates rule: DoNotCallOverridableMethodsInConstructors.
        DoSomething()
    End Sub 'New 

    ' This will be overridden in the derived type. 
    Public Overridable Sub DoSomething()
        Console.WriteLine("Base DoSomething")
    End Sub 'DoSomething
End Class 'BadlyConstructedType


Public Class DerivedType
    Inherits BadlyConstructedType

    Public Sub New()
        Console.WriteLine("Calling derived ctor.")
        initialized = "Yes" 
    End Sub 'New 

    Public Overrides Sub DoSomething()
        Console.WriteLine("Derived DoSomething is called - initialized ? {0}", initialized)
    End Sub 'DoSomething
End Class 'DerivedType


Public Class TestBadlyConstructedType

    Public Shared Sub Main()
        Dim derivedInstance As New DerivedType()
    End Sub 'Main
End Class  
End Namespace

This example produces the following output.

Calling base ctor. Derived DoSomething is called - initialized ? No Calling derived ctor.

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