Do not call overridable methods in constructors

Article
08/20/2007

TypeName	DoNotCallOverridableMethodsInConstructors
CheckId	CA2214
Category	Microsoft.Usage
Breaking Change	NonBreaking

Cause

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

Rule Description

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.

How to Fix Violations

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

When to Exclude Warnings

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

Example

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

using System;

namespace UsageLibrary
{
    public class BadlyConstructedType
    {
        protected  string initialized = "No";
        
        public BadlyConstructedType()
        {
            Console.WriteLine("Calling base ctor.");
            // Violates rule: DoNotCallOverridableMethodsInConstructors.
            DoSomething();
        }
        // This will be overridden in the derived type.
        public virtual void DoSomething()
        {
            Console.WriteLine ("Base DoSomething");
        }
    }
    
    public class DerivedType : BadlyConstructedType
    {
        public DerivedType ()
        {
            Console.WriteLine("Calling derived ctor.");
            initialized = "Yes";
        }
        public override void DoSomething()
        {
            Console.WriteLine("Derived DoSomething is called - initialized ? {0}", initialized);
        }
    }
 
    public class TestBadlyConstructedType
    {
        public static void Main()
        {
            DerivedType derivedInstance = new DerivedType();
        }
    }
}

This example produces the following output.

Output

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

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