C# Language

Constructors and Finalizers

Introduction#

Constructors are methods in a class that are invoked when an instance of that class is created. Their main responsibility is to leave the new object in a useful and consistent state.

Destructors/Finalizers are methods in a class that are invoked when an instance of that is destroyed. In C# they are rarely explicitely written/used.

Remarks#

C# does not actually have destructors, but rather Finalizers which use C++ style destructor syntax. Specifying a destructor overrides the Object.Finalize() method which cannot be called directly.

Unlike other languages with similar syntax, these methods are not called when objects go out of scope, but are called when the Garbage Collector runs, which occurs under certain conditions. As such, they are not guaranteed to run in any particular order.

Finalizers should be responsible for cleaning up unmanaged resources only (pointers acquired via the Marshal class, received through p/Invoke (system calls) or raw pointers used within unsafe blocks). To clean up managed resources, please review IDisposable, the Dispose pattern and the using statement.

(Further reading: When should I create a destructor?)

Default Constructor

When a type is defined without a constructor:

public class Animal
{
}

then the compiler generates a default constructor equivalent to the following:

public class Animal
{
    public Animal() {}
}

The definition of any constructor for the type will suppress the default constructor generation. If the type were defined as follows:

public class Animal
{
    public Animal(string name) {}
}

then an Animal could only be created by calling the declared constructor.

// This is valid
var myAnimal = new Animal("Fluffy");
// This fails to compile
var unnamedAnimal = new Animal();

For the second example, the compiler will display an error message:

‘Animal’ does not contain a constructor that takes 0 arguments

If you want a class to have both a parameterless constructor and a constructor that takes a parameter, you can do it by explicitly implementing both constructors.

public class Animal
{
    
    public Animal() {} //Equivalent to a default constructor.
    public Animal(string name) {}
}

The compiler will not be able to generate a default constructor if the class extends another class which doesn’t have a parameterless constructor. For example, if we had a class Creature:

public class Creature
{
    public Creature(Genus genus) {}
}

then Animal defined as class Animal : Creature {} would not compile.

Calling a constructor from another constructor

public class Animal
{
    public string Name { get; set; }

    public Animal() : this("Dog")
    {
    }

    public Animal(string name)
    {
        Name = name;
    }
}

var dog = new Animal();      // dog.Name will be set to "Dog" by default.
var cat = new Animal("Cat"); // cat.Name is "Cat", the empty constructor is not called.

Static constructor

A static constructor is called the first time any member of a type is initialized, a static class member is called or a static method. The static constructor is thread safe. A static constructor is commonly used to:

  • Initialize static state, that is state which is shared across different instances of the same class.
  • Create a singleton

Example:

class Animal
{
    // * A static constructor is executed only once,
    //   when a class is first accessed.
    // * A static constructor cannot have any access modifiers
    // * A static constructor cannot have any parameters
    static Animal()
    {
        Console.WriteLine("Animal initialized");
    }

    // Instance constructor, this is executed every time the class is created
    public Animal()
    {
        Console.WriteLine("Animal created");
    }

    public static void Yawn()
    {
        Console.WriteLine("Yawn!");
    }
}

var turtle = new Animal();
var giraffe = new Animal();

Output:

Animal initialized
Animal created
Animal created

View Demo

If the first call is to a static method, the static constructor is invoked without the instance constructor. This is OK, because the static method can’t access instance state anyways.

Animal.Yawn();

This will output:

Animal initialized
Yawn!

See also Exceptions in static constructors and Generic Static Constructors .

Singleton example:

public class SessionManager
{
    public static SessionManager Instance;

    static SessionManager()
    {
        Instance = new SessionManager();
    }
}

Calling the base class constructor

A constructor of a base class is called before a constructor of a derived class is executed. For example, if Mammal extends Animal, then the code contained in the constructor of Animal is called first when creating an instance of a Mammal.

If a derived class doesn’t explicitly specify which constructor of the base class should be called, the compiler assumes the parameterless constructor.

public class Animal
{
    public Animal() { Console.WriteLine("An unknown animal gets born."); }
    public Animal(string name) { Console.WriteLine(name + " gets born"); }
}

public class Mammal : Animal
{
    public Mammal(string name)
    {
        Console.WriteLine(name + " is a mammal.");
    }
}

In this case, instantiating a Mammal by calling new Mammal("George the Cat") will print

An unknown animal gets born.
George the Cat is a mammal.

View Demo

Calling a different constructor of the base class is done by placing : base(args) between the constructor’s signature and its body:

public class Mammal : Animal
{
    public Mammal(string name) : base(name)
    {
        Console.WriteLine(name + " is a mammal.");
    }
}

Calling new Mammal("George the Cat") will now print:

George the Cat gets born.
George the Cat is a mammal.

View Demo

Finalizers on derived classes

When an object graph is finalized, the order is the reverse of the construction. E.g. the super-type is finalized before the base-type as the following code demonstrates:

class TheBaseClass
{
    ~TheBaseClass() 
    {
        Console.WriteLine("Base class finalized!");
    }
}

class TheDerivedClass : TheBaseClass
{
    ~TheDerivedClass() 
    {
        Console.WriteLine("Derived class finalized!");
    }
}

//Don't assign to a variable
//to make the object unreachable
new TheDerivedClass();

//Just to make the example work;
//this is otherwise NOT recommended!
GC.Collect();

//Derived class finalized!
//Base class finalized!

Singleton constructor pattern

public class SingletonClass
{
    public static SingletonClass Instance { get; } = new SingletonClass();

    private SingletonClass()
    {
        // Put custom constructor code here
    }    
}

Because the constructor is private, no new instances of SingletonClass can be made by consuming code. The only way to access the single instance of SingletonClass is by using the static property SingletonClass.Instance.

The Instance property is assigned by a static constructor that the C# compiler generates. The .NET runtime guarantees that the static constructor is run at most once and is run before Instance is first read. Therefore, all synchronization and initialization concerns are carried out by the runtime.

Note, that if the static constructor fails the Singleton class becomes permanently unusable for the life of the AppDomain.

Also, the static constructor is not guaranteed to run at the time of the first access of Instance. Rather, it will run at some point before that. This makes the time at which initialization happens non-deterministic. In practical cases the JIT often calls the static constructor during compilation (not execution) of a method referencing Instance. This is a performance optimization.

See the Singleton Implementations page for other ways to implement the singleton pattern.

Forcing a static constructor to be called

While static constructors are always called before the first usage of a type it’s sometimes useful to be able to force them to be called and the RuntimeHelpers class provide an helper for it:

using System.Runtime.CompilerServices;    
// ...
RuntimeHelpers.RunClassConstructor(typeof(Foo).TypeHandle);

Remark: All static initialization (fields initializers for example) will run, not only the constructor itself.

Potential usages: Forcing initialization during the splash screen in an UI application or ensuring that a static constructor doesn’t fail in an unit test.

Calling virtual methods in constructor

Unlike C++ in C# you can call a virtual method from class constructor (OK, you can also in C++ but behavior at first is surprising). For example:

abstract class Base
{
    protected Base()
    {
        _obj = CreateAnother();
    }

    protected virtual AnotherBase CreateAnother()
    {
        return new AnotherBase();
    }

    private readonly AnotherBase _obj;
}

sealed class Derived : Base
{
    public Derived() { }

    protected override AnotherBase CreateAnother()
    {
        return new AnotherDerived();
    }
}

var test = new Derived();
// test._obj is AnotherDerived

If you come from a C++ background this is surprising, base class constructor already sees derived class virtual method table!

Be careful: derived class may not been fully initialized yet (its constructor will be executed after base class constructor) and this technique is dangerous (there is also a StyleCop warning for this). Usually this is regarded as bad practice.

Generic Static Constructors

If the type on which the static constructor is declared is generic, the static constructor will be called once for each unique combination of generic arguments.

class Animal<T>
{
    static Animal()
    {
        Console.WriteLine(typeof(T).FullName);
    }

    public static void Yawn() { }
}

Animal<Object>.Yawn();
Animal<String>.Yawn();

This will output:

System.Object
System.String

See also How do static constructors for generic types work ?

Exceptions in static constructors

If a static constructor throws an exception, it is never retried. The type is unusable for the lifetime of the AppDomain. Any further usages of the type will raise a TypeInitializationException wrapped around the original exception.

public class Animal
{
    static Animal()
    {
        Console.WriteLine("Static ctor");
        throw new Exception();
    }

    public static void Yawn() {}
}

try
{
    Animal.Yawn();
}
catch (Exception e)
{
    Console.WriteLine(e.ToString());
}

try
{
    Animal.Yawn();
}
catch (Exception e)
{
    Console.WriteLine(e.ToString());
}

This will output:

Static ctor

System.TypeInitializationException: The type initializer for ‘Animal’ threw an exception. ---> System.Exception: Exception of type ‘System.Exception’ was thrown.

[…]

System.TypeInitializationException: The type initializer for ‘Animal’ threw an exception. ---> System.Exception: Exception of type ‘System.Exception’ was thrown.

where you can see that the actual constructor is only executed once, and the exception is re-used.

Constructor and Property Initialization

Shall the property value’s assignment be executed before or after the class’ constructor?

public class TestClass 
{
    public int TestProperty { get; set; } = 2;
    
    public TestClass() 
    {
        if (TestProperty == 1) 
        {
            Console.WriteLine("Shall this be executed?");
        }

        if (TestProperty == 2) 
        {
            Console.WriteLine("Or shall this be executed");
        }
    }
}

var testInstance = new TestClass() { TestProperty = 1 };

In the example above, shall the TestProperty value be 1 in the class’ constructor or after the class constructor?

Assigning property values in the instance creation like this:

var testInstance = new TestClass() {TestProperty = 1};

Will be executed after the constructor is run. However, initializing the property value in the class’ property in C# 6.0 like this:

public class TestClass 
{
    public int TestProperty { get; set; } = 2;

    public TestClass() 
    {
    }
}

will be done before the constructor is run.

Combining the two concepts above in a single example:

public class TestClass 
{
    public int TestProperty { get; set; } = 2;
    
    public TestClass() 
    {
        if (TestProperty == 1) 
        {
            Console.WriteLine("Shall this be executed?");
        }

        if (TestProperty == 2) 
        {
            Console.WriteLine("Or shall this be executed");
        }
    }
}

static void Main(string[] args) 
{
    var testInstance = new TestClass() { TestProperty = 1 };
    Console.WriteLine(testInstance.TestProperty); //resulting in 1
}

Final result:

"Or shall this be executed"
"1"

Explanation:

The TestProperty value will first be assigned as 2, then the TestClass constructor will be run, resulting in printing of

"Or shall this be executed"

And then the TestProperty will be assigned as 1 due to new TestClass() { TestProperty = 1 }, making the final value for the TestProperty printed by Console.WriteLine(testInstance.TestProperty) to be

"1"
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