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How to write a copy constructor (C# Programming Guide)

  • Article

C# records provide a copy constructor for objects, but for classes you have to write one yourself.

Important

Writing copy constructors that work for all derived types in a class hierarchy can be difficult. If your class isn't sealed, you should strongly consider creating a hierarchy of record class types to use the compiler-synthesized copy constructor.

Example

In the following example, the Person class defines a copy constructor that takes, as its argument, an instance of Person. The values of the properties of the argument are assigned to the properties of the new instance of Person. The code contains an alternative copy constructor that sends the Name and Age properties of the instance that you want to copy to the instance constructor of the class. The Person class is sealed, so no derived types can be declared that could introduce errors by copying only the base class.

public sealed class Person
{
    // Copy constructor.
    public Person(Person previousPerson)
    {
        Name = previousPerson.Name;
        Age = previousPerson.Age;
    }

    //// Alternate copy constructor calls the instance constructor.
    //public Person(Person previousPerson)
    //    : this(previousPerson.Name, previousPerson.Age)
    //{
    //}

    // Instance constructor.
    public Person(string name, int age)
    {
        Name = name;
        Age = age;
    }

    public int Age { get; set; }

    public string Name { get; set; }

    public string Details()
    {
        return Name + " is " + Age.ToString();
    }
}

class TestPerson
{
    static void Main()
    {
        // Create a Person object by using the instance constructor.
        Person person1 = new Person("George", 40);

        // Create another Person object, copying person1.
        Person person2 = new Person(person1);

        // Change each person's age.
        person1.Age = 39;
        person2.Age = 41;

        // Change person2's name.
        person2.Name = "Charles";

        // Show details to verify that the name and age fields are distinct.
        Console.WriteLine(person1.Details());
        Console.WriteLine(person2.Details());

        // Keep the console window open in debug mode.
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }
}
// Output:
// George is 39
// Charles is 41

See also