C# Interview Questions on constructors

What is a constructor in C#?
Constructor is a class method that is executed when an object of a class is created. Constructor has the same name as the class, and usually used to initialize the data members of the new object.

In C#, What will happen if you do not explicitly provide a constructor for a class?
If you do not provide a constructor explicitly for your class, C# will create one by default that instantiates the object and sets all the member variables to their default values.

Structs are not reference types. Can structs have constructors?
Yes, even though Structs are not reference types, structs can have constructors.

We cannot create instances of static classes. Can we have constructors for static classes?
Yes, static classes can also have constructors.

Can you prevent a class from being instantiated?
Yes, a class can be prevented from being instantiated by using a private constructor as shown in the example below.

using System;
namespace TestConsole
{
  class Program
  {
    public static void Main()
    {
      //Error cannot create instance of a class with private constructor
      SampleClass SC = new SampleClass();
    }
  }
  class SampleClass
  {
    double PI = 3.141;
    private SampleClass()
    {
    }
  }
}


Can a class or a struct have multiple constructors?
Yes, a class or a struct can have multiple constructors. Constructors in csharp can be overloaded.

Can a child class call the constructor of a base class?
Yes, a child class can call the constructor of a base class by using the base keyword as shown in the example below.

using System;
namespace TestConsole
{
  class BaseClass
  {
    public BaseClass(string str)
    {
      Console.WriteLine(str);
    }
  }

  class ChildClass : BaseClass
  {
    public ChildClass(string str): base(str)
    {
    }

    public static void Main()
    {
      ChildClass CC = new ChildClass("Calling base class constructor from child class");
    }
  }
}

If a child class instance is created, which class constructor is called first - base class or child class?
When an instance of a child class is created, the base class constructor is called before the child class constructor. An example is shown below.

using System;
namespace TestConsole
{
  class BaseClass
  {
    public BaseClass()
    {
      Console.WriteLine("I am a base class constructor");
    }
  }
  class ChildClass : BaseClass
  {
    public ChildClass()
    {
      Console.WriteLine("I am a child class constructor");
    }
    public static void Main()
    {
      ChildClass CC = new ChildClass();
    }
  }
}

Will the following code compile?
using System;
namespace TestConsole
{
  class BaseClass
  {
    public BaseClass(string str)
    {
      Console.WriteLine(str);
    }
  }
  class ChildClass : BaseClass
  {
    public ChildClass()
    {
      Console.WriteLine("I am a child class constructor");
    }
    public static void Main()
    {
      ChildClass CC = new ChildClass();
    }
  }
}

No, the above code will not compile. This is because, if a base class does not offer a default constructor, the derived class must make an explicit call to a base class constructor by using the base keyword as shown in the example below.

using System;
namespace TestConsole
{
  class BaseClass
  {
    public BaseClass(string str)
    {
      Console.WriteLine(str);
    }
  }
  class ChildClass : BaseClass
  {
    //Call the base class contructor from child class
    public ChildClass() : base("A call to base class constructor")
    {
      Console.WriteLine("I am a child class constructor");
    }
    public static void Main()
    {
      ChildClass CC = new ChildClass();
    }
  }
}

Can a class have static constructor?
Yes, a class can have static constructor. Static constructors are called automatically, immediately before any static fields are accessed, and are generally used to initialize static class members. It is called automatically before the first instance is created or any static members are referenced. Static constructors are called before instance constructors. An example is shown below.

using System;
namespace TestConsole
{
  class Program
  {
    static int I;
    static Program()
    {
      I = 100;
      Console.WriteLine("Static Constructor called");
    }
    public Program()
    {
      Console.WriteLine("Instance Constructor called");
    }
    public static void Main()
    {
      Program P = new Program();
    }
  }
}

Can you mark static constructor with access modifiers?
No, we cannot use access modifiers on static constructor.

Can you have parameters for static constructors?
No, static constructors cannot have parameters.

What happens if a static constructor throws an exception?
If a static constructor throws an exception, the runtime will not invoke it a second time, and the type will remain uninitialized for the lifetime of the application domain in which your program is running.

Give 2 scenarios where static constructors can be used?
1. A typical use of static constructors is when the class is using a log file and the constructor is used to write entries to this file.
2. Static constructors are also useful when creating wrapper classes for unmanaged code, when the constructor can call the LoadLibrary method.