Object #

object (an alias for System.Object) is the base class for all types. Every type can be upcast to this type. object itself is a reference type - however, value types also inherit from it, thanks to the unified type system. This is not a perfect solution, though.

Boxing #

Casting a value type to/from an object involves a so-called boxing/unboxing operation. Downcasting requires an explicit cast; if it fails, the Runtime throws an InvalidCastException. The boxing operation consists of allocating the value type on the heap and copying it there. This is a costly operation and should be avoided if possible. Unboxing consists of copying the value type back to the stack.

int num = 42;
object obj = num; // Boxing
int unboxedNum = (int)obj; // Unboxing

Definition of the object type #

The definition of this type in the standard library looks as follows. Methods marked as virtual are intended to be overridden in derived classes.

public class Object
{
    public Object();
    public extern Type GetType();
    public virtual bool Equals(object obj);
    public static bool Equals(object objA, object objB);
    public static bool ReferenceEquals(object objA, object objB);
    public virtual int GetHashCode();
    public virtual string ToString();
    protected virtual void Finalize();
    protected extern object MemberwiseClone();
}

Methods in the object type #

Since all types inherit from object, the methods (e.g., ToString(), GetType(), GetHashCode(), and Equals(object)) defined in object are also available in other types. Calling these methods (except for GetType) on a value type does not cause a boxing operation.

int i = 15;
Console.WriteLine(i.ToString());
Console.WriteLine(i.GetType());
Console.WriteLine(i.GetHashCode());
Console.WriteLine(3.Equals(i));

ToString #

The ToString method returns a string representation of the object. All built-in types override it. If a type did not override it, then ToString returns the type name. You can override this method as follows:

Person alice = new Person {Name = "Alice", Age = 30};
Console.WriteLine(alice); // Alice, 30

class Person
{
    public string Name;
    public int Age;
    public override string ToString() => $"{Name}, {Age}";
}

GetType and the typeof operator #

The typeof operator allows you to obtain type information at compile time, while the GetType method does so at runtime. Apart from that, both methods work identically: they return an object that is a representation of the type.

DateTime now = DateTime.Now;

Console.WriteLine(now.GetType()); // evaluated at runtime
Console.WriteLine(typeof(DateTime)); // evaluated at compile time

Console.WriteLine(typeof(DateTime) == now.GetType());