Default value for user defined class in C#

Question

I see some code will return the default value, so I am wondering for a user defined class, how will the compiler define its default value?

Answer 1

To chime in with the rest, it will be null, but I should also add that you can get the default value of any type, using default

default(MyClass) // null
default(int) // 0

It can be especially useful when working with generics; you might want to return default(T), if your return type is T and you don't want to assume that it's nullable.

Answer 2

The default value for class is a null

Answer 3

Note: A DefaultValueAttribute will not cause a member to be automatically initialized with the attribute's value. You must set the initial value in your code.

You can decorate your properties with the DefaultValueAttribute.

private bool myVal = false;

[DefaultValue(false)]
public bool MyProperty
{
    get
    {
       return myVal;
    }
    set
    {
       myVal = value;
    }
 }

I know this doesn't answer your question, just wanted to add this as relevant information.

For more info see http://msdn.microsoft.com/en-us/library/system.componentmodel.defaultvalueattribute.aspx

Answer 4

The default value for classes is null. For structures, the default value is the same as you get when you instantiate the default parameterless constructor of the structure (which can't be overriden by the way). The same rule is applied recursively to all the fields contained inside the class or structure.

Answer 5

I would make this "default" class instance a field rather than property, like how System.String.Empty looks:

public class Person 
{
    public string Name { get; set; }
    public string Address { get; set; }

    public static readonly Person Default = new Person() 
    {
        Name = "Some Name",
        Address = "Some Address"
    };
}

...

public static void Main(string[] args)
{
    string address = String.Empty;

    Person person = Person.Default;

    //the rest of your code
}