Do properties always have a value when unset?

Question

I have a property like this:

public Tuple<String, String>[] Breadcrumbs { get; set; }

and I have a test in one of my methods like this:

if (Breadcrumbs != null && Breadcrumbs.Length > 0) { }

Depending on when this method is called, Breadcrumbs may not have been set. In one test, Breadcrumbs == null evaulates to true.

Will unset properties always have a value? (Will it always be null?)

Answer 1

An automatically-implemented property which hasn't been explicitly set by any code will always have the default value for the property type - which is null for reference types. (For int it would be 0, for char it would be '\0' etc).

An automatically implemented property like this is just equivalent to:

private PropertyType property;
public PropertyType Property
{
    get { return property; }
    set { property = value; }
}

... except that the backing variable has an unspeakable name (you can't refer to it in code) so it will always start off with the default value for the type.

Answer 2

Auto-properties use backing fields and are compiled to regular properties.

If the Property-Type is a reference type the value would be null, if not the value would be the default value.

Answer 3

Class member variables (called fields), and thus backing variables of properties, are always initialized to their default value, if they are not initialized explicitly, which is null for reference types. The default value for all types is the value whose binary representation consists of all bits set to 0.

On the other hand, C# requires you to initialize local variables explicitly. These are: variables declared in methods, constructors and property accessors and out method parameters; i.e. they are treated as undefined until you assign them a value.