Passing by enum class variables in C# via parameters to a function

Question

I'm very confused here. Suppose I have two programs in Visual Studios - I'm using C# WPF. Suppose I have two programs where Program 1 has Program 2 as a reference - and similarly Program 2 has Program 1 as a reference.

Program1.sol

p1.xaml.cs contents:

public void checkStuff(myThing x)
{
   // Do stuff (irrelevant)
}

public enum myThing
{
  stuff,
  stuff2,
  stuff3,
  stuff4
}

Program2.sol

p2.xaml.cs contents:

// Let's say I call that function
p1 p1content = new p1();
p1content.checkStuff(0); // <-- Why does this work??

My question is, why does the number 0 work as a parameter? This may be trivial but what exactly does the program think I am passing in that function when I pass in 0? At first I thought it was the index of the variables located in my enum class, but it can't be because I can't pass in 1,2, or 3, etc..

Answer 1

By default in C# the underlying type of enum s are int . See the MSDN documentation .

As mentioned in the comment in this SO post :

The enum keyword is used to declare an enumeration, a distinct type consisting of a set of named constants called the enumerator list. Every enumeration type has an underlying type, which can be any integral type except char.

An enum has an integer-based value as base (int, byte, ulong, etc). The compiler only accepts an assigment of zero to guarantee an variable of an enum-based type to be initialized even when no explicit value has been assigned to the enum attributes. The compiler however does not accept any other values. This is a build-in safety, but more so a compromise of addressing a flaw in the compiler design to resolve previous inconsistent behavior with enums. This little story by Eric Lippert provides an insight into how this came into existence.

Answer 2

When you declare an enum such as the one in your question, what actually happens under the hood(ish) is this:

public enum myThing : int
{
  stuff = 0,
  stuff2 = 1,
  stuff3 = 2,
  stuff4 = 3
}

ie, the default, underlying type of an enum is an int , and the fields are started sequentially from zero.

Strangely, this only works for the zero element, for example this works:

checkStuff(0);

but this will not

checkStuff(1); //error

You would need to be explicit

checkStuff((myThing)1); //cast int to myThing