C# – Meaning of Question Mark and Colon in Statements

It's the null coalescing operator, and quite like the ternary (immediate-if) operator. See also ?? Operator - MSDN.

FormsAuth = formsAuth ?? new FormsAuthenticationWrapper();

expands to:

FormsAuth = formsAuth != null ? formsAuth : new FormsAuthenticationWrapper();

which further expands to:

if(formsAuth != null)
    FormsAuth = formsAuth;
else
    FormsAuth = new FormsAuthenticationWrapper();

In English, it means "If whatever is to the left is not null, use that, otherwise use what's to the right."

Note that you can use any number of these in sequence. The following statement will assign the first non-null Answer# to Answer (if all Answers are null then the Answer is null):

string Answer = Answer1 ?? Answer2 ?? Answer3 ?? Answer4;

Also it's worth mentioning while the expansion above is conceptually equivalent, the result of each expression is only evaluated once. This is important if for example an expression is a method call with side effects. (Credit to @Joey for pointing this out.)

Your code is using what's called a nullable type. An enum, much like an int or a DateTime, is what is known as a "value type", which is required to always have some value. Nullable types allow you to treat value types as if they do allow null values.

For example, this code is invalid and won't compile because enums cannot be null:

E_Week week = null;

But this code is valid:

E_Week? week = null;

And it's exactly the same as this:

Nullable<E_Week> week = null;