What Does Question Mark and Dot Operator . Mean in C# 6.0

What does question mark and dot operator ?. mean in C# 6.0?

It's the null conditional operator. It basically means:

"Evaluate the first operand; if that's null, stop, with a result of null. Otherwise, evaluate the second operand (as a member access of the first operand)."

In your example, the point is that if a is null, then a?.PropertyOfA will evaluate to null rather than throwing an exception - it will then compare that null reference with foo (using string's == overload), find they're not equal and execution will go into the body of the if statement.

In other words, it's like this:

string bar = (a == null ? null : a.PropertyOfA);
if (bar != foo)
{
    ...
}

... except that a is only evaluated once.

Note that this can change the type of the expression, too. For example, consider FileInfo.Length. That's a property of type long, but if you use it with the null conditional operator, you end up with an expression of type long?:

FileInfo fi = ...; // fi could be null
long? length = fi?.Length; // If fi is null, length will be null

what is ? operator in NotifyPropertyChanged

This is the syntax for null propagation which was introduced in c# 6.0. Here the ? is the null-conditional operator. This code is equivalent to:

if(!(PropertyChanged is null))
    PropertyChanged.Invoke(this, new PropertyChangedEventArgs(propName));

c# dynamic variable with ?? and ? operator

It essentially means

if name is not null
  set name to name
else
  if data is null
    set name to null
  else
    set name to data.name

The second operator (?.) avoids a NullReferenceException by returning null instead of trying to use the access modifier. The first (??) returns the first operand if the value is not null, otherwise it returns the second.

Note that neither of these are specific to dynamic or Azure.

It could also have been written as

if ((name == null) && (data != null))
{
   name = data.name;
}

Check null/empty at multiple levels using question mark operator c#

Use the following:

return dummy?.list?.FirstOrDefault()?.A;

You should avoid using index on a List as Lists aren't deterministic. If you have to know the position of an item in an a collection, use an array.

What is the purpose of a question mark after a value type (for example: int? myVariable)?

It means that the value type in question is a nullable type

Nullable types are instances of the System.Nullable struct. A
nullable type can represent the correct range of values for its
underlying value type, plus an additional null value. For example, a
Nullable<Int32>, pronounced "Nullable of Int32," can be assigned any
value from -2147483648 to 2147483647, or it can be assigned the null
value. A Nullable<bool> can be assigned the values true, false, or
null. The ability to assign null to numeric and Boolean types is
especially useful when you are dealing with databases and other data
types that contain elements that may not be assigned a value. For
example, a Boolean field in a database can store the values true or
false, or it may be undefined.
class NullableExample
{
  static void Main()
  {
      int? num = null;

      // Is the HasValue property true?
      if (num.HasValue)
      {
          System.Console.WriteLine("num = " + num.Value);
      }
      else
      {
          System.Console.WriteLine("num = Null");
      }

      // y is set to zero
      int y = num.GetValueOrDefault();

      // num.Value throws an InvalidOperationException if num.HasValue is false
      try
      {
          y = num.Value;
      }
      catch (System.InvalidOperationException e)
      {
          System.Console.WriteLine(e.Message);
      }
  }
}

What do two question marks together mean in C#?

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.)