The in operator check if the left-hand value side is in the Array and Collections, Set or Flags Enum on the right, and returns true or false

var x: set of Color;
if Color.Red in x then

For Set and Flags Enum types, the operartor directly checks whether the value is included. For Flags, that is the equivalent of applying the and bitwise operator and checking for equality.

For Arrays, Collections and Sequences, the in operator invokes the LINQ Contains method, if available.

Not In

For ease of readability, the in operatore can be combined with not, to negate the result.

if Color.Red not in x then

Which is more naturally readable than the equivalent:

if not (Color.Red in x) then

See Also