Check

Code 

Description

•  This syntax supports static analysis such that later code will know whether variable is null or not.
•  Doesn’t produce a warning even when comparing against a non-nullable value type making the code to check pointless.
•  Requires C# 7.0 because it leverages type pattern matching.

•  This syntax supports static analysis such that later code will know whether variable is null or not.
•  Requires C# 8.0 since this is the method for checking for not null using property pattern matching.

•  Triggers a warning when comparing a non-nullable value type which could never be null
•  This syntax works with C# 8.0’s static analysis so later code will know that variable has been checked for null.
• Checks if the value not null by testing whether it is of type object.  (Relies on the fact that null values are not of type object.)

•  The only way to check for null prior to C# 7.0.
•  However, because the equality operator can be overridden, this has the (remote) possibility of failing or introducing a performance issue.

•  The only way to check for not null prior to C# 7.0.
•  Since the not-equal operator can be overridden, this has the (remote) possibility of failing or introducing a performance issue.