If a variable is dereferenced, for example as the qualifier in a method call, and the variable has a null value on all possible execution paths leading to the dereferencing, the dereferencing is guaranteed to result in a NullReferenceException.

Ensure that the variable does not have a null value when it is dereferenced.

In the following examples, the condition s.Length > 0 is only executed if s is null.

In the revised example, the condition is guarded correctly by using && instead of ||.

Microsoft, NullReferenceException Class.