Why is this an issue?

Calling ToString() on an object should always return a string. Thus, overriding the ToString method should never return Nothing, as it breaks the method’s implicit contract, and as a result the consumer’s expectations.

Public Overrides Function ToString() As String
    Return Nothing ' Noncompliant
End Function

Public Overrides Function ToString() As String
    Return ""
End Function

Resources

Documentation

• MITRE, CWE-476 - NULL Pointer Dereference
• Object.ToString Method