CodeQL documentation
CodeQL resources

Dereferenced variable is always null

ID: cs/dereferenced-value-is-always-null Kind: problem Security severity: Severity: error Precision: very-high Tags: - quality - reliability - correctness - exceptions - external/cwe/cwe-476 Query suites: - csharp-code-quality.qls - csharp-security-and-quality.qls

Click to see the query in the CodeQL repository

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.

Recommendation

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

Example

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

using System; namespace NullAlways {  class Bad  {  void DoPrint(string s)  {  if (s != null || s.Length > 0)  Console.WriteLine(s);  }  } }

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

using System; namespace NullAlways {  class Good  {  void DoPrint(string s)  {  if (s != null && s.Length > 0)  Console.WriteLine(s);  }  } }

References