CodeQL documentation

Dereferenced variable is always null

ID: cs/dereferenced-value-is-always-null
Kind: problem
Severity: error
Precision: very-high
Tags:
   - reliability
   - correctness
   - exceptions
   - external/cwe/cwe-476
Query suites:
   - 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