CSHARP.NULL.DEREF : Null Pointer Dereference (C#)

If the null value gets dereferenced, C# runs into a NullReferenceException. For this reason, programmers must ensure that the content of expressions dereferenced in their programs is never null. Solving this problem is in general hard. This checker provides a coverage of the most frequent scenarios when null might end up being dereferenced. For a sound alternative to this checker, that covers all possible situations, see the Nullness checker. However, BasicNullness is much faster than Nullness and issues a more restricted set of false alarms, hence it is often the best solution for a rapid identification of the most frequent null-pointer errors in a program.

Strict and Non-Strict Checking

When CSHARP_ANALYSIS_STRICT_MODE=No, warnings of this class will not be issued if there are indications that the possibility of a NullReferenceException has been recognized and accounted for. For example, warnings will not be issued for code inside a try-catch block that explicitly catches NullReferenceException, or for a JUnit test that is annotated as expecting this exception.

プロパティ

例

Example 1

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace TestGetFieldFromNullWarning
{
    public class Bean
    {
        private String field = "value";

        public String getField()
        {
            return field;
        }

        public void setField(String field)
        {
            this.field = field;
        }

    }

    public class TestGetFieldFromNullWarning
    {
        public bool condition;

        public void getFieldFromNullWarning()
        {
            Bean bean = null;

            if (condition)
                bean = new Bean();

            if (bean.getField().Equals("")) // Null Pointer Dereference (C#) warning issued here
                                            // - receiver bean of getField() is never checked for nullness
            {
                Console.WriteLine("Field is empty");
            }
        }

    }
}

Example 2

Look for variables which value is always null

Highlighting variables which value is always null may help discovering dead code or simply logic that is no more required.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace TestVariableCanOnlyBeNullWarning
{
    public class TestVariableCanOnlyBeNullWarning
    {

        public bool variableCanOnlyBeNullWarning()
        {
            int[] var = null;

            if (var == null) // Null Pointer Dereference (C#) warning issued here 
                             // - variable 'var' is always null at this point
                return false;

            for (int i = 0; i < var.Length; i++)
                Console.WriteLine("pippo");

            return true;
        }

    }
}

Example 3

using System;

public class MyMainClass 
{

    public static void Main(string[] args) 
    {
        
        Place p = null;
        p.h = "These's no place like home!";  // Null Pointer Dereference (C#) warning issued here 

    }

    private static class Place
    {
        
        public string h;
        
        // ...
        
    }

}

Example 4

using System;

public class MyExceptionHandler 
{
    
    public void Handle(Exception e)
    {

        if (e != null)
        {
            Console.WriteLine("Exception found: " + e.GetType());
        }
        
        throw e; // Null Pointer Dereference (C#) warning issued here 
                 // - e may be null 
     }

}

Example 5

public class CountingStars 
{

      int stars;
      
      object _lock;
      
      public void add(int value)
      {

            lock(_lock)  // Null Pointer Dereference (C#) warning issued here
                         // - synchronizing on null value
            {
               this.stars += value;   
            }
      }
}

public class CountingStars 
{

      int stars;
      
      readonly object lock = new object();
      
      public void Add(int value)
      {

            lock(_lock)
            {
               this.stars += value;   
            }
      }
}

Example 6

public class ArrayProgram
{

        public static void Length()
        {
            int[] arr1 = null;
            int arrayLength = arr1.Length; // "Null Pointer Dereference (C#)" warning issued here
        }

        public static void Load()
        {
            int[] arr2 = null;
            int n = arr2[0];               // "Null Pointer Dereference (C#)" warning issued here
        }

        public static void Store()
        {
            int[] arr3 = null;
            arr3[0] = 3;                   // "Null Pointer Dereference (C#)" warning issued here
        }
}

Example 7

using System;

namespace DocumentationExamples
{

    public class BasicNullness
    {

        public static void Main(string[] args)
        { }

        string f = null;
        public void Test1(string s)
        {
            if ((DateTime.Now.Millisecond % 2) == 0 || f != null)
                Console.WriteLine(f.Length);     // Null Pointer Dereference (C#) warning issued here 
                                                 //  - f is always null 
            if (s != null)
                Console.WriteLine("s01 is null");
            Console.WriteLine(s.Length);         // Null Pointer Dereference (C#) warning issued here 
                                                 // - Either the previous null-check of s is redundant (and should be removed)
                                                 //   or there should be one for this call too. 
        }
        public void Test2(string par)
        {
            string s = DateTime.Now.Millisecond % 2 == 0 ? null : "ciao";
            BasicNullnessSupport ts = new BasicNullnessSupport();
            int n01 = ts.M01(s);                 // Two Null Parameter Dereference (C#) warning instances issued here 
                                                 // - one because s may have been assigned null 
                                                 // - one because there is a subsequent null-check of s: either that check is redundant (and should be removed)
                                                 //    or there should be one for this call too. 
            int n02 = ts.M02(s);
            if (s != null)
                ts.M01(s);
            ts.M01(par);
            Console.WriteLine(n01 + n02);
        }
        public void Test3(string par)
        {
            if (par != null)
                Console.WriteLine("is non-null");
            new BasicNullnessSupport().M01(par); // Null Parameter Dereference (C#) warning issued here
                                                 // - either the previous null-check of s is redundant (and should be removed)
                                                 //   or there should be one for this call too.
        }
    }
    
    public class BasicNullnessSupport
    {
        public int M01(string s)
        {
            return s.Length;
        }
        public int M02(string s)
        {
            if (s != null)
                return s.Length;
            return -1;
        }
    }
}

解決法

Check if the warning corresponds to a situation where null might actually be dereferenced at runtime. If that is the case, add a nullness check for the value being dereferenced, or change the logic of the code. Sometimes, a warning of this checker corresponds to a spurious nullness check, that can be removed.

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