CA1062: Validate arguments of public methods

Cause

An externally visible method dereferences one of its reference arguments without verifying whether that argument is null (Nothing in Visual Basic).

Rule Description

All reference arguments that are passed to externally visible methods should be checked against null. If appropriate, throw a ArgumentNullException when the argument is null.

If a method can be called from an unknown assembly because it is declared public or protected, you should validate all parameters of the method. If the method is designed to be called only by known assemblies, you should make the method internal and apply the InternalsVisibleToAttribute attribute to the assembly that contains the method.

How to Fix Violations

To fix a violation of this rule, validate each reference argument against null.

When to Suppress Warnings

You can suppress a warning from this rule if you are sure that the dereferenced parameter has been validated by another method call in the function.

Example

The following example shows a method that violates the rule and a method that satisfies the rule.

Imports System

Namespace DesignLibrary

    Public Class Test

        ' This method violates the rule. 
        Sub DoNotValidate(ByVal input As String)

            If input.Length <> 0 Then
                Console.WriteLine(input)
            End If 

        End Sub 

        ' This method satisfies the rule. 
        Sub Validate(ByVal input As String)

            If input Is Nothing Then 
                Throw New ArgumentNullException("input")
            End If 

            If input.Length <> 0 Then
                Console.WriteLine(input)
            End If 

        End Sub 

    End Class 

End Namespace

public class Person
{
    public string Name { get; private set; }
    public int Age { get; private set; }

    public Person(string name, int age)
    {
        Name = name;
        Age = age;
    }

    // Copy constructor CA1062 fires because other is dereferenced 
    // without being checked for null 
    public Person(Person other)
        : this(other.Name, other.Age)
    {
    }
}

using System;

namespace DesignLibrary
{
    public class Test
    {
        // This method violates the rule. 
        public void DoNotValidate(string input)
        {
            if (input.Length != 0)
            {
                Console.WriteLine(input);
            }
        }

        // This method satisfies the rule. 
        public void Validate(string input)
        {
            if (input == null)
            {
                throw new ArgumentNullException("input");
            }
            if (input.Length != 0)
            {
                Console.WriteLine(input);
            }
        }
    }
}

In Visual Studio 2005, this rule does not detect that parameters are being passed to another method that does the validation.

Public Function Method(ByVal value As String) As String
    EnsureNotNull(value)

    ' Fires incorrectly     
    Return value.ToString()
End Function 

Private Sub EnsureNotNull(ByVal value As String)
    If value Is Nothing Then 
        Throw (New ArgumentNullException("value"))
    End If 
End Sub

public class Person
{
    public string Name { get; private set; }
    public int Age { get; private set; }

    public Person(string name, int age)
    {
        Name = name;
        Age = age;
    }

    // Copy constructor 
    public Person(Person other)
        : this(PassThroughNonNull(other).Name,
          PassThroughNonNull(other).Age)
    {
    }

    // Null check method 
    private static Person PassThroughNonNull(Person person)
    {
        if (person == null)
            throw new ArgumentNullException("person");
        return person;
    }
}

public string Method(string value)
{
    EnsureNotNull(value);

    // Fires incorrectly     
    return value.ToString();
}

private void EnsureNotNull(string value)
{
    if (value == null)
        throw new ArgumentNullException("value");
}

Copy constructors that populate field or properties that are reference objects can also violate the CA1062 rule. The violation occurs because the copied object that is passed to the copy constructor might be null (Nothing in Visual Basic). To resolve the violation, use a static (Shared in Visual Basic) method to check that the copied object is not null.

In the following Person class example, the other object that is passed to the Person copy constructor might be null.

public class Person
{
    public string Name { get; private set; }
    public int Age { get; private set; }

    public Person(string name, int age)
    {
        Name = name;
        Age = age;
    }

    // Copy constructor CA1062 fires because other is dereferenced
    // without being checked for null
    public Person(Person other)
        : this(other.Name, other.Age)
    {
    }
}

In the following revised Person example, the other object that is passed to the copy constructor is first checked for null in the PassThroughNonNull method.

public class Person
{
    public string Name { get; private set; }
    public int Age { get; private set; }

    public Person(string name, int age)
    {
        Name = name;
        Age = age;
    }

    // Copy constructor
    public Person(Person other)
        : this(PassThroughNonNull(other).Name, 
          PassThroughNonNull(other).Age)
    { 
    }

    // Null check method
    private static Person PassThroughNonNull(Person person)
    {
        if (person == null)
            throw new ArgumentNullException("person");
        return person;
    }
}