A publicly accessible method can be called from anywhere, which means you should validate parameters to be within the expected constraints. In general, checking against ``++null++`` is recommended defensive programming.


This rule raises an issue when a parameter of a publicly accessible method is not validated against ``++null++`` before being dereferenced.

== Noncompliant Code Example

----
public class MyClass
{
    private MyOtherClass other;

    public void Foo(MyOtherClass other) 
    {
        this.other = other; // Compliant: other not being dereferenced
    }

    public void Bar(MyOtherClass other) 
    {
        this.other = other.Clone(); // Noncompliant
    }

    protected void FooBar(MyOtherClass other)
    {
        this.other = other.Clone(); // Noncompliant
    }
}
----

== Compliant Solution

----
public class MyClass
{
    private MyOtherClass other;

    public void Foo(MyOtherClass other) 
    {
        this.other = other;
    }

    public void Bar(MyOtherClass other) 
    {
        if (other != null) 
        {
            this.other = other.Clone();
        }
    }

    protected void FooBar(MyOtherClass other) 
    {
        if (other != null) 
        {
            this.other = other.Clone();
        }
    }
}
----

== Exceptions

To create a custom null validation method declare an attribute with name ``++ValidatedNotNullAttribute++`` and mark the parameter that is validated for null in your method declaration with it:

----
using System;

public sealed class ValidatedNotNullAttribute : Attribute { }

public static class Guard
{
    public static void NotNull<T>([ValidatedNotNullAttribute] this T value, string name) where T : class
    {
        if (value == null)
            throw new ArgumentNullException(name);
    }
}

public static class Utils
{
    public static string ToUpper(string value)
    {
        Guard.NotNull(value, nameof(value));
        if (value == null)
        {
            return value.ToString();
        }
        return value.ToUpper(); // Compliant
    }
}
----