The documentation for the protected keyword contained two significant errors that provided misleading information about C# protected member access rules.

Issues Fixed

1. Incorrect Code Comment

Before: The code comment stated that the CS1540 error occurs "because x can only be accessed by classes derived from A" - but class B is derived from A, making this explanation incorrect.

After: Updated to accurately explain that "x can only be accessed through the derived class type, not through the base class type."

2. Incorrect Documentation Explanation

Before: The documentation claimed the error occurs "because it is made within the static method Main, and not an instance of class B" - but the static nature of the method is irrelevant to protected access rules.

After: Corrected to explain that the error occurs "because it accesses the protected member through a base class reference (a is of type A). Protected members can only be accessed through the derived class type or types derived from it."

The Correct Rule

Protected members can only be accessed through references of the derived class type (or types derived from it), not through references of the base class type. This is why:

class B : A 
{
    static void Main() 
    {
        var a = new A();
        var b = new B();
        
        a.x = 10; // ❌ CS1540 - accessing through base class reference (A)
        b.x = 10; // ✅ OK - accessing through derived class reference (B)
    }
}

The compiler error message confirms this: "Cannot access protected member 'A.x' via a qualifier of type 'A'; the qualifier must be of type 'B' (or derived from it)"

Changes Made

• Updated code comment in csrefKeywordsModifiers.cs
• Updated explanation text in protected.md
• Both changes are minimal and surgical, preserving all existing functionality

Fixes #17445.

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