Private Protected Method

using System;
using System.Runtime.CompilerServices;

// Optional: Grants internal access to Assembly B
// [assembly: InternalsVisibleTo("AssemblyB")]

// =========================================
// Assembly A (e.g., CoreLibrary.dll)
// =========================================
public class BaseClass
{
    // Declaring the private protected method
    private protected void ExecuteCoreLogic()
    {
        Console.WriteLine("Executing logic...");
    }

    public void CallFromDeclaringType()
    {
        // VALID: Accessed directly from within the declaring class.
        ExecuteCoreLogic();
    }

    public class NestedClass
    {
        public void CallFromNested(BaseClass instance)
        {
            // VALID: Accessed from a type nested within the declaring class.
            instance.ExecuteCoreLogic();
        }
    }
}

public class DerivedInSameAssembly : BaseClass
{
    public void TestAccess()
    {
        // VALID: Caller is a derived class AND resides in Assembly A.
        ExecuteCoreLogic(); 
    }
}

public class NonDerivedInSameAssembly
{
    public void TestAccess(BaseClass instance)
    {
        // ERROR CS0122: 'BaseClass.ExecuteCoreLogic()' is inaccessible 
        // due to its protection level.
        // Reason: Resides in Assembly A, but does NOT inherit from BaseClass.
        // instance.ExecuteCoreLogic(); 
    }
}

// =========================================
// Assembly B (e.g., ExternalApp.exe)
// Requires project reference to Assembly A
// =========================================
public class DerivedInDifferentAssembly : BaseClass
{
    public void TestAccess()
    {
        // ERROR CS0122: 'BaseClass.ExecuteCoreLogic()' is inaccessible 
        // due to its protection level.
        // Reason: Inherits from BaseClass, but resides in Assembly B.
        // (NOTE: This WOULD compile successfully if Assembly A declared 
        // [assembly: InternalsVisibleTo("AssemblyB")])
        // ExecuteCoreLogic(); 
    }
}