Page 80 - CSharp/C#

same class and derived classes only:

public class Foo()
{
protected void SomeFooMethod()
{
//do something
}

protected class Thing
{
private string blah;
public int N { get; set; }
}
}

public class Bar() : Foo
{
private void someBarMethod()
{
SomeFooMethod(); // inside derived class
var thing = new Thing(); // can use nested class
}
}

public class Baz()
{
private void someBazMethod()
{
var foo = new Foo();
foo.SomeFooMethod(); //not accessible due to protected modifier
}
}

protected internal

The protected internal keyword marks field, methods, properties and nested classes for use
inside the same assembly or derived classes in another assembly:

Assembly 1

public class Foo
{
public string MyPublicProperty { get; set; }
protected internal string MyProtectedInternalProperty { get; set; }

protected internal class MyProtectedInternalNestedClass
{
private string blah;
public int N { get; set; }
}
}

public class Bar
{
void MyMethod1()
{
Foo foo = new Foo();

https://riptutorial.com/ 26