技术咨询：protected与internal的区别是什么？二者仅在项目内使用场景有差异吗？

Hey Tal, these are fantastic questions—access modifiers can feel tricky at first, especially when their scopes overlap a bit. Let’s break this down step by step.

1. Core Differences Between Protected and Internal #

Let’s start with the basics of each modifier, then contrast their scopes clearly:

• protected: This modifier grants access to:

  • The class that defines the member
  • Any subclass that inherits from the defining class, regardless of which assembly (project/DLL) the subclass lives in
    Think of it as "family-only" access—any class that’s part of the inheritance tree can reach it, even if it’s in a separate project.

• internal: This modifier grants access to:

  • All code within the same assembly (project/DLL) as the defining class
  • No code outside that assembly, even if it’s a subclass of the defining class
    Think of it as "team-only" access—everyone in the same project can use it, no inheritance required.

Quick Code Example to Illustrate #

Suppose we have a base class in Project A:

public class BaseClass
{
    protected int ProtectedNumber = 42;
    internal int InternalNumber = 100;
}

• In Project B (separate assembly): If we create DerivedClass : BaseClass, we can access ProtectedNumber (thanks to inheritance), but we can’t touch InternalNumber (it’s locked to Project A).
• In Project A: A totally unrelated class RandomClass (no inheritance from BaseClass) can access InternalNumber, but it can’t access ProtectedNumber (no inheritance link).

2. Is Their Only Difference In-Project Usage? #

Absolutely not—their core distinction is rooted in two entirely different access rules, not just project boundaries:

• Protected’s scope is inheritance-based, not assembly-based. The key question is "is this class a subclass?"—not "is it in the same project?" A subclass in a totally separate DLL can still access protected members.
• Internal’s scope is assembly-based, not inheritance-based. The key question is "is this code in the same project?"—not "is it a subclass?" Even a class with no relationship to the defining class can access internal members if they’re in the same assembly.

So in-project usage is just one piece of the puzzle. Protected cares about class hierarchies, internal cares about project/assembly boundaries—those are fundamentally different rules.

内容的提问来源于stack exchange，提问作者Tal Shloman