I don't agree with the "you should only be interested in testing the external interface" philosophy. It's a bit like saying that a car repair shop should only have tests to see if the wheels turn. Yes, ultimately I'm interested in the external behavior but I like my own, private, internal tests to be a bit more specific and to the point. Yes, if I refactor, I may have to change some of the tests, but unless it's a massive refactor, I'll only have to change a few and the fact that the other (unchanged) internal tests still work is a great indicator that the refactoring has been successful.

You can try to cover all internal cases using only the public interface and theoretically it's possible to test every internal method (or at least every one that matters) entirely by using the public interface but you may have to end up standing on your head to achieve this and the connection between the test cases being run through the public interface and the internal portion of the solution they're designed to test may be difficult or impossible to discern. Having pointed, individual tests that guarantee that the internal machinery is working properly is well worth the minor test changes that come about with refactoring - at least that's been my experience. If you have to make huge changes to your tests for every refactoring, then maybe this doesn't make sense, but in that case, maybe you ought to rethink your design entirely. A good design should be flexible enough to allow for most changes without massive redesigns.

首先，您不应该测试代码的私有方法。你应该测试“公共接口”或API，即类的公共内容。API是所有公开给外部调用者的公共方法。

原因是一旦您开始测试类的私有方法和内部结构，您就将类的实现(私有的东西)耦合到您的测试中。这意味着当您决定更改实现细节时，您也必须更改您的测试。

出于这个原因，你应该避免使用internalsvisibletoattribute。

以下是Ian Cooper关于这个主题的演讲:Ian Cooper: TDD，哪里出了问题

如果您想对私有方法进行单元测试，那么可能会出现一些错误。单元测试(一般来说)用于测试类的接口，即类的公共(和受保护)方法。你当然可以“破解”一个解决方案(即使只是通过公开方法)，但你可能还想考虑:

如果您想测试的方法确实值得测试，那么将它移到自己的类中可能是值得的。 向调用私有方法的公共方法添加更多测试，测试私有方法的功能。(正如评论者所指出的，只有当这些私有方法的功能确实是公共接口的一部分时，才应该这样做。如果它们实际上执行对用户隐藏的功能(即单元测试)，这可能是不好的)。

将它们声明为内部的，然后使用InternalsVisibleToAttribute允许单元测试程序集看到它们。

我很惊讶还没有人说过这一点，但我采用的一个解决方案是在类内部创建一个静态方法来测试自身。这使您可以访问用于测试的所有公共和私有内容。

此外，在脚本语言(具有OO功能，如Python、Ruby和PHP)中，您可以在运行时对文件本身进行测试。确保您的更改不会破坏任何东西的快速方法。这显然为测试所有类提供了可伸缩的解决方案:只需运行所有类即可。(你也可以用void main在其他语言中这样做，它也总是运行它的测试)。

MS Test内置了一个很好的特性，通过创建一个名为VSCodeGenAccessors的文件，可以在项目中使用私有成员和方法

[System.Diagnostics.DebuggerStepThrough()]
    [System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.VisualStudio.TestTools.UnitTestGeneration", "1.0.0.0")]
    internal class BaseAccessor
    {

        protected Microsoft.VisualStudio.TestTools.UnitTesting.PrivateObject m_privateObject;

        protected BaseAccessor(object target, Microsoft.VisualStudio.TestTools.UnitTesting.PrivateType type)
        {
            m_privateObject = new Microsoft.VisualStudio.TestTools.UnitTesting.PrivateObject(target, type);
        }

        protected BaseAccessor(Microsoft.VisualStudio.TestTools.UnitTesting.PrivateType type)
            :
                this(null, type)
        {
        }

        internal virtual object Target
        {
            get
            {
                return m_privateObject.Target;
            }
        }

        public override string ToString()
        {
            return this.Target.ToString();
        }

        public override bool Equals(object obj)
        {
            if (typeof(BaseAccessor).IsInstanceOfType(obj))
            {
                obj = ((BaseAccessor)(obj)).Target;
            }
            return this.Target.Equals(obj);
        }

        public override int GetHashCode()
        {
            return this.Target.GetHashCode();
        }
    }

使用派生自BaseAccessor的类

如

[System.Diagnostics.DebuggerStepThrough()]
[System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.VisualStudio.TestTools.UnitTestGeneration", "1.0.0.0")]
internal class SomeClassAccessor : BaseAccessor
{

    protected static Microsoft.VisualStudio.TestTools.UnitTesting.PrivateType m_privateType = new Microsoft.VisualStudio.TestTools.UnitTesting.PrivateType(typeof(global::Namespace.SomeClass));

    internal SomeClassAccessor(global::Namespace.Someclass target)
        : base(target, m_privateType)
    {
    }

    internal static string STATIC_STRING
    {
        get
        {
            string ret = ((string)(m_privateType.GetStaticField("STATIC_STRING")));
            return ret;
        }
        set
        {
            m_privateType.SetStaticField("STATIC_STRING", value);
        }
    }

    internal int memberVar    {
        get
        {
            int ret = ((int)(m_privateObject.GetField("memberVar")));
            return ret;
        }
        set
        {
            m_privateObject.SetField("memberVar", value);
        }
    }

    internal int PrivateMethodName(int paramName)
    {
        object[] args = new object[] {
            paramName};
        int ret = (int)(m_privateObject.Invoke("PrivateMethodName", new System.Type[] {
                typeof(int)}, args)));
        return ret;
    }