public static class PrivateMethodTester
{
    public static object InvokePrivateMethodWithReturnType<T>(this T testObject, string methodName, Type[] methodParamTypes, object[] parameters)
    {
        //shows that we want the nonpublic, static, or instance methods.
        var flags = BindingFlags.Static | BindingFlags.NonPublic | BindingFlags.Instance;

        //gets the method, but we need the methodparamtypes so that we don't accidentally get an ambiguous method with different params.
        MethodInfo methodInfo = testObject.GetType().GetMethod(methodName, flags, null, methodParamTypes, null);
        if (methodInfo == null)
        {
            throw new Exception("Unable to find method.");
        }

        //invokes our method on our object with the parameters.
        var result = methodInfo.Invoke(testObject, parameters);
        if (result is Task task)
        {
            //if it is a task, it won't resolve without forcing it to resolve, which means we won't get our exceptions.
            task.GetAwaiter().GetResult();
        }

        return result;
    }
}

这样称呼它:

        Type[] paramTypes = new Type[] { typeof(OrderTender), typeof(string) };
        var parameters = new object[] { orderTender, OrderErrorReasonNames.FailedToCloneTransaction };

        myClass.InvokePrivateMethodWithReturnType("myPrivateMethodName", paramTypes, parameters);

如果PrivateObject不可用，并且被测试的类不是密封类，则可以将想要公开的方法和属性设置为受保护的。在单元测试文件中创建一个带有内部方法的继承类，这些内部方法公开了测试中的私有方法/属性。

如果被测试的类是:

class MyClass{private string GetStr(string x, int y) => $"Success! {x} {y}";}

改为:

class MyClass{protected string GetStr(string x, int y) => $"Success! {x} {y}";}

在单元测试文件中创建一个继承类，如下所示:

class MyClassExposed: MyClass 
{
    internal string ExposedGetStr(string x, int y)
    {
        return base.GetStr(x, y);
    }
}

现在您可以使用继承的类MyClassExposed来测试公开的方法和属性。

这里的另一个想法是将测试扩展到“内部”类/方法，给这种测试更多的白盒意义。您可以在程序集上使用InternalsVisibleTo属性将它们公开给单独的单元测试模块。

结合密封类，你可以达到这样的封装，测试方法只能从单元测试程序集你的方法可见。考虑到密封类中的受保护方法实际上是私有的。

[assembly: InternalsVisibleTo("MyCode.UnitTests")]
namespace MyCode.MyWatch
{
    #pragma warning disable CS0628 //invalid because of InternalsVisibleTo
    public sealed class MyWatch
    {
        Func<DateTime> _getNow = delegate () { return DateTime.Now; };
    

       //construktor for testing purposes where you "can change DateTime.Now"
       internal protected MyWatch(Func<DateTime> getNow)
       {
           _getNow = getNow;
       }

       public MyWatch()
       {            
       }
   }
}

和单元测试:

namespace MyCode.UnitTests
{

[TestMethod]
public void TestminuteChanged()
{
    //watch for traviling in time
    DateTime baseTime = DateTime.Now;
    DateTime nowforTesting = baseTime;
    Func<DateTime> _getNowForTesting = delegate () { return nowforTesting; };

    MyWatch myWatch= new MyWatch(_getNowForTesting );
    nowforTesting = baseTime.AddMinute(1); //skip minute
    //TODO check myWatch
}

[TestMethod]
public void TestStabilityOnFebruary29()
{
    Func<DateTime> _getNowForTesting = delegate () { return new DateTime(2024, 2, 29); };
    MyWatch myWatch= new MyWatch(_getNowForTesting );
    //component does not crash in overlap year
}
}

测试私有方法的一种方法是通过反射。这也适用于NUnit和XUnit:

MyObject objUnderTest = new MyObject();
MethodInfo methodInfo = typeof(MyObject).GetMethod("SomePrivateMethod", BindingFlags.NonPublic | BindingFlags.Instance);
object[] parameters = {"parameters here"};
methodInfo.Invoke(objUnderTest, parameters);

你可以使用PrivateObject类:

Class target = new Class();
PrivateObject obj = new PrivateObject(target);
var retVal = obj.Invoke("PrivateMethod");
Assert.AreEqual(expectedVal, retVal);

注意:PrivateObject和PrivateType不适用于针对netcoreapp2.0 - GitHub Issue 366的项目

遗憾的是。net6中没有PrivateObject类

不过，我写了一个小型扩展方法，能够使用反射调用私有方法。

看一下示例代码:

class Test
{
  private string GetStr(string x, int y) => $"Success! {x} {y}";
}

var test = new Test();
var res = test.Invoke<string>("GetStr", "testparam", 123);
Console.WriteLine(res); // "Success! testparam 123"

下面是扩展方法的实现:

/// <summary>
/// Invokes a private/public method on an object. Useful for unit testing.
/// </summary>
/// <typeparam name="T">Specifies the method invocation result type.</typeparam>
/// <param name="obj">The object containing the method.</param>
/// <param name="methodName">Name of the method.</param>
/// <param name="parameters">Parameters to pass to the method.</param>
/// <returns>The result of the method invocation.</returns>
/// <exception cref="ArgumentException">When no such method exists on the object.</exception>
/// <exception cref="ArgumentException">When the method invocation resulted in an object of different type, as the type param T.</exception>
/// <example>
/// class Test
/// {
///   private string GetStr(string x, int y) => $"Success! {x} {y}";
/// }
///
/// var test = new Test();
/// var res = test.Invoke&lt;string&gt;("GetStr", "testparam", 123);
/// Console.WriteLine(res); // "Success! testparam 123"
/// </example>
public static T Invoke<T>(this object obj, string methodName, params object[] parameters)
{
  var method = obj.GetType().GetMethod(methodName, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
  if (method == null)
  {
    throw new ArgumentException($"No private method \"{methodName}\" found in class \"{obj.GetType().Name}\"");
  }

  var res = method.Invoke(obj, parameters);
  if (res is T)
  {
    return (T)res;
  }

  throw new ArgumentException($"Bad type parameter. Type parameter is of type \"{typeof(T).Name}\", whereas method invocation result is of type \"{res.GetType().Name}\"");
}