Visual Studio允许通过自动生成的访问器类对私有方法进行单元测试。我已经编写了一个私有方法的测试，它编译成功，但在运行时失败。一个相当小的版本的代码和测试是:
//in project MyProj
class TypeA
{
private List<TypeB> myList = new List<TypeB>();
private class TypeB
{
public TypeB()
{
}
}
public TypeA()
{
}
private void MyFunc()
{
//processing of myList that changes state of instance
}
}
//in project TestMyProj
public void MyFuncTest()
{
TypeA_Accessor target = new TypeA_Accessor();
//following line is the one that throws exception
target.myList.Add(new TypeA_Accessor.TypeB());
target.MyFunc();
//check changed state of target
}
运行时错误为:
Object of type System.Collections.Generic.List`1[MyProj.TypeA.TypeA_Accessor+TypeB]' cannot be converted to type 'System.Collections.Generic.List`1[MyProj.TypeA.TypeA+TypeB]'.
根据智能感知-因此我猜编译器-目标类型是TypeA_Accessor。但是在运行时它的类型是TypeA，因此列表添加失败。
有什么方法可以停止这个错误吗?或者，更有可能的是，其他人有什么其他的建议(我预测可能是“不要测试私有方法”和“不要使用单元测试来操纵对象的状态”)。
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) => $"Success! {x} {y}";
/// }
///
/// var test = new Test();
/// var res = test.Invoke<string>("GetStr", "testparam", 123);
/// Console.WriteLine(res); // "Success! testparam 123"
/// </example>
public static T Invoke<T>(this object obj, string methodName, params object[] parameters)
{
var method = obj.GetType().GetMethod(methodName, BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);
if (method == null)
{
throw new ArgumentException($"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}\"");
}
我使用这个helper(对象类型扩展)
public static TReturn CallPrivateMethod<TReturn>(
this object instance,
string methodName,
params object[] parameters)
{
Type type = instance.GetType();
BindingFlags bindingAttr = BindingFlags.NonPublic | BindingFlags.Instance;
MethodInfo method = type.GetMethod(methodName, bindingAttr);
return (TReturn)method.Invoke(instance, parameters);
}
你可以这样叫它
Calculator systemUnderTest = new Calculator();
int result = systemUnderTest.CallPrivateMethod<int>("PrivateAdd",1,8);
优点之一是它使用泛型来预先确定返回类型。
