I'm new to mock objects, but I understand that I need to have my classes implement interfaces in order to mock them.
The problem I'm having is that in my data access layer, I want to have static methods, but I can't put a static method in an interface.
What's the best way around this? Should I just use instance methods (which seems wrong) or is there another solution?
Yes, you use instance methods. Static methods basically say, "There is one way to accomplish this functionality - it's not polymorphic." Mocking relies on polymorphism.
Now, if your static methods logically don't care about what implementation you're using, they might be able to take the interfaces as parameters, or perhaps work without interacting with state at all - but otherwise you should be using instances (and probably dependency injection to wire everything together).
I found a blog via google with some great examples on how to do this:
Refactor class to be an instance class and implement an interface.
You have already stated that you don't want to do this.
Use a wrapper instance class with delegates for static classes members
Doing this you can simulate a static interface via delegates.
Use a wrapper instance class with protected members which call the static class
This is probably the easiest to mock/manage without refactoring as it can just be inherited from and extended.
I would use a method object pattern. Have a static instance of this, and call it in the static method. It should be possible to subclass for testing, depending on your mocking framework.
i.e. in your class with the static method have:
private static final MethodObject methodObject = new MethodObject();
public static void doSomething(){
methodObject.doSomething();
}
and your method object can be a very simple, easily-tested:
public class MethodObject {
public void doSomething() {
// do your thang
}
}
You might be trying to test at too deep a starting point. A test does not need to be created to test each and every method individually; private and static methods should be tested by calling the public methods that then call the private and static ones in turn.
So lets say your code is like this:
public object GetData()
{
object obj1 = GetDataFromWherever();
object obj2 = TransformData(obj1);
return obj2;
}
private static object TransformData(object obj)
{
//Do whatever
}
You do not need to write a test against the TransformData method (and you can't). Instead write a test for the GetData method that tests the work done in TransformData.
Use instance methods where possible.
Use public static Func[T, U] (static function references that can be substituted for mock functions) where instance methods are not possible.
A simple solution is to allow to change the static class's implementation via a setter:
class ClassWithStatics {
private IClassWithStaticsImpl implementation = new DefaultClassWithStaticsImpl();
// Should only be invoked for testing purposes
public static void overrideImplementation(IClassWithStaticsImpl implementation) {
ClassWithStatics.implementation = implementation;
}
public static Foo someMethod() {
return implementation.someMethod();
}
}
So in the setup of your tests, you call overrideImplementation with some mocked interface. The benefit is that you don't need to change clients of your static class. The downside is that you probably will have a little duplicated code, because you'll have to repeat the methods of the static class and it's implementation. But some times the static methods can use a ligther interface which provide base funcionality.
The problem you have is when you're using 3rd party code and it's called from one of your methods. What we ended up doing is wrapping it in an object, and calling passing it in with dep inj, and then your unit test can mock 3rd party static method call the setter with it.
Related
I'm trying to mock a static class and static method in C# using NSubstitute. I obviously don't want to Generate an excel file for every unit test. So I definitely want to mock. But since this is a static class, I'm unable to mock as this is a static class.
public static class FileGenerator
{
public static async Task GenerateExcelFile(string filename, IEnumerable<T> content)
{
.....
}
}
var mockobj = Substitute.For<FileGenerator>(); // Returns error
Any hint?
It is a good idea to use static only very sparringly. It is one of those mechanics that is easily and often missused as a shortcut. But those always end up causing issues down the line.
For constants or runtime constants it is okay to use a static. But do not use it for changing data. Something called FileGenerator? Right there with things you should not be making static.
Instead:
create a normal class that needs instantiation
create a instance and assign it to a static field
Now you can easily have multiple instance in paralell. You can also extract a interface/abstract class and then create a different implementation. Including a mock implementation.
Now you can use source generators to produce interface and its implementation for a static class. For example I do such thing for Clipboard class.
The example of my source generator is here: https://github.com/Tum4ik/just-clipboard-manager/blob/main/Tum4ik.SorceGenerators/StaticClassInterfaceAndImplementationGenerator.cs
So, to generate interface and implementation for the Clipboard static class I just need to write this:
[IIForStaticClass(typeof(Clipboard), "ClipboardWrapper")]
internal partial interface IClipboard
{
}
I know how to make pluggable things in c#. Define an Interface, Activator.CreateInstance(<class>), etc. Or I can have a code path that explicitly creates an instance of the plugged class. Many ways.
But what if the service I want to make pluggable is static (I know I could refactor so that it's not but that's not the point of the question)
Concrete example. I have a class that provides disk I/O abstraction (Read File, List Directory,....). Now I want different implementations of this abstraction that serves up files from , say, a real FS, a database.
Based on Olivier Jacot-Descombes reply, I will have a FileSystem class (that is real) like this
public static class FileSystem
{
static IFSImplemenation s_imple;
static FileSystem()
{
if(<some system setting>)
// converted to singleton instead of static
s_imple = new OldFileSystem()
else
s_imple = new DbFileSystem();
}
public static byte[] ReadFile(string path)
{
return s_imple.ReadFile(path);
}
...
}
To reiterate - I have a large body of code that I dont want to change so it was important to keep the calling signature the same - this solution achieves that
You can't, and that's a limitation of the type system in .NET and most object-oriented systems/languages.
The reason being that "pluggable things" (as you refer to them) require polymorphism in order to be effective.
You define a contract (an interface, in .NET-speak), and then you implement that contract. Your system only ever deals with the contract.
Static members are not polymorphic in .NET, so you can never get them to implement a contract.
For your example of disk I/O abstraction, you wouldn't create a static class for this, you'd create an interface, and implement the interface, passing around the interface.
Of course, the benefits of using an interface are that it's easier to test both sides of the implementation of your contract:
Where the contract is a client, you can mock the interface easily and pass the mock to the consumers of the contract (this is where you might start thinking about dependency injection)
You can test the implementation separately from the rest of your system.
In the case of your disk I/O abstraction, for everything that calls your contract, you would never have to worry about actually touching the file system, you simply have to make the contract behave (through proper mock setup) as if it were touching the file system.
If you have an existing service that is exposed through static members, and you want the ability to swap that out with another implementation, then you'll have to do the following:
Create an abstraction (contract/abstract type) which defines the operations on your service.
Create an implementation that forwards the calls from the instance of the implementation of the abstraction to the static methods.
Rewire all of the static calls to the service with instances of the implementation. At this point, you really must use dependency injection of some sort; if you don't, you're just creating the concrete implementation at the call site, and that's pretty much the same (you'd have to change every call site again if you wanted to use another implementation).
Use your static class as facade for non-static implementations
public static class DB
{
private static IDbInterface _implementation;
public static void SetImplementation(IDbInterface implementation)
{
_implementation = implementation;
}
public static Customer GetCustomerByID(int custId)
{
return _implementation.GetCustomerByID(custId);
}
...
}
Static classes are not "pluggable". If you want to be able to swap it out, you need to refactor it to use interfaces that you can inject.
Some classes have a static Current or Default parameter, such as ServiceLocator. You then use ServiceLocator.Current to access the current IServiceLocator in a static context.
You can take this a little further and hide the Current object. While I don't recommend doing this, it will prevent the need to refactor all of your code. However, the maintenance is much higher with this method, as Foo does not implement IFoo.
Suppose you have a static class Foo
static class Foo
{
public static string GetBar() { return "Bar"; }
}
You could make an interface IFoo
interface IFoo
{
string GetBar();
}
Now change your class Foo to the following
static class Foo
{
private static IFoo _foo;
public static void SetFoo(IFoo foo) { _foo = foo; }
public static string GetBar() { return _foo.GetBar(); }
}
Make you static class residenting in host application and being actually a factory: returning implementation provided by a plugin by interface.
I need to know how to prevent a class from being Instantiated in .net?
I know few methods like making the class Abstract and Static.
Is there any more way to achieve this?
Making the class static is the best approach, if you absolutely don't want any instances. This stops anyone from creating instances. The class will be both sealed and abstract, and won't have any constructors.
Additionally, the language will notice that it's a static class and stop you from using it in various places which imply instances, e.g. type arguments and variables. This indicates the intention more clearly than just having a private constructor - which could mean that there are instances, created within that class (e.g. for a singleton implementation).
Oh, and making the class static will stop you from introducing any pointless instance members in the class, too :)
See MSDN for more information about static classes.
Mark the constructor(s) private, protected or if in used from another assembly, internal
Marking the constructor private. Of course, this doesn't prevent the class from instantiating itself through a static method, for example...
More practically, what's the purpose of disallowing class instantiation. If it's to have a singleton, then a private constructor is appropriate. If it's to force subclassing, making the class abstract is better; if it's to have a class with utility methods, making it static is one way (then you can only have static methods).
I need to know how to prevent a class from being Instantiated in .net?
Your question is not clear.
Do you mean instantiated at runtime? Make the class abstract or static.
Do you mean that the constructor is not accessible in code? Make the constructor private. But note that someone could still use reflection to grab a handle on a constructor and instantiate an instance at runtime.
So, which do you mean?
If the question is:
How can you make your class not be instanced without having your class
be static or abstract?
Then the answer to this is to implement the singleton pattern, which in .NET 4+ this is done easily with:
public sealed class myClass
{
private static readonly Lazy<myClass> lazyInstance =
new Lazy<myClass>(() => new myClass());
public static Instance
{
get
{
return lazyInstance.Value;
}
}
private myClass()
{
// constructor logic here
}
}
The singleton pattern allows you to pass your class around as a reference to methods, while still ensuring that you have only a single instance of your class. It also makes testing much easier as you can have a ImyClass instance which myClass implements, this is very helpful when making mock objects.
Without .NET4 you can still implement the singleton pattern, for example:
private static readonly myClass instance = new myClass();
public static Instance
{
get
{
return instance;
}
}
// rest of code remains the same
Which doesn't have deferred loading until it's called, there's lots of other ways as well (I think about 6 different ways), but the above two are the most common ones.
In summary the question is likely asking if you know the singleton pattern and if you recognise it's importance over static classes for unit tests and mock objects.
As others have already pointed out, static fields, even those marked readonly can be set with reflection, in addition the private constructor can be called using reflection. If you need to prevent these, either you need to make the calling code run in a less trusted app-domain space, or you will need to implement your class as static.
Generally though, people don't bother with such levels of reflection to get around your constraints unless they 'really need to' for example, writing obscure / extreme fringe case unit tests.
I know that virtual and static methods are opposing concepts, but I think that it could make sense sometimes to use them together. There have been quite a bunch of similiar question on SO on this topic, but the following scenario has not been covered yet.
There's a C# interface that looks like this:
interface IVertexMeshLoader
{
VertexMesh LoadFromFile(string fname);
}
An implementation of that could look like this:
class VertexMeshLoaderObj : IVertexMeshLoader
{
public VertexMesh LoadFromFile(string fname) { .. }
}
Now I would like to be able to call method without an object instance, but I cannot make the LoadFromFile() method static, because it implements the interface.
The best solution I worked out so far is to write a static method LoadFromFileStatic() that contains the actual code. The LoadFromFile() then just calls it. Not very pretty, imho.
I could also create an instance of VertexMeshLoadObj every time I want to call the method, but that is even worse.
Are there better ways? Thanks :-)
Here's another option. Provide an explicit implementation of the interface which just calls the static method. It allows them to have the same name
class VertexMeshLoaderObj : IVertexMeshLoader
{
VertexMesh IVertexMeshLoader.LoadFromFile(string fname) {
LoadFromFile(fname);
}
public static VertexMesh LoadFromFile(fname) {
...
}
}
If you must do this, create a singleton instance of IVertexMeshLoader and access that
Even if you're only interested in the vtable of the object and not in the actual members, you need an object instance to know what method to call.
So even if the actual implementation doesn't seem to depend on the instance, it in fact does.
You're not using "static virtual" methods but really virtual ones.
In C#, I have base class Product and derived class Widget.
Product contains a static method MyMethod().
I want to call static method Product.MyMethod() from static method Widget.MyMethod().
I can't use the base keyword, because that only works with instance methods.
I can call Product.MyMethod() explicitly, but if I later change Widget to derive from another class, I have to revise the method.
Is there some syntax in C# similar to base that allows me to call a static method from a base class from a static method of a derived class?
static methods are basically a method to fallback from object oriented concepts. As a consequence, they are not very flexible in inheritance hierarchies and it's not possible to do such a thing directly.
The closest thing I can think of is a using directive.
using mybaseclass = Namespace.BaseClass;
class MyClass : mybaseclass {
static void MyMethod() { mybaseclass.BaseStaticMethod(); }
}
It can be done, but I don't recommend it.
public class Parent1
{
public static void Foo()
{
Console.WriteLine("Parent1");
}
}
public class Child : Parent1
{
public new static void Foo()
{
Type parent = typeof(Child).BaseType;
MethodInfo[] methods = parent.GetMethods();
MethodInfo foo = methods.First(m => m.Name == "Foo");
foo.Invoke(null, null);
}
}
Calling a static method using reflection is exactly the same as calling an instance method except that you pass null for the instance. You need FlattenHierarchy because it's defined in an ancestor.
var type = assy.GetType("MyNamespace.MyType");
MethodInfo mi = type.GetMethod("MyStaticMethod",
BindingFlags.Static | BindingFlags.Public | BindingFlags.FlattenHierarchy);
mi.Invoke(null, null);
Further reading and thinking leaves me asking the same questions as others who have responded: why use static methods like this? Are you trying to do functional programming, and if so why not use lambda expressions instead? If you want polymophic behaviours with shared state, instance methods would be better.
It can be done:
public class Parent1
{
protected static void Foo()
{
Console.WriteLine("Parent1");
}
}
public class Child : Parent1
{
public static void Foo()
{
return Parent1.Foo();
}
}
Can be useful for unit testing protected static methods (for example).
First and foremost, if you're worried about re-parenting a class, then you're probably doing inheritance wrong. Inheritance should be used to establish "is-a" relationships, not simply foster code reuse. If you need code re-use alone, consider using delegation, rather than inheritance. I suppose you could introduce an intermediate type between a sub-type and its parent, but I would let that possibility drive my design.
Second, if you need to use functionality from the base class but extend it AND the use case calls for a static method, then you might want to consider using some external class to hold the functionality. The classic case for this in my mind is the Factory pattern. One way to implement the Factory pattern is through Factory Methods, a static method on a class that constructs an instance of that class. Usually the constructor is protected so that the factory method is the only way to build the class from outside.
One way to approach re-use with Factory Methods in an inheritance hierarchy would be to put the common code in a protected method and call that method from the Factory Method rather than directly call the base class Factory Method from a sub-types Factory Method. A better implementation might use the same technique but move the Factory Methods to a Factory class and use the constructor logic (internal now, not private), perhaps in conjunction with an initialization method(s), to create the object. If the behavior you are inheriting is external from the class (decryption/validation/etc), you can use shared methods (or composition) within the Factory to allow re-use between the Factory methods.
Without knowing the goal of your use of static methods it's difficult to give you an exact direction, but hopefully this will help.
Static methods are not polymorphic, so what you want to do is impossible.
Trying to find a way to treat static methods as polymorphic is possible but dangerous, because the language itself doesn't support it.
Some suggestions:
Reflection
Aliasing the base class (such as Mehrdad's example)
Having into account that a Static method shoudn't relay in instance data... you should have a static "MyMethod" that behaves diferent based on a parameter or something like that.
Remember that the static method defined in one class is just a way to order your code... but it is the same if that method is placed elsewhere... because it don't relay on the object where it is placed.
EDIT: I think your best choise is to use Product.MyMethod explicitly... If you think it... it should't be probable that your Widget change its base clase... and also in that case it is a minor change in code.
Static methods are "Class-Level" methods. They are intended to be methods that apply to all instances of a specific class. Thus, inheritance of this method does not make sense as it would change the meaning to apply to instances of the class. For instance, if you were looping through a collection of Products (some of Widget, some not) and called MyMethod on each Product then the method that was called would be determined by the instance of the class. This violates the purpose of static methods.
You can probably do something like you want more cleanly by simply not using static methods at all, because in your example it does not seem like MyMethod applies to all instances of Product. However, you can achieve the same affect you are describing by using an interface class like ‘IMyMethod’. This approach is still not using a static method. I guess I’m not seeing a need for a static method. Why do you want to use a static method to begin with?
It's very simple. Much simpler than using aliasing, reflections, etc. Maybe it's been made easier in newer additions of .NET, IDK, but this works perfectly fine. Just like with instance methods, accessing base methods doesn't require base be used, it is optional, usually necessary when the inheriting and base class have a method of the same name. Even without the base keyword, you can access the static methods of a base class as if they were in the class you are calling from. I only tested this when calling a base static method from a derived class's static method. Might not work if you are calling from an instance method to a static method.
public class BaseThings
{
protected static void AssertPermissions()
{
//something
}
}
public class Person:BaseThings
{
public static void ValidatePerson(Person person)
{
//just call the base static method as if it were in this class.
AssertPermissions();
}
}