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Doing a simple unit test in Visual Studio in a Windows Forms application [duplicate]

Visual Studio allows unit testing of private methods via an automatically generated accessor class. I have written a test of a private method that compiles successfully, but it fails at runtime. A fairly minimal version of the code and the test is:
//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
}
The runtime error is:
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]'.
According to intellisense - and hence I guess the compiler - target is of type TypeA_Accessor. But at runtime it is of type TypeA, and hence the list add fails.
Is there any way I can stop this error? Or, perhaps more likely, what other advice do other people have (I predict maybe "don't test private methods" and "don't have unit tests manipulate the state of objects").

You can use the PrivateObject class:
Class target = new Class();
PrivateObject obj = new PrivateObject(target);
var retVal = obj.Invoke("PrivateMethod");
Assert.AreEqual(expectedVal, retVal);
Note: PrivateObject and PrivateType are not available for projects targeting netcoreapp2.0 - GitHub Issue 366

“There is nothing called as standard or best practice, probably they are just popular opinions”.
Same holds true for this discussion as well.
It all depends on what you think is a unit , if you think UNIT is a class then you will only hit the public method. If you think UNIT is lines of code hitting private methods will not make you feel guilty.
If you want to invoke private methods you can use "PrivateObject" class and call the invoke method. You can watch this indepth youtube video ( http://www.youtube.com/watch?v=Vq6Gcs9LrPQ ) which shows how to use "PrivateObject" and also discusses if testing of private methods are logical or not.

Another thought here is to extend testing to "internal" classes/methods, giving more of a white-box sense of this testing. You can use InternalsVisibleTo attribute on the assembly to expose these to separate unit testing modules.
In combination with sealed class you can approach such encapsulation that test method are visible only from unittest assembly your methods. Consider that protected method in sealed class is de facto private.
[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()
{
}
}
}
And unit test:
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
}
}

One way to test private methods is through reflection. This applies to NUnit and XUnit, too:
MyObject objUnderTest = new MyObject();
MethodInfo methodInfo = typeof(MyObject).GetMethod("SomePrivateMethod", BindingFlags.NonPublic | BindingFlags.Instance);
object[] parameters = {"parameters here"};
methodInfo.Invoke(objUnderTest, parameters);

Ermh... Came along here with exactly the same problem: Test a simple, but pivotal private method. After reading this thread, it appears to be like "I want to drill this simple hole in this simple piece of metal, and I want to make sure the quality meets the specs", and then comes "Okay, this is not to easy. First of all, there is no proper tool to do so, but you could build a gravitational-wave observatory in your garden. Read my article at http://foobar.brigther-than-einstein.org/ First, of course, you have to attend some advanced quantum physics courses, then you need tons of ultra-cool nitrogenium, and then, of course, my book available at Amazon"...
In other words...
No, first things first.
Each and every method, may it private, internal, protected, public has to be testable. There has to be a way to implement such tests without such ado as was presented here.
Why? Exactly because of the architectural mentions done so far by some contributors. Perhaps a simple reiteration of software principles may clear up some missunderstandings.
In this case, the usual suspects are: OCP, SRP, and, as always, KIS.
But wait a minute. The idea of making everything publicly available is more of less political and a kind of an attitude. But. When it comes to code, even in then Open Source Community, this is no dogma. Instead, "hiding" something is good practice to make it easier to come familiar with a certain API. You would hide, for example, the very core calculations of your new-to-market digital thermometer building block--not to hide the maths behind the real measured curve to curious code readers, but to prevent your code from becoming dependent on some, perhaps suddenly important users who could not resist using your formerly private, internal, protected code to implement their own ideas.
What am I talking about?
private double TranslateMeasurementIntoLinear(double actualMeasurement);
It's easy to proclaim the Age of Aquarius or what is is been called nowadays, but if my piece of sensor gets from 1.0 to 2.0, the implementation of Translate... might change from a simple linear equation that is easily understandable and "re-usable" for everybody, to a pretty sophisticated calculation that uses analysis or whatever, and so I would break other's code. Why? Because they didn't understand the very priciples of software coding, not even KIS.
To make this fairy tale short: We need a simple way to test private methods--without ado.
First: Happy new year everyone!
Second: Rehearse your architect lessons.
Third: The "public" modifier is religion, not a solution.

Another option that has not been mentioned is just creating the unit test class as a child of the object that you are testing. NUnit Example:
[TestFixture]
public class UnitTests : ObjectWithPrivateMethods
{
[Test]
public void TestSomeProtectedMethod()
{
Assert.IsTrue(this.SomeProtectedMethod() == true, "Failed test, result false");
}
}
This would allow easy testing of private and protected (but not inherited private) methods, and it would allow you to keep all your tests separate from the real code so you aren't deploying test assemblies to production. Switching your private methods to protected methods would be acceptable in a lot of inherited objects, and it is a pretty simple change to make.
HOWEVER...
While this is an interesting approach to solving the problem of how to test hidden methods, I am unsure that I would advocate that this is the correct solution to the problem in all cases. It seems a little odd to be internally testing an object, and I suspect there might be some scenarios that this approach will blow up on you. (Immutable objects for example, might make some tests really hard).
While I mention this approach, I would suggest that this is more of a brainstormed suggestion than a legitimate solution. Take it with a grain of salt.
EDIT: I find it truly hilarious that people are voting this answer down, since I explicitly describe this as a bad idea. Does that mean that people are agreeing with me? I am so confused.....

From the book Working Effectively with Legacy Code:
"If we need to test a private method, we should make it public. If
making it public bothers us, in most cases, it means that our class is
doing too much and we ought to fix it."
The way to fix it, according to the author, is by creating a new class and adding the method as public.
The author explains further:
"Good design is testable, and design that isn't testable is bad."
So, within these limits, your only real option is to make the method public, either in the current or a new class.

I use this helper (object type extension)
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);
}
You can call it like this
Calculator systemUnderTest = new Calculator();
int result = systemUnderTest.CallPrivateMethod<int>("PrivateAdd",1,8);
One of the advantages is that it uses generics to pre-determine return type.

It's 2022 now!
...and we have .NET6
While this does not really answer the question, my preferred approach these days is to collocate code and test in the same C# project, with naming convention like <ClassName>.Tests.cs. Then I use internal access modifier instead of private.
In the project file, I have something like this:
<ItemGroup Condition="'$(Configuration)' == 'Release'">
<Compile Remove="**\*.Tests.cs" />
</ItemGroup>
to exclude the test files in release builds. Modify as needed.
FAQ 1: But sometimes you want to also test code in Release (optimized) build.
Answer: I find it unnecessary. I trust that the compiler will do its job without messing up my intent. So far, I've had no reason to question its ability to do so.
FAQ 2: But I really want to keep the method (or class) private.
Answer: Lots of excellent solutions in this page to try out. In my experience, having access modifier set to internal is usually more than enough since the method (or class) won't be visible outside the project it's defined. Beyond that, there's nothing more to hide.

Extract private method to another class, test on that class; read more about SRP principle (Single Responsibility Principle)
It seem that you need extract to the private method to another class; in this should be public. Instead of trying to test on the private method, you should test public method of this another class.
We has the following scenario:
Class A
+ outputFile: Stream
- _someLogic(arg1, arg2)
We need to test the logic of _someLogic; but it seem that Class A take more role than it need(violate the SRP principle); just refactor into two classes
Class A1
+ A1(logicHandler: A2) # take A2 for handle logic
+ outputFile: Stream
Class A2
+ someLogic(arg1, arg2)
In this way someLogic could be test on A2; in A1 just create some fake A2 then inject to constructor to test that A2 is called to the function named someLogic.

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;
}
}
Call it this way:
Type[] paramTypes = new Type[] { typeof(OrderTender), typeof(string) };
var parameters = new object[] { orderTender, OrderErrorReasonNames.FailedToCloneTransaction };
myClass.InvokePrivateMethodWithReturnType("myPrivateMethodName", paramTypes, parameters);

In VS 2005/2008 you can use private accessor to test private member,but this way was disappear in later version of VS

You can use nested classes to test private methods. For example (NUnit v3 is used):
internal static class A
{
// ... other code
private static Int32 Sum(Int32 a, Int32 b) => a + b;
[TestFixture]
private static class UnitTests
{
[Test]
public static void OnePlusTwoEqualsThree()
{
Assert.AreEqual(3, Sum(1, 2));
}
}
}
Furthermore tests related code can be moved to another file using 'partial class' feature, excluded from release builds using 'conditional compilation', etc. Advanced example:
File A.cs
internal static partial class A
{
// ... other code
private static Int32 Sum(Int32 a, Int32 b) => a + b;
}
File A.UnitTests.cs
#if UNIT_TESTING
partial class A
{
[TestFixture]
private static class UnitTests
{
[Test]
public static void OnePlusTwoEqualsThree()
{
Assert.AreEqual(3, Sum(1, 2));
}
}
}
#endif

I had another approach that it works for me. because I always run my tests in debug mode so I used #if DEBUG to add public before my private method. so my private method is like this:
public class Test
{
#if (DEBUG)
public
#endif
string PrivateMehtod()
{
return "PrivateMehtod called";
}
}

Sadly there is no PrivateObject class in .net6
However I wrote a small extension method capable of invoking private methods using reflection.
Have a look at the sample code:
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"
And here is the implementation of the extension method:
/// <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}\"");
}

If PrivateObject is not available and if the class under test is not a sealed class, you can make the methods and properties you want to expose protected. Create an inherited class in the unit test file with internal methods that expose the private methods/properties under test.
If the class under test is:
class MyClass{private string GetStr(string x, int y) => $"Success! {x} {y}";}
Change it to:
class MyClass{protected string GetStr(string x, int y) => $"Success! {x} {y}";}
In your unit test file create an inherited class something like this:
class MyClassExposed: MyClass
{
internal string ExposedGetStr(string x, int y)
{
return base.GetStr(x, y);
}
}
Now you can use the inherited class MyClassExposed to test the exposed methods and properties.

.NET doesn't allow use of Accessors anymore. You can use the code I posted here for an answer to a similar question.
How do you unit test private methods?

How to mock static method in static class in C#? [duplicate]

Recently, I've begun to use Moq to unit test. I use Moq to mock out classes that I don't need to test.
How do you typically deal with static methods?
public void foo(string filePath)
{
File f = StaticClass.GetFile(filePath);
}
How could this static method, StaticClass.GetFile() get mocked?
P.S. I'd appreciate any reading materials you recommend on Moq and Unit Testing.

#Pure.Krome: good response but I will add a few details
#Kevin: You have to choose a solution depending on the changes that you can bring to the code.
If you can change it, some dependency injection make the code more testable.
If you can't, you need a good isolation.
With free mocking framework (Moq, RhinoMocks, NMock...) you can only mock delegates, interfaces and virtual methods. So, for static, sealed and non-virtual methods you have 3 solutions:
TypeMock Isolator (can mock everything but it's expensive)
JustMock of Telerik (new comer, less expensive but still not
free)
Moles of Microsoft (the only free solution for isolation)
I recommend Moles, because it's free, efficient and use lambda expressions like Moq. Just one important detail: Moles provide stubs, not mocks. So you may still use Moq for interface and delegates ;)
Mock: a class that implements an interface and allows the ability to dynamically set the values to return/exceptions to throw from particular methods and provides the ability to check if particular methods have been called/not called.
Stub: Like a mock class, except that it doesn't provide the ability to verify that methods have been called/not called.

Mocking frameworks like Moq or Rhinomocks can only create mock instances of objects, this means mocking static methods is not possible.
You can also search Google for more info.
Also, there's a few questions previously asked on StackOverflow here, here and here.

There is a possibility in .NET excluding MOQ and any other mocking library. You have to right click on solution explorer on assembly containing static method you want to mock and choose Add Fakes Assembly. Next you can freely mock that assembly static methods.
Assume that you want to mock System.DateTime.Now static method. Do this for instance this way:
using (ShimsContext.Create())
{
System.Fakes.ShimDateTime.NowGet = () => new DateTime(1837, 1, 1);
Assert.AreEqual(DateTime.Now.Year, 1837);
}
You have similar property for each static property and method.

You can achieve this with Pose library available from nuget. It allows you to mock, among other things, static methods. In your test method write this:
Shim shim = Shim.Replace(() => StaticClass.GetFile(Is.A<string>()))
.With((string name) => /*Here return your mocked value for test*/);
var sut = new Service();
PoseContext.Isolate(() =>
result = sut.foo("filename") /*Here the foo will take your mocked implementation of GetFile*/, shim);
For further reading refer here https://medium.com/#tonerdo/unit-testing-datetime-now-in-c-without-using-interfaces-978d372478e8

I liked Pose but couldn't get it to stop throwing InvalidProgramException which appears to be a known issue. Now I'm using Smocks like this:
Smock.Run(context =>
{
context.Setup(() => DateTime.Now).Returns(new DateTime(2000, 1, 1));
// Outputs "2000"
Console.WriteLine(DateTime.Now.Year);
});

I've been playing around with a concept of refactoring the static methods to invoke a delegate which you can externally set for testing purposes.
This would not use any testing framework and would be a completely bespoke solution however the refactor will not influence the signature of your caller and so it would be a relatively safe.
For this to work, you would need to have access to the static method, so it wouldn't work for any external libraries such as System.DateTime.
Heres an example I've been playing with where I've created a couple of static methods, one with a return type that takes in two parameters and one generic which has no return type.
The main static class:
public static class LegacyStaticClass
{
// A static constructor sets up all the delegates so production keeps working as usual
static LegacyStaticClass()
{
ResetDelegates();
}
public static void ResetDelegates()
{
// All the logic that used to be in the body of the static method goes into the delegates instead.
ThrowMeDelegate = input => throw input;
SumDelegate = (a, b) => a + b;
}
public static Action<Exception> ThrowMeDelegate;
public static Func<int, int, int> SumDelegate;
public static void ThrowMe<TException>() where TException : Exception, new()
=> ThrowMeDelegate(new TException());
public static int Sum(int a, int b)
=> SumDelegate(a, b);
}
The Unit Tests (xUnit and Shouldly)
public class Class1Tests : IDisposable
{
[Fact]
public void ThrowMe_NoMocking_Throws()
{
Should.Throw<Exception>(() => LegacyStaticClass.ThrowMe<Exception>());
}
[Fact]
public void ThrowMe_EmptyMocking_DoesNotThrow()
{
LegacyStaticClass.ThrowMeDelegate = input => { };
LegacyStaticClass.ThrowMe<Exception>();
true.ShouldBeTrue();
}
[Fact]
public void Sum_NoMocking_AddsValues()
{
LegacyStaticClass.Sum(5, 6).ShouldBe(11);
}
[Fact]
public void Sum_MockingReturnValue_ReturnsMockedValue()
{
LegacyStaticClass.SumDelegate = (a, b) => 6;
LegacyStaticClass.Sum(5, 6).ShouldBe(6);
}
public void Dispose()
{
LegacyStaticClass.ResetDelegates();
}
}

Using Microsoft Fakes:
Add the fakes assembly, then if you have this static method...
//code under test
public static class MyClass {
public static int MyMethod() {
...
}
}
... you can mock it like this:
// unit test code
using (ShimsContext.Create())
{
ShimMyClass.MyMethod = () => 5;
}
Source: https://learn.microsoft.com/en-us/visualstudio/test/using-shims-to-isolate-your-application-from-other-assemblies-for-unit-testing?view=vs-2019#static-methods

How to mock IMemoryCache instance methods using Moq that are being hidden by extension methods in C#? [duplicate]

I have a preexisting Interface...
public interface ISomeInterface
{
void SomeMethod();
}
and I've extended this intreface using a mixin...
public static class SomeInterfaceExtensions
{
public static void AnotherMethod(this ISomeInterface someInterface)
{
// Implementation here
}
}
I have a class thats calling this which I want to test...
public class Caller
{
private readonly ISomeInterface someInterface;
public Caller(ISomeInterface someInterface)
{
this.someInterface = someInterface;
}
public void Main()
{
someInterface.AnotherMethod();
}
}
and a test where I'd like to mock the interface and verify the call to the extension method...
[Test]
public void Main_BasicCall_CallsAnotherMethod()
{
// Arrange
var someInterfaceMock = new Mock<ISomeInterface>();
someInterfaceMock.Setup(x => x.AnotherMethod()).Verifiable();
var caller = new Caller(someInterfaceMock.Object);
// Act
caller.Main();
// Assert
someInterfaceMock.Verify();
}
Running this test however generates an exception...
System.ArgumentException: Invalid setup on a non-member method:
x => x.AnotherMethod()
My question is, is there a nice way to mock out the mixin call?

I have used a Wrapper to get around this problem. Create a wrapper object and pass your mocked method.
See Mocking Static Methods for Unit Testing by Paul Irwin, it has nice examples.

You can't "directly" mock static method (hence extension method) with mocking framework. You can try Moles (http://research.microsoft.com/en-us/projects/pex/downloads.aspx), a free tool from Microsoft that implements a different approach.
Here is the description of the tool:
Moles is a lightweight framework for test stubs and detours in .NET that is based on delegates.
Moles may be used to detour any .NET method, including non-virtual/static methods in sealed types.
You can use Moles with any testing framework (it's independent about that).

I found that I had to discover the inside of the extension method I was trying to mock the input for, and mock what was going on inside the extension.
I viewed using an extension as adding code directly to your method. This meant I needed to mock what happens inside the extension rather than the extension itself.

I like to use the wrapper (adapter pattern) when I am wrapping the object itself. I'm not sure I'd use that for wrapping an extension method, which is not part of the object.
I use an internal Lazy Injectable Property of either type Action, Func, Predicate, or delegate and allow for injecting (swapping out) the method during a unit test.
internal Func<IMyObject, string, object> DoWorkMethod
{
[ExcludeFromCodeCoverage]
get { return _DoWorkMethod ?? (_DoWorkMethod = (obj, val) => { return obj.DoWork(val); }); }
set { _DoWorkMethod = value; }
} private Func<IMyObject, string, object> _DoWorkMethod;
Then you call the Func instead of the actual method.
public object SomeFunction()
{
var val = "doesn't matter for this example";
return DoWorkMethod.Invoke(MyObjectProperty, val);
}
For a more complete example, check out http://www.rhyous.com/2016/08/11/unit-testing-calls-to-complex-extension-methods/

If you just want to make sure that the extension method was invoked, and you aren't trying to setup a return value, then you can check the Invocations property on the mocked object.
Like this:
var invocationsCount = mockedObject.Invocations.Count;
invocationsCount.Should().BeGreaterThan(0);

Reason why it is not possible to mock an extension method is already given in good answers. I am just trying to give another possible solution with this answer: Extract a protected, virtual method with the call to the extension method and create a setup for this method in the test class/method by using a proxy.
public class Foo
{
public void Method()
=> CallToStaticMethod();
protected virtual void CallToStaticMethod()
=> StaticClass.StaticMethod();
}
and test
[TestMethod]
public void MyTestMethod()
{
var expected = new Exception("container exception");
var proxy = new Mock<Foo>();
proxy.Protected().Setup("CallToStaticMethod").Throws(expected);
var actual = Assert.ThrowsException<Exception>(() => proxy.Object.Foo());
Assert.AreEqual(expected, actual);
}

In my case extension method is a method around some public method of my class. So I checked call of that internal method. That approach is similar to Alvis answer (above).

So if you are using Moq, and want to mock the result of an Extension method, then you can use SetupReturnsDefault<ReturnTypeOfExtensionMethod>(new ConcreteInstanceToReturn()) on the instance of the mock class that has the extension method you are trying to mock.
It is not perfect, but for unit testing purposes it works well.

When to declare the entire class as static

I have a math helper class where every single function is static, i.e., params fed in as arguments, value returned. Should I declare the entire class as static? Would adding the static modifier to the class make a difference in performance?
Also, I am not sure what this guideline means in: "do not treat static classes as a miscellaneous bucket." - I have a few classes that are just a bunch of miscellaneous static functions...

It's perfectly fine to make classes like that static, in fact if you look at System.Math you'll see it's static as well:
public static class Math
What the guideline is trying to say is you should not put every static method you have to one static class which would do everything and play a role of a bucket for static methods. Instead, if it's appropriate, create smaller util classes with methods related to the same functionality, like it's done with System.Math and couple more within BCL as well.

Should I declare the entire class as static?
Yes. Adding static to a class says that it contains only static members and that you can't ever instantiate it. Without it, users of your class might get confused and try to create an instance or variable of your class. With static, that's not possible.
It seems like this is exactly your case.
Would adding the static modifier to the class make a difference in performance?
No, call to a static method will always have the same performance characteristics, it doesn't matter whether the containing class is static or not. Actually, the whole concept of static classes doesn't exist at the CIL level, they're just sealed abstract classes (a combination that wouldn't compile in C#).
But even if there was a difference, it would be tiny. Don't optimize prematurely, especially when it comes to micro-optimizations.

Helper classes are normally static classes, so that you don't need to instantiate them. There is no great cost in instantiating a managed .NET object (especially helper classes), it is just a matter of convenience.
It is extremely tempting to just put together a static class with minimal helper methods and get the job done. They have their place in code, and can be used especially when there is deterministic input/output. e.g. ComputeHash of a string, Find Average of numbers etc.
But the one reason, Static classes are discouraged is because they normally interfere with unit testing and present all sorts of problems. (Fakes, Moles, Private Accessors etc.)
An interfaced based approach for even helper classes, helps with the unit testing of the overall code. This is especially true for big projects which involve workflows such that the static helper methods are only a part of the workflow.
e.g. Suppose you need to check if the current year is a leap year. It is tempting to write a quick static method.
public static class DateHelper
{
public static bool IsLeapYear()
{
var currentDate = DateTime.UtcNow;
// check if currentDate's year is a leap year using some unicorn logic
return true; // or false
}
}
and if this method is used in your code somewhere like:
public class Birthday
{
public int GetLeapYearDaysData()
{
// some self-logic..
// now call our static method
var isLeapYear = DateHelper.IsLeapYear();
// based on this value, you might return 100 or 200.
if (isLeapYear)
{
return 100;
}
return 200;
}
}
Now, if you go and try to unit test this method public int GetLeapYearDaysData(), you might end up in trouble since the return value is indeterminate.. i.e. depends on the current year and it is not recommended to have unit tests behaving unpredictably/deteriorate as time progresses.
// this unit test is flaky
[Test]
public void TestGetLeapYearDaysData()
{
var expected = 100;
// we don't know if this method will return 100 or 200.
var actual = new Birthday().GetLeapYearDaysData();
Assert.AreEqual(expected, actual);
}
The above problem happens because we cannot control/mock the method IsLeapYear() in the above code. so we're at its mercy.
Now imagine the following design:
public interface IDateHelper
{
bool IsLeapYear();
}
public class DateHelper : IDateHelper
{
public bool IsLeapYear()
{
var currentDate = DateTime.UtcNow;
// check if currentDate's year is a leap year using some unicorn logic
return true; // or false
}
}
Now our birthday class can be injected with a helper:
public class Birthday
{
private IDateHelper _dateHelper;
// any caller can inject their own version of dateHelper.
public Birthday(IDateHelper dateHelper)
{
this._dateHelper = dateHelper;
}
public int GetLeapYearDaysData()
{
// some self-logic..
// now call our injected helper's method.
var isLeapYear = this._dateHelper.IsLeapYear();
// based on this value, you might return 100 or 200.
if (isLeapYear)
{
return 100;
}
return 200;
}
}
// now see how are unit tests can be more robust and reliable
// this unit test is more robust
[Test]
public void TestGetLeapYearDaysData()
{
var expected = 100;
// use any mocking framework or stubbed class
// to reliably tell the unit test that 100 needs to be returned.
var mockDateHelper = new Mock<IDateHelper>();
// make the mock helper return true for leap year check.
// we're no longer at the mercy of current date time.
mockDateHelper.Setup(m=>m.IsLeapYear()).Returns(true);
// inject this mock DateHelper in our BirthDay class
// we know for sure the value that'll be returned.
var actual = new Birthday(mockDateHelper).GetLeapYearDaysData();
Assert.AreEqual(expected, actual);
}
As you can see, the moment the helper methods were Interface based, they were easily testable. Over the course of a big project, many such smaller static methods ultimately result in bottlenecks in testing key functional flows.
So it pays to be aware of this pitfall in advance and make the additional investment upfront. Basically identify what classes/methods need to be static and what shouldn't be.

It all starts from when should I have a static method, and that is when you don't have any dependency on instance variables.
Now that said if none of you methods are depending on instance variable, you can make your class static.
Static class serve several benefits, and many more.

Mocking Static Methods

Recently, I've begun to use Moq to unit test. I use Moq to mock out classes that I don't need to test.
How do you typically deal with static methods?
public void foo(string filePath)
{
File f = StaticClass.GetFile(filePath);
}
How could this static method, StaticClass.GetFile() get mocked?
P.S. I'd appreciate any reading materials you recommend on Moq and Unit Testing.

#Pure.Krome: good response but I will add a few details
#Kevin: You have to choose a solution depending on the changes that you can bring to the code.
If you can change it, some dependency injection make the code more testable.
If you can't, you need a good isolation.
With free mocking framework (Moq, RhinoMocks, NMock...) you can only mock delegates, interfaces and virtual methods. So, for static, sealed and non-virtual methods you have 3 solutions:
TypeMock Isolator (can mock everything but it's expensive)
JustMock of Telerik (new comer, less expensive but still not
free)
Moles of Microsoft (the only free solution for isolation)
I recommend Moles, because it's free, efficient and use lambda expressions like Moq. Just one important detail: Moles provide stubs, not mocks. So you may still use Moq for interface and delegates ;)
Mock: a class that implements an interface and allows the ability to dynamically set the values to return/exceptions to throw from particular methods and provides the ability to check if particular methods have been called/not called.
Stub: Like a mock class, except that it doesn't provide the ability to verify that methods have been called/not called.

Mocking frameworks like Moq or Rhinomocks can only create mock instances of objects, this means mocking static methods is not possible.
You can also search Google for more info.
Also, there's a few questions previously asked on StackOverflow here, here and here.

There is a possibility in .NET excluding MOQ and any other mocking library. You have to right click on solution explorer on assembly containing static method you want to mock and choose Add Fakes Assembly. Next you can freely mock that assembly static methods.
Assume that you want to mock System.DateTime.Now static method. Do this for instance this way:
using (ShimsContext.Create())
{
System.Fakes.ShimDateTime.NowGet = () => new DateTime(1837, 1, 1);
Assert.AreEqual(DateTime.Now.Year, 1837);
}
You have similar property for each static property and method.

You can achieve this with Pose library available from nuget. It allows you to mock, among other things, static methods. In your test method write this:
Shim shim = Shim.Replace(() => StaticClass.GetFile(Is.A<string>()))
.With((string name) => /*Here return your mocked value for test*/);
var sut = new Service();
PoseContext.Isolate(() =>
result = sut.foo("filename") /*Here the foo will take your mocked implementation of GetFile*/, shim);
For further reading refer here https://medium.com/#tonerdo/unit-testing-datetime-now-in-c-without-using-interfaces-978d372478e8

I liked Pose but couldn't get it to stop throwing InvalidProgramException which appears to be a known issue. Now I'm using Smocks like this:
Smock.Run(context =>
{
context.Setup(() => DateTime.Now).Returns(new DateTime(2000, 1, 1));
// Outputs "2000"
Console.WriteLine(DateTime.Now.Year);
});

I've been playing around with a concept of refactoring the static methods to invoke a delegate which you can externally set for testing purposes.
This would not use any testing framework and would be a completely bespoke solution however the refactor will not influence the signature of your caller and so it would be a relatively safe.
For this to work, you would need to have access to the static method, so it wouldn't work for any external libraries such as System.DateTime.
Heres an example I've been playing with where I've created a couple of static methods, one with a return type that takes in two parameters and one generic which has no return type.
The main static class:
public static class LegacyStaticClass
{
// A static constructor sets up all the delegates so production keeps working as usual
static LegacyStaticClass()
{
ResetDelegates();
}
public static void ResetDelegates()
{
// All the logic that used to be in the body of the static method goes into the delegates instead.
ThrowMeDelegate = input => throw input;
SumDelegate = (a, b) => a + b;
}
public static Action<Exception> ThrowMeDelegate;
public static Func<int, int, int> SumDelegate;
public static void ThrowMe<TException>() where TException : Exception, new()
=> ThrowMeDelegate(new TException());
public static int Sum(int a, int b)
=> SumDelegate(a, b);
}
The Unit Tests (xUnit and Shouldly)
public class Class1Tests : IDisposable
{
[Fact]
public void ThrowMe_NoMocking_Throws()
{
Should.Throw<Exception>(() => LegacyStaticClass.ThrowMe<Exception>());
}
[Fact]
public void ThrowMe_EmptyMocking_DoesNotThrow()
{
LegacyStaticClass.ThrowMeDelegate = input => { };
LegacyStaticClass.ThrowMe<Exception>();
true.ShouldBeTrue();
}
[Fact]
public void Sum_NoMocking_AddsValues()
{
LegacyStaticClass.Sum(5, 6).ShouldBe(11);
}
[Fact]
public void Sum_MockingReturnValue_ReturnsMockedValue()
{
LegacyStaticClass.SumDelegate = (a, b) => 6;
LegacyStaticClass.Sum(5, 6).ShouldBe(6);
}
public void Dispose()
{
LegacyStaticClass.ResetDelegates();
}
}

Using Microsoft Fakes:
Add the fakes assembly, then if you have this static method...
//code under test
public static class MyClass {
public static int MyMethod() {
...
}
}
... you can mock it like this:
// unit test code
using (ShimsContext.Create())
{
ShimMyClass.MyMethod = () => 5;
}
Source: https://learn.microsoft.com/en-us/visualstudio/test/using-shims-to-isolate-your-application-from-other-assemblies-for-unit-testing?view=vs-2019#static-methods