Stubs and Shims

Stubs and Shims

Stubs: Stubs replace the functionality with the different implementation. One should have interface based programming i.e. we cannot directly instantiate a class, instead you should be using the interface (which helps in replacing the original functionality with the stub, based on the same interface).

 

Shims: Replace the functionality provided by .NET framework. It modifies the complied code at runtime to replace the method call.

 

 

Stubs Example: The functionality in class EmployeeRepositrywill search for an employee in the list. This class implement the interface IEmployeeRepositry

 

public class Employee

    {

public string FirstName { get; set; }

public string LastName { get; set; }

    }

 

public interface IEmployeeRepositry

    {

Employee FindEmployee(Employee emp);

    }

 

public class EmployeeRepositry : IEmployeeRepositry

    {

 

    List<Employee> list;

    public EmployeeRepositry()

        {

             list = new List<Employee>{new Employee{FirstName = "Girish",LastName = "Goudar"  },

                                    new Employee{FirstName = "Deepak",LastName = "H" }};

        }

    public Employee FindEmployee(Employee emp)

        {

            return list.Find(item =>item.FirstName.Equals(emp.FirstName));

        }

 

    }

**Unit testing without stubs:**Unit test calls the Employee repository FindEmployeemethod to search for an employee

** **

**           ** publicvoid TestMethod1()

        {

            Employeeemp = newEmployee();

            emp.FirstName = "Girish";

            emp.LastName = "Singh";

            IEmployeeRepositryrepositry = newEmployeeRepositry();

            Employeeemployee = repositry.FindEmployee(emp);

            Assert.AreEqual(emp.FirstName,employee.FirstName);

 

        }

    }

Unit testing with stubs: Generate the fakes assembly using the Microsoft fakes framework and the stub methods will be generated as shown.

** **

 

** **

The stub method is created as “FindEmployeeEmployee”.The name convention used will be first the name of the method and then the type of parameter name

 

** **

** **

The Interface IEmployeeRepositry this time will use the stub employee repository and provide its own implementation of the FindEmployee method.

** **

** **

**           ** publicvoid TestMethod1()

        {

        Employee emp = new Employee();

        emp.FirstName = "Girish";

        emp.LastName = "Singh";

**        IEmployeeRepositry repositry = new StubIEmployeeRepositry()**

**            {**

**                FindEmployeeEmployee = emp1 =>**

**                    {**

**                        return new Employee { FirstName ="Mohan",LastName ="v"};**

**                    }**

**            };**

        Employee employee = repositry.FindEmployee(emp);

        Assert.AreEqual(emp.FirstName,employee.FirstName);

 

        }

 

**Shims Example:**The functionality in class EmployeeRepositrywill search for an employee in the list.Shims can be used to replace calls to assemblies that you cannot modify, such .NET assemblies.

public class EmployeeRepositry

    {

 List<Employee> list;

public EmployeeRepositry()

        {

             list = newList<Employee>{newEmployee{FirstName = "Girish",LastName = "Goudar" },

                                    newEmployee{FirstName = "Deepak",LastName = "H" }};

        }

public Employee FindEmployee(Employee emp)

        {

returnlist.Find(item =>item.FirstName.Equals(emp.FirstName));

        }

**Unit testing with shims:**Consider  the EmployeeRepositryClass to be part of3rd party solution or legacy code which cannot be refactored by us. Use the shims created by Microsoft  fakes framework as shown.

publicvoid TestMethod1()

        {

 

    Employee emp = new Employee();

    emp.FirstName = "Girish";

    emp.LastName = "Singh";

    using (ShimsContext.Create())

            {

            EmployeeRepositryrepositry = new ShimEmployeeRepositry()

                {

                FindEmployeeEmployee = emp1 =>

                        {

                            return new Employee { FirstName = "Girish", LastName = "v" };

                        }

                };

                Employee employee = repositry.FindEmployee(emp);

                Assert.AreEqual(emp.FirstName, employee.FirstName);

            }

        }

Difference between stubs and shims

** **

	Stubs	Shims
Static methods, sealed types	You can only use stubs to implement interfaces. Therefore, stub types cannot be used for static methods, non-virtual methods, sealed virtual methods, methods in sealed types	Shims can be used for Static methods and sealed types
Internal types	Stubs can be used with internal types that are made accessible by using the assembly attribute InternalsVisibleToAttribute.	Shims can be used with internal types that are made accessible by using the assembly attribute InternalsVisibleToAttribute.
Private methods	Stubs can only replace visible methods.	Shims can replace calls to private methods if all the types on the method signature are visible.
Interfaces and abstract methods	Stubs provide implementations of interfaces and abstract methods that can be used in testing	Shims can’t instrument interfaces and abstract methods, because they don’t have method bodies.
Performance	Stubs do not have this performance overhead and are as fast as virtual methods can go	Shims run slower because they rewrite your code at run time