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What Is Unit test?

A unit test verifies that a function or set of functions meet the requirements.  Unit tests inspect both black boxes or white boxes. NUnitis a unit-testing framework for all .Net languages.
Here is an example. Let’s take a look this class:

namespace Bank
{
    public class Account
    {
        private float balance;

        public void Deposit(float amount)
        {
            balance += amount;
        }
        public void Withdraw(float amount)
        {
            balance -= amount;
        }
        public void TransferFunds(Account destination, float amount)
        {
            destination.Deposit(amount);
            Withdraw(amount);

        }
        public float Balance
        {
            get { return balance; }
        }
    }
}

for this class we make corresponding unit test class like this:

using NUnit.Framework;
using Bank;

namespace Test
{
    [TestFixture]
    public class AccountTests
    {
        [Test]
        public void TestTransferFunds()
        {
            Account source = new Account();
            source.Deposit(200.00F);
            Account destination = new Account();
            destination.Deposit(150.00F);

            source.TransferFunds(destination, 100.00F);

            //Here are assertions
            //we check actual value with expected value
            Assert.AreEqual(250.00F, destination.Balance);
            Assert.AreEqual(100.00F, source.Balance);

        }
  }
}

Assertions are central part of unit testing in any of the xUnit frameworks. NUnit provides a rich set of assertions as static methods of the Assert class.
Example:
Assert.AreEqual( int expected, int actual );
There are:

• StringAssert
• CollectionAssert
• FileAssert

From version 2.0 on, NUnit has used custom attributes.
All NUnit attributes are contained in the NUnit.Framework namespace. Each source file that contains tests must include a using statement for that namespace and the project must reference the framework assembly, nunit.framework.dll.

SetUpFixture Attribute

This is the attribute that marks a class that contains the one-time setup or teardown methods for all the test fixtures under a given namespace. The class may contain at most one method marked with the SetUpAttribute and one method marked with the TearDownAttribute.
Example:

[SetUpFixture]
public class MySetUpClass
{
  [SetUp]
  RunBeforeAnyTests()
  {
    // ...
  } 

  [TearDown]
  RunAfterAnyTests()
  {
    // ...
  }
}

Test Attribute (NUnit 2.0)

The Test attribute marks a specific method inside a class that has already been marked as a TestFixture, as a test method. The signature for a test method is defined as follows:
public void MethodName()

Example:

[Test]
public void Add()
{
	 /* ... */
}

SetUpAttribute

This attribute is used inside a TestFixture to provide a common set of functions that are performed just before each test method is called. A TestFixture can have only one SetUp method. If more than one is defined the TestFixture will compile successfully, but its tests will not run.

Example:

[TestFixture]
public class SuccessTests
{
  [SetUp]
  public void Init()
  {
 	/* ... */
  }
}

TearDownAttribute

This attribute is used inside a TestFixture to provide a common set of functions that are performed after each test method is run. A TestFixture can have only one TearDown method. If more than one is defined the TestFixture will compile successfully, but its tests will not run.
Example:

  [TestFixture]
  public class SuccessTests
  {
    [SetUp]
    public void Init()
    {
	/* ... */
    } 

    [TearDown]
    public void Dispose()
     {
	 /* ... */
     } 

     [Test]
     public void Add()
    {
        /* ... */

    }
}

At the end I want to tell you about ExpectedException Attribute.
We use it when we want to test some exception.
This is the way to specify that the execution of a test will throw an exception.

Example:

  [ExpectedException( typeof( ArgumentException ) )]
  public void TestMethod()
  {
 	...
  }

  [ExpectedException( typeof( ArgumentException ), ExpectedMessage="expected message" )]
  public void TestMethod()
  {
 	...
  }