Introduction

Unit testing is testing a small module, or unit of our code in order to validate that the code is working as expected. Sometimes we throw a specific exception manually in our code and we have to write a unit test to validate that exception. In this article, we are going to explore how to write a unit test for exception and console log in C#.

xUnit

Sample Program

Calculator.cs

public class Calculator
{
    public int Division(int num1, int num2)
    {
        if (num2 == 0)
            throw new ArithmeticException("Divide By Zero Error");

        return num1 / num2;
    }
}

We are going to write a unit test for both positive and negative (exception) cases of the "Division" method.

How to Create xUnit Test Project

Follow the below steps to create xUnit Test Project,

Step 1

Right-click the Solution in Solution Explorer Window. Select Add -> New Project.

Step 2

Find the project type "xUnit Test Project (.NET Core)". Select it, then click "Next".

Step 3

Enter Project Name is "Simple_Program_Test" and click "Create".

Test for Positive case using xUnit

We are going to test the below positive cases,

  1. Both num1 and num2 are non zero number
  2. num1 is zero and num2 is a non-zero number

In the positive cases, we are going to call the "Division" method with two input values and checks if the expected value is equal to the actual value (the result of the Division method) or not using Assert.

Test Case Inputs Expected Output
Both num1 and num2 are non zero number num1 = 10
num2 = 2		 5
num1 is zero and num2 is non-zero number num1 = 0
num2 = 2		 0

CalculatorTest.cs

[Fact]
public void Check_Divide_By_Non_Zero_Number()
{
    var num1 = 10;
    var num2 = 2;
    var expectedValue = 5;

    Calculator calculator = new Calculator();
    var res = calculator.Division(num1, num2);

    Assert.Equal(expectedValue, res);
}

[Fact]
public void Check_Divide_By_Non_Zero_Number_1()
{
    var num1 = 0;
    var num2 = 2;
    var expectedValue = 0;

    Calculator calculator = new Calculator();
    var res = calculator.Division(num1, num2);

    Assert.Equal(expectedValue, res);
}

Test for Exceptions using xUnit

Test Case Inputs Expected Output
num1 is non-zero and num2 is zero num1 = 1
num2 = 0		 "ArithmeticException" Exception Thrown
Both num1 and num2 are zero num1 = 0
num2 = 0		 "ArithmeticException" Exception Thrown

CalculatorTest.cs

[Fact]
public void Check_Divide_By_Zero()
{
    var num1 = 1;
    var num2 = 0;
    string expectedErrorMessage = "Divide By Zero Error";

    Calculator calculator = new Calculator();
    var ex = Assert.Throws<ArithmeticException>(() => calculator.Division(num1, num2));

    Assert.Equal(expectedErrorMessage, ex.Message);
}

[Fact]
public void Check_Divide_By_Both_Zero()
{
    var num1 = 0;
    var num2 = 0;
    string expectedErrorMessage = "Divide By Zero Error";

    Calculator calculator = new Calculator();
    var ex = Assert.Throws<ArithmeticException>(() => calculator.Division(num1, num2));

    Assert.Equal(expectedErrorMessage, ex.Message);
}

How to Run the Unit Test

Go to 'Test' menu -> Select 'Test Explorer'.

In the 'Test Explorer window, we can run either all test cases or a particular test case.

Now we are going to run all test cases and the results are shown below.

Test for Console.log

Logger.cs

public class Logger
{
    public void Log(Exception ex)
    {
        Console.WriteLine("Error Message: " + ex.Message);
        Console.WriteLine("StackTrace: " + ex.StackTrace);
    }
}

We are going to write a unit test for capturing the console output and validate the expected value with the console value.

In the test, we are using Console.SetOut with the fake writer (for example StringWriter) to check the console value.

LoggerTest.cs

[Fact]
public void Log_Test()
{
    Logger logger = new Logger();
    var output = new StringWriter();
    Console.SetOut(output);
    var expectedValue = "Error Message: Test Exception";
    logger.Log(new Exception("Test Exception"));
    Assert.Contains(expectedValue, output.ToString());
}

Summary

We can validate exceptions and console values using the xUnit framework for .NET. I hope you have liked it and know about how to write a unit test for exception and console log in C#.