Looking at the latest Visual Studio 2015 CTP release I noticed the new Smart Unit Tests feature. This is an evolution of Microsoft Research’s Pex project and to quote Microsoft:

Smart Unit Tests explores your .NET code to generate test data and a suite of unit tests

That sounds pretty amazing so I fired VS 2015 to have a look. I grabbed a piece of sample code that I wrote a while back for an interview, you can find it on github if you want to follow along:

public class RomanNumeralGenerator : IRomanNumeralGenerator 
  static Dictionary<int, string> numerals = new Dictionary<int, string> 
    { 1000, "M" }, { 900, "CM" }, { 500, "D" }, { 400, "CD" }, 
    { 100, "C" }, { 90, "XC" }, { 50, "L" }, { 40, "XL" }, { 10, "X" }, 
    { 9, "IX" }, { 5, "V" }, { 4, "IV" }, { 1, "I" } 
  public string Generate(int number) 
    StringBuilder numeralBuilder = new StringBuilder();
    foreach (var numeral in numerals) 
      while (number >= numeral.Key) 
        number -= numeral.Key; 
    return numeralBuilder.ToString(); 
  void Validate(int number) 
    if (number < 1 || number > 3999) 
      throw new ArgumentOutOfRangeException(); 

RomanNumeralGenerator generates the roman numeral string equivalent of an integer, it throws an exception if the integer is above 3999 or below 1. To generate the test for this code I just right click inside the class file and select Smart Unit Tests.
This fires up the Smart Unit Tests Exploration generates a set of tests and lets you view details about them.
This is actually really neat, tests have been created that trigger the exceptions as well as two tests that confirm correct strings are returned. Technically this class now has 100% code coverage, although really we would need to write a much more in depth test suite to cover all the potential bugs, and if I select one of the tests I can see what test code was generated.
I can also explore the events that generated the tests, modify how those tests are generated and finally select any or all of the tests and save them.
This generates a new unit test project, MSTest only at the moment, and adds it to your solution.

My first impressions

This seems like a great way to backport tests onto existing code but I hope that people don’t think that now there’s no longer any need to write tests because Visual Studio will write them for you. Smart Unit Tests are going to be a great tool for refactoring currently untested code, creating baseline tests so you can change with more confidence. I’m looking forward to trying out using them on a more complicated code base next.