Question

The below code lives in an instance class with no constructor, I don’t know from where to start writing unit tests for it.

E.g price is “2/9” so secondPart is 9 also the _random is an object of Random class

    public string RefreshPrice(string price, out bool isUpOrDown, out bool isChanged)
    {
        if (string.IsNullOrEmpty(price))
        {
            isUpOrDown = false;
            isChanged = false;
            return price;
        }

        int secondPart;
        string[] priceParts = price.Split('/');
        int newPrice = 0;
        if(int.TryParse(priceParts[1], out secondPart))
        {
            newPrice = secondPart >= 2 ? _random.Next(2, 20) : 2;
        }

        isUpOrDown = (newPrice > secondPart);
        isChanged = (newPrice != secondPart);

        secondPart = newPrice;
        return string.Format("{0}/{1}", priceParts[0], secondPart);
    }

Answer 1

an instance class with no constructor

So this class will have a default ctor:

Legacy legacyUnderTest = new Legacy();

If _random is not set up in the default ctor, you should create a helper method getLegacyUnderTest to set _random, either how it is set in production or have a look at this stack overflow answer for accessing privates during tests using PrivateObject.

private Legacy getLegacyUnderTest()
{
    Legacy result = new Legacy();
    //set _random however it is set in production for the general need of your test
    return result;
}

Since they’re out, you’ll need to declare your bool flags as part of your arrange step for the test, then you’re good to go:

[TestMethod]
public void RefreshPrice_EmptyPriceIsNotUpDownIsNotChange_ReturnsInitalPrice()
{
    //Arrange
    Legacy legacyUnderTest = new this.getLegacyUnderTest();
    bool isUpDown = false;
    bool isChange = false;
    string initialPrice = string.Empty;

    //Act
    string result = legacyUnderTest.RefreshPrice(initalPrice, isUpDown, isChange);

    //Assert
    Debug.IsEqual(initalPrice, result);
}

Now (after fixing the problems I’ve inevitably made by writing that snippet without VS in front of me) you’ll just have to vary the values of isUpDown, isChange and initialPrice to cover all the possibilities you need to test and you should be good.

This should be the different formats of initial price (valid, null, empty and invalid formats) combined with the different true/false combinations of those flags.