How to simulate throwing an exception in Unit tests?

Question

How can I simulate an exception being thrown in C# unit tests?

I want to be able to have 100% coverage of my code, but I can\'t test the code with exceptions that may oc

Answer 1

What you need is stub - an object that will simulate certain conditions for your code. For testing purposes, you usually replace real object implementation with stub (or other type of faked object). In your case, consider:

public class MyClass
{
    private IDataProvider dataProvider;

    public void MyMethod()
    {
        try
        {
            this.dataProvider.GetData();
        }
        catch (OutOfMemoryException)
        {
        }
    }
}

Now, class you are testing should be configurable at some level - so that you can easily replace real DataProvider implementation with stubbed/faked one when testing (like you said, you don't want to destroy your DB - nobody wants!). This can be achieved for example by constructor injection (or in fact, any other dependency injection technique).

Your test then is trivial (some made-up requirement to test when exception is thrown):

[Test]
public void MyMethod_DataProviderThrowsOutOfMemoryException_LogsError()
{
    var dataProviderMock = new Mock<IDataProvider>();
    dataProviderMock
        .Setup(dp => dp.GetData())
        .Throws<OutOfMemoryException>();
    var myClass = new MyClass(dataProviderMock);

    myClass.MyMethod();

    // assert whatever needs to be checked
}

Answer 2

You need to create a mock object that stands in for the real objects that can throw these exceptions. Then you can create tests that simply are something like this:

public void ExampleMethod()
{
    throw new OutOfMemoryException();
}

If you are using a dependency injection framework it makes replacing the real code with the mock code much easier.