How to test Assert for base class exception implementation using Microsoft Unit testing framework?-CodePudding

Here is specific case. I want to test this Assert.ThrowsException<DbException>(() => { MyMethod(0, 100); });

The problem is that exception thrown is SqlException. Why do I want to test base exception? Because I want a unified test for any of my 3 DB providers. The exception sometimes is OracleException or MySqlException. All 3 providers exceptions derive from DbException

Is there a way to test it nicely, using the Microsoft Unit Test framework?

CodePudding user response：

According to the documentation for Assert.ThrowsException this is expected. However, if you're able to use Fluent Assertions, this is in fact very easy to achieve.

Action action = () => MyMethod(0, 100);
action.Should().Throw<DbException>(); // add some more checks

// this would mimic the behavior of Assert.ThrowsException, which should fail.
action.Should().ThrowExactly<DbException>();

See also the tips on improving assertions under Tips/Exceptions.

The alternative would be to use regular try...catch like in the example below. You need to make sure that you capture all the code path' and don't accidentally succeed:

    /// <summary>
    /// Catching a base exception class just by using <see cref="Assert"/>.
    /// Do not use <see cref="ExpectedExceptionAttribute"/>, since it doesn't
    /// provide any type of validation.
    /// </summary>
    [TestMethod]
    public void CatchAderivedExceptionTheAssertWay()
    {
        try
        {
            // A test method that throws an exception.
            Action action = () => throw new ArgumentNullException("param1");
            action();
        }
        catch (ArgumentException e)
        {
            // This will catch any exception derived from ArgumentException.
            // Do some validation to ensure the right thing is caught,
            // like checking the parameter name.
            if (!e.ParamName.Equals("param1", StringComparison.Ordinal))
            {
                Assert.Fail("Reason why the validation failed.");
            }

            // Otherwise jump out of the test.
            return;
        }

        Assert.Fail("Make sure the test fails, in case the code inside try doesn't throw at all.");
    }

For completeness sake, there's also the ExpectedException attribute that can be added to a [TestMethod]. Due to the lack of validation options I would not recommend using it.

I'll add this question to my list of interview questions. Such a nice example.

CodePudding user response：

Thanks for all the leads, one from @StephanAdler

Considering limitations of given testing framework my sensible solution was this

Exception retEx = null;

try
{
    MyMethod(0, 100);;
}
catch (Exception ex)
{
    retEx = ex;
}

Assert.IsNotNull(retEx);
Assert.IsInstanceOfType(retEx, typeof(DbException));

Assert.IsInstanceOfType works perfectly with the base type