如何在C＃中对NpgsqlDataReader扩展进行单元测试

Question

I've been playing around with some ADO.NET extensions inspired by a PostgreSQL Pluralsight article by Rob Conery.

I'm new to unit testing and have been trying to learn some more about it by writing some tests around a couple of extension methods I've written for the NpgsqlDataReader class.

An example of a method I'm trying to test:

    public static T ToEntity<T>(this NpgsqlDataReader reader) where T : new()
    {
        var result = new T();
        var properties = typeof(T).GetProperties();

        foreach (var prop in properties)
        {
            for (var i = 0; i < reader.FieldCount; i++)
            {
                if (reader.GetName(i).Replace("_", "").Equals(prop.Name, StringComparison.InvariantCultureIgnoreCase))
                {
                    var val = reader.GetValue(i) != DBNull.Value ? reader.GetValue(i) : null;
                    prop.SetValue(result, val);
                }
            }
        }

        return result;
    }

I'm not sure how to even start mocking the database connection, as all I want to test is functionality of iterating over the returned result object and mapping the columns to the properties of a generic entity class.

How would I go about testing the entity mapping, without accessing the database?

Thanks

Update:

The code I've been using to call the extension above is another extension on the NpgsqlCommand class:

public static IEnumerable<T> ToList<T>(this NpgsqlCommand cmd) where T : new()
{
    var results = new List<T>();
    var reader = cmd.ExecuteReader();

    while (reader.Read())
    {
        results.Add(reader.ToEntity<T>());
    }

    reader.Close();
    reader.Dispose();

    return results;
}

I'll be using the same techniques posted in the answer below to test this method as well.

Answer 1

First of all, the method implementation does not depend on the concrete implementation of NpgsqlDataReader. So you can change the signature to:

public static T ToEntity<T>(this IDataReader reader) where T : new()

Next, the signature is not logical. A datareader is used to loop over a result set and yields a list of rows (with 0, 1 or more elements), not a single result. So a more logical signature and implementation would be:

public static IEnumerable<T> ToEntity<T>(this IDataReader reader) where T : new()
{
    var properties = typeof(T).GetProperties();

    while (reader.Read())
    {
        var result = new T();
        foreach (var prop in properties)
        {
            for (var i = 0; i < reader.FieldCount; i++)
            {
                if (reader.GetName(i).Replace("_", "").Equals(prop.Name, StringComparison.InvariantCultureIgnoreCase))
                {
                    var val = reader.GetValue(i) != DBNull.Value ? reader.GetValue(i) : null;
                    prop.SetValue(result, val);
                }
            }
        }

        yield return result;
    }
}

Now you can provide a fake implementation of IDataReader. Either you can use a mocking framework such as NSubstitute, but if you're new to unit testing a simpler way is to manually implement one. For example, in this gist (incomplete implementation) I implemented IDataReader on an in-memory generic list (and an extension method to facilitate usage).

Now you can write a test method like this:

// note the anonymous type here
var expectedEntity = new {MY_PROPERTY = "SomeValue"};
var reader = new[]
{
    expectedEntity
}.AsDataReader();
var entity = reader.ToEntity<MyEntity>().First();
Assert.AreEqual("SomeValue", entity.MyProperty);

Finally, your solution using reflection works, but may end up rather slow. Keep in mind that this kind of code is a solved problem; have alook at light-weight ORM's such as PetaPoco, dapper.net ao