Method not getting called correctly when passing Func<T1, T2> as argument

Question

I've been trying to get this method to work since yesterday and have taken to pulling my hair out -

Basically, I have a method that retrieves articles from a database. Below is the method code:

public static IEnumerable<Article> RetrieveTopTenArticles()
{
    using(var dbConn = new SqlConnection(Settings.Instance.DbConnectionString))
    {
        const String query = "SELECT TOP 10 Title, Content FROM Article";
        Func<SqlDataReader, Article> operation = reader => reader.ToArticle();
        return dbConn.SqlRetrieveMultiple(query, operation);
    }
}

And in a separate "Extensions" class -

public static IEnumerable<T> SqlRetrieveMultiple<T>(this SqlConnection sqlConnection, String query, Func<SqlDataReader, T> operation, params SqlParameter[] parameters)
{
    sqlConnection.Open();
    using (var command = new SqlCommand(query, sqlConnection))
    {
        if (parameters.Length > 0)
        {
            command.Parameters.AddRange(parameters);
        }
        using (SqlDataReader reader = command.ExecuteReader())
        {
            while (reader.Read())
            {
                yield return operation.Invoke(reader);
            }
        }
    }
}

As you can see, I have an extension method that performs a SQL select statement, and iterates over the results, invoking the operation and converting each SqlDataReader row into my Article type (also done with an extension method). I believe I've used this same exact extension method in the past with no issues.

When I debug this and it gets to the dbConn.SqlRetrieveMultiple(query, operation); row, if I hit F11 to step into, it steps over the row completely. It continues to walk down the chain of ending nested brackets and eventually I get an exception on the sqlConnection.Open().

The exception I get is:

The ConnectionString property has not been initialized.

I have found that the function will step into just fine if I remove my Func delegate. I feel like this has to be something stupid / unrelated to extension methods or SqlConnection because I get the same behavior if I put it in a regular private static method within the same class. If I turn on .NET framework source stepping at the sqlConnection.Open(); line, it jumps into a ctor method of the .NET framework... I can post up more code if requested but I think I've got everything necessary. Please let me know if you've got any ideas.

Answer 1

You're returning the result of your iterator method instead of enumerating it. Iterator methods use deferred execution. As a result, it is not actually doing anything until well after the connection it will use has been disposed. That's why it "just stepped over" the method and why it failed at some arbitrary later time.

You can fix this problem in two ways. Either get all of the results ahead of time and return them, eagerly getting the items before disposal occurs, using a method like ToArray:

return dbConn.SqlRetrieveMultiple(query, operation).ToArray();

or you can ensure disposal is delayed until after the items have been lazily retrieved, by making your outer method an iterator method as well:

using(var dbConn = new SqlConnection(Settings.Instance.DbConnectionString))
{
    const String query = "SELECT TOP 10 Title, Content FROM Article";
    Func<SqlDataReader, Article> operation = reader => reader.ToArticle();
    foreach (var e in dbConn.SqlRetrieveMultiple(query, operation))
        yield return e;
}

However, keep in mind that if users iterate this lazy version twice, they will end up opening two connections. That may or may not be what you want.

Answer 2

The exception given means that something is wrong with your connection string. Post it.

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Also I would use your func this way:

while (reader.Read())
{
    yield return operation(reader);
}