Improve data access layer select method Pattern

Question

Lately I find myself writing data access layer select methods where the code all takes this general form:

public static DataTable GetSomeData( ... arguments)         


        

Answer 1

Similar to what I posted here

public IEnumerable Get(string query, Action parameterizer, 
                             Func selector)
{
    using (var conn = new T()) //your connection object
    {
        using (var cmd = conn.CreateCommand())
        {
            if (parameterizer != null)
                parameterizer(cmd);
            cmd.CommandText = query;
            cmd.Connection.ConnectionString = _connectionString;
            cmd.Connection.Open();
            using (var r = cmd.ExecuteReader())
                while (r.Read())
                    yield return selector(r);
        }
    }
}

I have these simple extension methods to aid ease of calling:

public static void Parameterize(this IDbCommand command, string name, object value)
{
    var parameter = command.CreateParameter();
    parameter.ParameterName = name;
    parameter.Value = value;
    command.Parameters.Add(parameter);
}

public static T To(this IDataRecord dr, int index, T defaultValue = default(T),
                      Func converter = null)
{
    return dr[index].To(defaultValue, converter);
}

static T To(this object obj, T defaultValue, Func converter)
{
    if (obj.IsNull())
        return defaultValue;

    return converter == null ? (T)obj : converter(obj);
}

public static bool IsNull(this T obj) where T : class
{
    return (object)obj == null || obj == DBNull.Value;
}

So now I can call:

var query = Get(sql, cmd =>
{
    cmd.Parameterize("saved", 1);
    cmd.Parameterize("name", "abel");
}, r => new User(r.To(0), r.To(1), r.To(2), r.To(3)));
foreach (var user in query)
{

}

This is fully generic, fits any model that comply to ado.net interfaces. The connection object and reader is disposed only after the collection is enumerated once.