我可以从DataReader创建匿名类型集合吗？

Question

我有这个代码:

var query = "SELECT * FROM Cats";

var conn = new SqlConnection(sqlConnectionString);

conn.Open();

var cmd = new SqlCommand(query);
var reader = cmd.ExecuteReader();

while (reader.Read())
{
    var CatName = reader.GetString(0);
    var CatDOB = reader.GetDateTime(1);
    var CatStatus = reader.GetInt32(2);
}

我想将行拉出到一个匿名类型集合中,我通常使用LINQ进行迭代,但我不确定是否可能由于.Read()每次调用下一行所需的方式.

有没有办法做到这一点？

Answer 1

您可以创建辅助泛型方法并让编译器推断类型参数:

private IEnumerable<T> Select<T>(DbDataReader reader, Func<DbDataReader, T> selector)
{
    while(reader.Read())
    {
        yield return selector(reader);
    }
}

用法:

var items = SelectFromReader(reader, r => new { CatName = r.GetString(0), CarDOB = r.GetDateTime(1), CatStatus = r.GetInt32(2) });

您甚至可以将该方法作为扩展方法DbDataReader:

public static IEnumerable<T> Select<T>(this DbDataReader reader, Func<DbDataReader, T> selector)
{
    while (reader.Read())
    {
        yield return selector(reader);
    }
}

并使用它:

var items = reader.Select(r => new { CatName = r.GetString(0), CarDOB = r.GetDateTime(1), CatStatus = r.GetInt32(2) });

Answer 2

这是一个使用动态(我认为更容易使用)的例子,但有些人可能觉得不遵守你的问题的字母.

像这样称呼它:

var result = SelectIntoList("SELECT * FROM Cats",sqlconnectionString);

你可以(像我一样)将它放在一个单独的文件中的静态类中,以便于维护.

public static IEnumerable<dynamic> SelectIntoList(string SQLselect, string connectionString, CommandType cType = CommandType.Text)
{
  using (SqlConnection conn = new SqlConnection(connectionString))
  {
    using (SqlCommand cmd = conn.CreateCommand())
    {
      cmd.CommandType = cType;
      cmd.CommandText = SQLselect;

      conn.Open();

      using (SqlDataReader reader = cmd.ExecuteReader())
      {

        if (reader.Read())  // read the first one to get the columns collection
        {
          var cols = reader.GetSchemaTable()
                       .Rows
                       .OfType<DataRow>()
                       .Select(r => r["ColumnName"]);

          do
          {
            dynamic t = new System.Dynamic.ExpandoObject();

            foreach (string col in cols)
            {
              ((IDictionary<System.String, System.Object>)t)[col] = reader[col];
            }

            yield return t;
          } while (reader.Read());
        }
      }

      conn.Close();
    }
  }
}