Can I create an anonymous type collection from a D

问题:

I have this code:

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);
}

I'd like to pull the rows out into an anonymous type collection, which I'd normally do using LINQ to iterate, but I an not sure if it's possible due to the way you have to call .Read() each time to get the next row.

Is there a way to do this?

回答1:

You can create helper generic method and let compiler infer type parameter:

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

usage:

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

You can even make the method an extension method on DbDataReader:

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

and use it like that:

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

回答2:

Here is an example of doing it with dynamic (which I think is easier to work with) but some may feel does not adhere to the letter of your question.

Call it like this:

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

You could (like I did) put it into a static class in a separate file for easier maintanence.

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();
    }
  }
}

回答3:

It's possible, although not particularly neat. We'll need to create a new method that will allow us to create an empty sequence that allows for type inference off of a dummy value for starters:

public static IEnumerable<T> Empty<T>(T dummyValue)
{
    return Enumerable.Empty<T>();
}

This lets us create a list of an anonymous type:

var list = Empty(new
{
    CatName = "",
    CatDOB = DateTime.Today,
    CatStatus = 0
}).ToList();

(The item here isn't used.)

Now we can add our anonymous types to this list:

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

while (reader.Read())
{
    list.Add(new
    {
        CatName = reader.GetString(0),
        CatDOB = reader.GetDateTime(1),
        CatStatus = reader.GetInt32(2),
    });
}

Of course, using a named type would likely be easier, so I would suggest using one unless there is a real compelling reason not to do so. That is especially true if you plan to use the list outside of the scope it's created in.

回答4:

Technically, it may not answer your question, but simply don't use a reader. Instead use a SqlDataAdapter to Fill a DataSet, if you can. Take the 0th Table of that DataSet, and select a new anonymous object from the Rows collection.

using System.Data; // and project must reference System.Data.DataSetExtensions

var ds = new DataSet();
using (var conn = DbContext.Database.GetDbConnection())
using (var cmd = conn.CreateCommand())
{
  cmd.CommandType = CommandType.Text;
  cmd.CommandText = sqlText;
  conn.Open();
  (new SqlDataAdapter(cmd)).Fill(ds);
}
var rows = ds.Tables[0].AsEnumerable(); // AsEnumerable() is the extension
var anons = rows
  .Select(r => new { Val = r["Val"] })
  .ToList();