Fill an array (or arraylist) from SqlDataReader

Is there a way to fill an array via a SqlDataReader (or any other C# ADO.NET object) without looping through all the items? I have a query that is returning a single column, and I want to put that into a string array (or ArrayList, or List, etc).

Solution 1:

It is possible. In .NET 2.0+, SqlDataReader inherits from DbDataReader, which implements IEnumerable (non-generic one). This means that you can use LINQ:

List<string> list = (from IDataRecord r in dataReader
                     select (string)r["FieldName"]
                    ).ToList();

That said, the loop is still there, it's just hidden in Enumerable.Select, rather than being explicit in your code.

Solution 2:

No, since SqlDataReader is a forward-only read-only stream of rows from a SQL Server database, the stream of rows will be looped through whether explicitly in your code or hidden in a framework implementation (such as DataTable's Load method).

It sounds like using a generic list and then returning the list as an array would be a good option. For example,

List<int> list = new List<int>();

using (SqlDataReader reader = cmd.ExecuteReader())
{
    while (reader.Read())
    {
         list.Add(reader.GetInt32(0));
    }    
}
return list.ToArray();

In response to your comment, calling ToArray() may be overhead, it depends. Do you need an array of objects to work with or would a generic collection (such as List<T> or ReadOnlyCollection<T>) be more useful?

Solution 3:

Apparently, ever since .NET 1.1 SqlDataReader had the following method:

int size;
object[] data = new object[]{};
size = reader.GetValues(data);

This populates data with the values of the current reader row, assigning into size the number of objects that were put into the array.