F# working with while loop

Question

I have a datareader and i want to return collection of rows from it, after reading books for like a day i am not able to find out best way to do it in f#. I can do it normal C# way in F# but that is not why i am using f#

Here is what i am trying to achieve

let values =
    while reader.Read() do
        yield reader.GetString(0), reader.GetInt64(1)

Above is how i am trying to do

• all values get collected into values, which could be dictinary or tuples or any collection
• yield can't be used in while loop but that is what i am trying to do

What could be the best way to achieve this

Answer 1

F# also provides list comprehension for arrays and sequences.

let records_as_list = 
    [
        while reader.Read() 
            do yield (reader.GetString(0), reader.GetInt64(1)) 
    ]

let records_as_array = 
    [|
        while reader.Read() 
            do yield (reader.GetString(0), reader.GetInt64(1)) 
    |]

let records_as_sequence = 
    seq {
        while reader.Read() 
            do yield (reader.GetString(0), reader.GetInt64(1)) 
    }

F# has a convenient dictionary function built in called dict

let records_as_IDictionary = dict records_as_sequence

Answer 2

You can use sequence expressions to implement enumerators:

let records = seq { while reader.NextResult() do yield (reader.GetString(0), reader.GetInt64(1)) }

If you need more than two fields, you can yield maps of column index (or name) -> field values (untested code):

let dbSchema = reader.GetSchemaTable()

let makeMap (rdr : IDataReader) (schema : DataTable) =
    schema.Columns |> Seq.cast<DataColumn> |> Seq.map (fun col -> (col.ColumnName, rdr.[col.ColumnName])) |> Map.ofSeq

let records = seq { while reader.NextResult() do yield (makeMap reader dbSchema) }