问题
In C# I use sql scripts to add data into a List where T would be a class with fields/properties mapped to the sql script.
How could I do this in F#? This piece uses a stored procedure in the standard way.
using (conn)
{
conn.Open();
using (SqlCommand cmd = new SqlCommand("dbo.query_here", conn))
{
cmd.CommandText = "dbo.query_here";
cmd.CommandType = System.Data.CommandType.StoredProcedure;
cmd.CommandTimeout = 600;
cmd.Parameters.Add(new SqlParameter("@x1", Convert.ToString(x)));
cmd.Parameters.Add(new SqlParameter("@y1", y));
cmd.Parameters.Add(new SqlParameter("@z1", z));
SqlDataReader reader = cmd.ExecuteReader();
while (reader.Read())
{
MyListOfClasses.Add(new MyDataClass(reader.GetInt32(reader.GetOrdinal("x"))
reader.GetDouble(reader.GetOrdinal("y")),
reader.GetDouble(reader.GetOrdinal("a")),
reader.GetDouble(reader.GetOrdinal("b"))));
}
reader.Close();
}
conn.Close();
I realise F# is not quite as straight forward perhaps as this, but I need to get this data into F# lists in a similar way. Also would prefer suggestions that were not functional in nature and followed a similar pattern to C# code.
In a previous thread someone suggested using records, but still this was not in relation to SqlDataReader. It would be preferable to have a list of classes so I can use getters and setters on each item.
I should add before the inevitable comments of "why not just use C#". Well obviously I can use C#, but I am exploring possibilities of writing algorithms in F# and to do that I need to get my raw data from SQL Server.
回答1:
If you would like to return results
in F# list, list comprehension is suitable to use here. And with use
keyword, you don't need to explicitly dispose objects.
use conn = (* Initialize sql connection here *)
conn.Open()
use cmd = new SqlCommand("dbo.query_here", conn)
cmd.CommandText <- "dbo.query_here"
cmd.CommandType <- System.Data.CommandType.StoredProcedure
cmd.CommandTimeout <- 600
cmd.Parameters.Add(new SqlParameter("@x1", Convert.ToString(x)))
cmd.Parameters.Add(new SqlParameter("@y1", y))
cmd.Parameters.Add(new SqlParameter("@z1", z))
use reader = cmd.ExecuteReader()
let results =
[ while reader.Read() do
yield new MyDataClass(reader.GetInt32(reader.GetOrdinal("x")),
reader.GetDouble(reader.GetOrdinal("y")),
reader.GetDouble(reader.GetOrdinal("a")),
reader.GetDouble(reader.GetOrdinal("b"))) ]
回答2:
Working with databases using plain old ADO.NET can be actually quite nice using the F# dynamic operator ?
. I wrote an MSDN article that implements a helper that lets you write something like:
let myListOfClasses =
[ let db = new DynamicDatabase(connectionString)
let rows = db.Query?query_here(string x, y, z)
for r in rows do
yield MyDataClass(row?x, row?y, row?a, row?b) ]
This uses the dynamic invoke operator ?
to call the stored procedure (db.Query?query_here
). When you call it, you can give it parameters as if it was a normal function call (it will add them as SQL parameters). Then you can also use row?x
to read the properties - this infers the type from the type of MyDataClass
arguments, so assuming these match, you do not need to write type conversions yourself.
Here are links to a MSDN tutorial that discusses this:
- How to: Dynamically Invoke a Stored Procedure
- Step 1: Create a Database and Show the Poll Options
- Step 2: Implement Voting for an Option
回答3:
You can read this
Link : http://www.codeproject.com/Articles/95656/Using-a-DataRader-like-a-List-in-F
let rdr = cmd.ExecuteReader()
rdr
|> SomeRecord.asSeq // project datareader into seq<SomeRecord>
|> Seq.sumBy (fun r-> r.val1 * r.val2) // now you can use all the Seq operators
// that everybody knows.
回答4:
Untested translation to F#
use conn = ???
conn.Open()
using new SqlCommand("dbo.query_here", conn)(fun cmd ->
cmd.CommandText <- "dbo.query_here"
cmd.CommandType <- System.Data.CommandType.StoredProcedure
cmd.CommandTimeout <- 600
cmd.Parameters.Add(new SqlParameter("@x1", Convert.ToString(x)))
cmd.Parameters.Add(new SqlParameter("@y1", y))
cmd.Parameters.Add(new SqlParameter("@z1", z))
let reader = cmd.ExecuteReader()
while (reader.Read()) do
MyListOfClasses.Add(new MyDataClass(reader.GetInt32(reader.GetOrdinal("x"))
reader.GetDouble(reader.GetOrdinal("y")),
reader.GetDouble(reader.GetOrdinal("a")),
reader.GetDouble(reader.GetOrdinal("b"))))
reader.Close()
)
conn.Close()
MyListOfClasses.ToArray() |> List.ofArray
来源:https://stackoverflow.com/questions/12543363/use-sqldatareader-in-f