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Need help with cast type for data function.
f#compiler

Dear All:
I have a seemingly easy

question: I create a data table and insert 3 values using SQL Server 2008, and

want to retrieve all the values into an array of float.

The table definition is:

Create TABLE Table1(Field1 [money] not null);

Insert into Table1

Values(1.0)

Insert into Table1

Values(2.0)

Insert into Table1

Values(3.0)

Here is my code:

let sql_ConnDB = "server=(local);Integrated Security=True;Database=localDB"

let connDB = new

SqlConnection(sql_ConnDB)

let sqlGetRecords cmdText =

use cmd = new

SqlCommand(cmdText, connDB)

connDB.Open()

let reader = cmd.ExecuteReader()

seq { while reader.Read() do

yield reader.GetValue(0)

reader.Close()

connDB.Close() }

let floatArray = sqlGetRecords "SELECT Field1 FROM TABLE1"

|> Seq.toArray

|> Array.map(fun (x) -> x :?> float)
When I run the code I get

the run-time error, the error appeared at x :?> float

System.InvalidCastException

was unhandled

Message: Specified cast is

not valid.

It means I can not cast [money] in SQL to float in F#.

Besides, if I use yield reader.GetFloat(0), then I got the compiler error:

The type 'float32' does not have any proper subtypes and

cannot be used as the source of a type test or runtime coercion.

Any idea? (I am using VS2010 Beta 2 with F# 1.9.7.4 and SQL

Server 2008 with SP1)

Thanks,

.

Hi,
there is one thing in F# that is really confusing when it comes to floating point types. The F# float type corresponds to System.Double (double in C#). The System.Single (float in C#) type is called float32 in F#.

This means that the "GetFloat" method actually returns float32. I'm not exactly sure how SQL data types map to .NET types, but I think that using "GetFloat" and then working with "float32" values should work. In that case, you'd get an array<float32> as the result (even without using Array.map).

If you wanted to convert a value of "float32" type to a value of "float" type, then you can use "float" function (which has the same name as the target type and works for various input types):

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let double = 1.0
let single = float32(double)
let double2 = float(single)

input |> Array.map(fun x -> float(x)) // converts input (array<float32>) to array<float>
// More simply:
input |> Array.map float // converts input (array<float32>) to array<float>
By on 1/13/2010 12:30 PM ()

Hello, Gary:
Do you have any code sample for LINQ 2 SQL for F# for my question?
By the way, reader.Getfloat(0) didn't work.
Thanks,

By on 1/13/2010 12:49 PM ()

I have already shown the code in another thread.

DataContext.translate()

it converts any sqlreader result sets to array of strong types using column name to property name mapping. you only need to define your strong type with the proper properties(and I said in this case, C# is actually better as it is shorter and cleaner). They are effectively your n-tuples but accessed with property rather than position.

that means 95% of the use cases.

By on 1/13/2010 1:05 PM ()

data conversion is hard, you need to cater for all the possible data type found in SQL server and map them properly to .NET type.

You need lots of reflection coding to handle them properly and that at the moment is too much for you.

Leave this task to others, LINQ 2 SQL has already done it for you.

By on 1/13/2010 12:05 PM ()

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