ASP.NET Web PDF Document Viewer/Editor Control Library

You can use the combination of .NET and F# reflection to provide generic implementations of language-related transformations. In this section, we give one example of this powerful technique. Listing 9-10 shows the definition of a generic schema reader compiler, where a data schema is described using F# types and the schema compiler helps convert untyped data from text files into this data schema. Listing 9-10. Using Types and Attributes to Guide Dynamic Schema Compilation open System open System.IO open Microsoft.FSharp.Reflection type ColumnAttribute(col:int) = inherit Attribute() member x.Column = col /// SchemaReader builds an object that automatically transforms lines of text /// files in comma-separated form into instances of the given type 'schema. /// 'schema must be an F# record type where each field is attributed with a /// ColumnAttribute attribute, indicating which column of the data the record /// field is drawn from. This simple version of the reader understands /// integer, string and DateTime values in the CSV format. type SchemaReader<'schema>() = // Grab the object for the type that describes the schema let schemaType = typeof<'schema> // Grab the fields from that type let fields = match Type.GetInfo(schemaType) with | RecordType(fields) -> fields | _ -> failwithf "this schema compiler expects a record type" // For each field find the ColumnAttribute and compute a function // to build a value for the field let schema = fields |> List.mapi (fun fldIdx (fieldName,fieldType) -> let fieldInfo = schemaType.GetProperty(fieldName) let fieldConverter = match fieldType with | ty when ty = typeof<string> -> (fun (s:string) -> box s) | ty when ty = typeof<int> -> (System.Int32.Parse >> box) | ty when ty = typeof<DateTime> -> (System.DateTime.Parse >> box) | _ -> failwithf "Unknown primitive type %A" fieldType

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db.ExecuteNonQuery(cw); boReturn.PubID = PubID; boReturn.Name = cw.GetParameterValue("@pub_name").ToString(); boReturn.City = cw.GetParameterValue("@city").ToString(); boReturn.State = cw.GetParameterValue("@state").ToString(); boReturn.Country = cw.GetParameterValue("@country").ToString(); return boReturn; } While using the Database and command wrapper objects largely replaces constructor semantics of managed code with factory method calls, this example really shows how the amount of code is reduced, as it has a single line of code to deal with each property value in use at the business object layer. The structure of the code is much simpler than the constructor and indexing code the Managed Provider requires.

To insert an object using a custom SQLData object, we first instantiate and set the attributes of the object using the Java object s constructor and setter methods. Once the object is ready in memory, we use the setObject() method to set the value in the placeholder of our CallableStatement or PreparedStatement strings. Finally, we execute the statement and do a commit. The method _demoInsert() illustrates these steps: private static void _demoInsert( Connection connection ) throws SQLException { As a first step, we initialize the MyAddress object: MyAddress myAddress = new MyAddress (); myAddress.setLine1("133 Ferry Rd"); myAddress.setLine2( "Apt # 24"); myAddress.setStreet( "Crypton St." ); myAddress.setCity( "Dallas"); myAddress.setState( "TX" ); myAddress.setZip( "75201" ); PreparedStatement pstmt = null; try {

let attrib = match fieldInfo.GetCustomAttributes(typeof<ColumnAttribute>, false) with | [| (: ColumnAttribute as attrib) |] -> attrib | _ -> failwithf "No column attribute found on field %s" fieldName (fldIdx,fieldName, attrib.Column, fieldConverter)) |> List.to_array // Compute the permutation defined by the ColumnAttributes indexes let columnToFldIdxPermutation = Permutation(schema.Length, schema |> Array.map (fun (fldIdx,_,colIdx,_) -> (colIdx,fldIdx))) // Drop the parts of the schema we don t need let schema = schema |> Array.map (fun (_,fldName,_ ,fldConv) -> (fldName,fldConv)) // Compute a function to build instances of the schema type. This uses an // F# library function. let objectBuilder = Reflection.Value.GetRecordConstructor(schemaType) // OK, now we re ready to implement a line reader member reader.ReadLine(textReader: TextReader) = let line = textReader.ReadLine() let words = line.Split([|','|]) |> Array.map(fun s -> s.Trim()) if words.Length <> schema.Length then failwith "unexpected number of columns in line %s" line let words = words |> Array.permute columnToFldIdxPermutation let convertColumn colText (fieldName, fieldConverter) = try fieldConverter colText with e -> failwithf "error converting '%s' to field '%s'" colText fieldName let obj = objectBuilder (Array.map2 convertColumn words schema) // OK, now we know we've dynamically built an object of the right type unbox<'schema>(obj) // OK, this read an entire file member reader.ReadFile(file) = seq { use textReader = File.OpenText(file) while not textReader.EndOfStream do yield reader.ReadLine(textReader) } The type of the SchemaReader is simple:

We use the setObject() method of the PreparedStatement interface to insert the object after creating the PreparedStatement object: String insertStmt = "insert into address_table values( )"; pstmt = connection.prepareStatement ( insertStmt ); pstmt.setObject (1, myAddress ); int rows = pstmt.executeUpdate(); System.out.println ( "Inserted " + rows + " row(s) " ); connection.commit(); } finally { JDBCUtil.close( pstmt ); } }

There is another option that s even simpler. The GetXYZCommandWrapper methods have another overloaded method signature that accepts a parameter array as a second parameter. This parameter array will accept all the values for the procedure s input parameters. The command wrapper factory then makes a request to the database for the schema definition of the procedure being executed. Using this information, it generates the set of parameters the procedure expects. Our last example could use this technique with the following code: private PublisherBO GetPublisherParamCache(string PubID) { PublisherBO boReturn = new PublisherBO(); Database db = DatabaseFactory.CreateDatabase(); DBCommandWrapper cw = db.GetStoredProcCommandWrapper("usp_GetPubDetails", PubID); db.ExecuteNonQuery(cw); boReturn.PubID = PubID; boReturn.Name = cw.GetParameterValue("@pub_name").ToString(); boReturn.City = cw.GetParameterValue("@city").ToString(); boReturn.State = cw.GetParameterValue("@state").ToString(); boReturn.Country = cw.GetParameterValue("@country").ToString(); return boReturn; } Here you ve replaced all of the calls to create parameters and simply added the PubID as a second argument passed to the factory method. Notice that even though you haven t explicitly