[BLACKBOX] a detail on delegates

From: VDI <bernhardt.gunnar{([at]})nowhere.xy>
Date: Fri, 22 Oct 2010 10:05:17 +0200

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Dear all,

on trying to familiarize myself with .NET I stumble upon a peculiarity
 in Gardens Point CP:

It is perfectly possible to add an instance method to an existing
 delegate:
You can either call a method which takes a procedure variable as
 parameter. The compiler warns you, that procedure variable are
 "deprecated" but the code works fine.

Or you can use the builtin procedures REGISTER, which is specific to
 the .NET implementation of CP. For instance:

(* ------- BEGIN Example 1 ------------- *)
IMPORT
  System := mscorlib_System,
  System_Xml := System_Xml_,
  System_Xml_Schema := System_Xml__Schema;


TYPE
  XmlValidationHandler = POINTER TO RECORD
    ErrorCount: INTEGER
  END;
  
  
PROCEDURE ( this: XmlValidationHandler ) HandleError( sender:
 System.Object; args: System_Xml_Schema.ValidationEventArgs ),NEW;
BEGIN
  INC( this.ErrorCount );
  (* handle the validation error*)
END HandleError;


PROCEDURE Validate( xmlFilename: ARRAY OF CHAR ): BOOLEAN;
VAR
  settings: System_Xml.XmlReaderSettings;
  handler: XmlValidationHandler;
  reader: System_Xml.XmlReader;
BEGIN
  NEW( handler ); handler.ErrorCount := 0;
  NEW( settings );
  settings.set_ProhibitDtd( FALSE );
  settings.set_ValidationType( System_Xml.ValidationType.DTD );
  REGISTER( settings.ValidationEventHandler, handler.HandleError );
  (* create the reader and read xmlFilename for validation *)
  (* ... *)
END Validate;
(* ------- END Example 2 ------------- *)



The compiler won't keep you either from doing the following, which
 results in a null reference exception at run time:
 
(* ------- BEGIN Example 2 ------------- *)
IMPORT
(* same as above *)

TYPE
  XmlValidationHandler = RECORD
    ErrorCount: INTEGER
  END;
  
  
PROCEDURE ( VAR this: XmlValidationHandler ) HandleError( sender:
 System.Object; args: System_Xml_Schema.ValidationEventArgs ),NEW;
(* same as above *)
END HandleError;


PROCEDURE Validate( xmlFilename: ARRAY OF CHAR ): BOOLEAN;
VAR
(* same as above *)
BEGIN
(* same as above *)
  REGISTER( settings.ValidationEventHandler, handler.HandleError );
 (* this fails at runtime with a null-reference exception *)
  (* ... *)
END Validate;
(* ------- END Example 2 ------------- *)


I suspect this could be correct, if CP treated RECORDs as value types
 in .NET.

What I Wonder is: should I report this as a compiler error, even if it
 may be just a minor inconvenience?


yours confusedly
gunnar

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Received on Fri Oct 22 2010 - 10:05:17 UTC