Jim Tierney

This post provides more information about DataSnap server method parameters and return types. This is my second post on this subject, following DataSnap Server Method Parameters. The previous post was about "Basic" types. This post is about "Basic DBXValue" types.  The sample client and client and server projects that go with this post can be downloaded here: http://cc.embarcadero.com/item/26734

The following table summarizes differences between "Basic" and "Basic DBXValue" types:

Supports null values

Support for null values is needed, for example, when a server method needs to pass values that come from a database column that allows NULL.    All DBXValue types have an IsNull property and a SetNull method.

Declaration

The var and out keywords can’t be used to specify the parameter direction of a DBXValue parameter.  The direction is always in/out.   A DBXValue type can’t be used as a function result.

Proxy Generator

The RAD Studio 2007 client proxy generator does not work properly for server methods with DBXValue parameters.  So you will need to hand code.   The sample client has code like this to call a server method:

// Non-generic version of TestSetNull
procedure TCallTestMethods.TestSetNullInt32(AConnection: TDBXConnection; I: Int32);
const
  LMethodName = 'TTestDBXInt32Value.SetNull';
var
  LIsNull: Boolean;
  LDBXCommand: TDBXCommand;
begin
  LDBXCommand := AConnection.CreateCommand;
  try
    try
      LDBXCommand.CommandType := TDBXCommandTypes.DSServerMethod;
      LDBXCommand.Text := LMethodName;
      LDBXCommand.Prepare;
      LDBXCommand.Parameters[0].Value.SetInt32(I);
      LDBXCommand.ExecuteUpdate;
      LIsNull := LDBXCommand.Parameters[0].Value.IsNull;
      LogTestResult(LMethodName, LIsNull);
    except
      on E: Exception do
      LogTestException(LMethodName, E);
    end;
  finally
    LDBXCommand.Free;
  end;
end;

Calling a server method with a TDBXInt32Value parameter is the same as calling a server method with an Int32 parameter, except that you can use SetNull and IsNull methods to work with null values.  

Accessing Values

The following table shows the methods that are used to access a non-null value from the different DBXValue types   A mismatched call, such as GetBcd on TDBXStringValue, will raise an exception. 

String Value Types

Of the five DBXValue types that represent strings, you need only use TDBXAnsCharsValue to pass AnsiString values and TDBXWideCharsValue to pass UnicodeString/string values. The following table shows that these two types are always passed to server methods even if the method is declared with one of the other string types.

Sample Client And Server Applications

The sample server has a few simple server methods for passing values (including null), and two informational server methods.  In order to support various DBXValue types, I’ve implemented a generic server method base class:

{$METHODINFO ON}
  TTestBasicDBXValueType = class(TComponent)
  strict protected
    function EqualValues(I: TValue; J: TValue): Boolean; virtual;
    function GetValue(I: T): TValue; virtual; abstract;
    procedure SetValue(I: T; Value: TValue); virtual; abstract;
  public
    // Server methods to pass values (including null)
    function IsEqual(I: T; J: T): Boolean;
    function Echo(I: T): TValue;
    procedure Swap(I: T; J: T);
    function IsNull(I: T): Boolean;
    procedure SetNull(I: T);
    // Server method to get the DBXValue parameter type names
    procedure GetTypeNames(I: T; ADeclaredName, AActualName: TDBXStringValue);
    // Server method to try DBXValue methods like GetInt32 and SetInt32
    function TryGetAndSet(I: T): TDataSet;
  end;
{$METHODINFO OFF}

Then I’ve used TTestBasicDBXValueType to declare and implement server methods classes for a variety of DBXValue types, such as TDBXInt32Value, TTestDBXDateValue , etc.

  TTestDBXInt32Value = class(TTestBasicDBXValueType)
  strict protected
    function GetValue(I: TDBXInt32Value): Int32; override;
    procedure SetValue(I: TDBXInt32Value; Value: Int32); override;
  end;

  TTestDBXDateValue = class(TTestBasicDBXValueType)
  strict protected
    function GetValue(I: TDBXDateValue): TDBXDate; override;
    procedure SetValue(I: TDBXDateValue; Value: TDBXDate); override;
  end;

Consult the sample server to see how I’ve implemented the "plumbing" to make these classes callable by a DataSnap client. As in the “Basic” types sample server, the implementation is unconventional because the TDSServerClass component is not used.

The sample client tests the generated methods by calling them with sample values and verifying the results. Consult the sample for implementation details. Here is screen shot of the running server and client:

The following table shows the parameter types and return types demonstrated in the sample client and server:

That’s all for now.  I plan on covering the collection types in my next post.

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