Loading and Saving Custom Variant Values

Go Up to Copying and Clearing Custom Variants

By default, when the custom variant is assigned as the value of a published property, it is typecast to a string when that property is saved to a form file, and converted back from a string when the property is read from a form file. You can, however, provide your own mechanism for loading and saving custom variant values in a more natural representation. To do so, the TCustomVariantType descendant must implement the System.Classes.IVarStreamable interface from Classes.pas.

IVarStreamable defines two methods, StreamIn and StreamOut, for reading a value of a variant from a stream and for writing the variant's value to the stream. For example, TComplexVariantType, in the VarCmplx unit, implements the IVarStreamable methods as follows:

 procedure TComplexVariantType.StreamIn(var Dest: TVarData; const Stream: TStream);
 begin
 with TReader.Create(Stream, 1024) do
 try
 with TComplexVarData(Dest) do
 begin
 VComplex := TComplexData.Create;
 VComplex.Real := ReadFloat;
 VComplex.Imaginary := ReadFloat;
 end;
 finally
 Free;
 end;
 end;
 procedure TComplexVariantType.StreamOut(const Source: TVarData; const Stream: TStream);
 begin
 with TWriter.Create(Stream, 1024) do
 try
 with TComplexVarData(Source).VComplex do
 begin
 WriteFloat(Real);
 WriteFloat(Imaginary);
 end;
 finally
 Free;
 end;
 end;

See Also

• Implementing Binary Operations
• Implementing Comparison Operations
• Implementing Unary Operations
• Enabling Casting
• Copying and Clearing Custom Variants
• Using the TCustomVariantType Descendant

• Windows Developer's Guide
• Delphi

• AMMigrate