如何将接口变量分配给Rtti.TValue类型的变量

Question

At the moment I give delphi2010 a trial and found the TValue type of the Rtti Unit. TValue have very interessting features, but I can't find a way to assign an interface.

I try the following

program Project1;
uses
  Classes, SysUtils, Rtti;
var
   list : IInterfaceList;
   value : TValue;
begin
  // all these assignments works
  value := 1;
  value := 'Hello';
  value := TObject.Create;

  // but nothing of these assignments works
  list := TInterfaceList.Create;
  value := list; // [DCC Fehler] Project1.dpr(15): E2010 incompatible types: 'TValue' and 'IInterfaceList'
  value.From[list]; // [DCC Fehler] Project1.dpr(16): E2531 Method 'From' requires explicit typarguments
  value.From<IInterfaceList>[list]; // [DCC Fehler] Project1.dpr(17): E2035 Not enough parameters
end.

I can't find any further information. Not in the delphi help system and not on the internet. What do I do wrong?

Answer 1

This is a working version of the program:

program Project1;
uses
  Classes, SysUtils, Rtti;
var
   list : IInterfaceList;
   value : TValue;
begin
  // all these assignments works
  value := 1;
  value := 'Hello';
  value := TObject.Create;

  // but nothing of these assignments works
  list := TInterfaceList.Create;
  value := TValue.From(list);
end.

Answer 2

Your last try is the closest. TValue.From is a class function that creates a TValue from a parameter. You probably put the square brackets in there because that's how CodeInsight showed it, right? That's actually a glitch in CodeInsight; it does that for generics-based functions, where you should be using parenthesis instead. The proper syntax looks like this:

Value := TValue.From<IInterfaceList>(list);