如何在 Delphi 中将特定类型的变量转换为泛型 T？

Question

Depending on the actual type of T, I want to return a specific value; here a simplified example (a record wrapper around some object):

function TMyRec.TryGet<T>(const Default: T): T;
begin
  if TypeInfo(T) = TypeInfo(TMyObject) then
    Result:= FMyObject {E2010 Incompatible types}
  else Result:= FMyObject.TryGet<T>(Default);
end;

It's clear that I cannot assign anything other than T to Result , and as T or T() don't work, when T is not restricted to class ...

It's also not possible to do an overload which differs only in the generic restriction:

function TryGet<T: TMyObject>(const Default: T): T; overload;
{...}
Result:= FMyObject as T; {here it would work}

---

To help understand the discussion in the comments below, here is the example from before my edit:

function TMyRec.TryGet<T>(const Default: T): T;
begin
  if TypeInfo(T) = TypeInfo(TMyObject) then Result:= FMyObject
  //else ...
  else Result:= Default;
end;

Answer 1

I believe this is the best method (performance-wise at least) to achieve what you want.

type 
  PMyObject = ^TMyObject;

function TMyRec.TryGet<T>(const Default: T): T;
begin
  if TypeInfo(T) = TypeInfo(TMyObject) then 
    PMyObject(@Result)^ := FMyObject
  else if TypeInfo(T) = TypeInfo(Integer) then 
    PInteger(@Result)^ := 1
  //else ...
  else
    Result := Default;
end;

Answer 2

The only way I found is using System.Rtti.TValue (exception if wrong type):

Result:= TValue.From(FMyObject).AsType<T>

or much shorter ;-) with automatic type (returns false if wrong type; you could ignore that here because you already checked TypeInfo):

TValue.From(FMyObject).TryAsType(Result)

Suggestion: rethink your concept; there are concerns that generics might not be a good solution, but we cannot tell without knowing more about the background.

Answer 3

I'm using Delphi XE5.

Change yoru sample to this:

function TryGet<T: class>(const Default: T): T; overload;
{...}
Result:= FMyObject as T; {here it will work}

The change is TMyObject to class . When you are not "class" it, you will get the compiler error E2089.