How can I call GetEnumName with a generic enumerated type?

Question

I have a Generic class which uses an Enum Generic Type. My problem how do I use GetEnumName on an instance of that type?

I've created a small demo class to illustrate the problem:

type
  TEnumSettings<TKey: record > = class
    private
      Key: TKey;
    public
      constructor Create(aKey: TKey);
      function ToString: string; override;
    end;

uses
  TypInfo;


{ TEnumSettings<TKey> }

constructor TEnumSettings<TKey>.Create(aKey: TKey);
begin
  if PTypeInfo(System.TypeInfo(TKey)).Kind <> tkEnumeration then
    Exception.Create(string(PTypeInfo(System.TypeInfo(TKey)).Name) + ' is not an Enumeration');
  Key := aKey;
end;

function TEnumSettings<TKey>.ToString: string;
begin
  Result := GetEnumName(System.TypeInfo(TKey), Integer(Key)) <== HERE I get a compile error: Invalid type cast
end;

I'm using Delphi XE. So can this be done? And if so how?

Answer 1

Personally, I would do this with a call to Move. I have the following type:

type
  TEnumeration<T: record> = class
  strict private
    class function TypeInfo: PTypeInfo; inline; static;
    class function TypeData: PTypeData; inline; static;
  public
    class function IsEnumeration: Boolean; static;
    class function ToOrdinal(Enum: T): Integer; inline; static;
    class function FromOrdinal(Value: Integer): T; inline; static;
    class function MinValue: Integer; inline; static;
    class function MaxValue: Integer; inline; static;
    class function InRange(Value: Integer): Boolean; inline; static;
    class function EnsureRange(Value: Integer): Integer; inline; static;
  end;

{ TEnumeration<T> }

class function TEnumeration<T>.TypeInfo: PTypeInfo;
begin
  Result := System.TypeInfo(T);
end;

class function TEnumeration<T>.TypeData: PTypeData;
begin
  Result := TypInfo.GetTypeData(TypeInfo);
end;

class function TEnumeration<T>.IsEnumeration: Boolean;
begin
  Result := TypeInfo.Kind=tkEnumeration;
end;

class function TEnumeration<T>.ToOrdinal(Enum: T): Integer;
begin
  Assert(IsEnumeration);
  Assert(SizeOf(Enum)<=SizeOf(Result));
  Result := 0; // needed when SizeOf(Enum) < SizeOf(Result)
  Move(Enum, Result, SizeOf(Enum));
  Assert(InRange(Result));
end;

class function TEnumeration<T>.FromOrdinal(Value: Integer): T;
begin
  Assert(IsEnumeration);
  Assert(InRange(Value));
  Assert(SizeOf(Result)<=SizeOf(Value));
  Move(Value, Result, SizeOf(Result));
end;

class function TEnumeration<T>.MinValue: Integer;
begin
  Assert(IsEnumeration);
  Result := TypeData.MinValue;
end;

class function TEnumeration<T>.MaxValue: Integer;
begin
  Assert(IsEnumeration);
  Result := TypeData.MaxValue;
end;

class function TEnumeration<T>.InRange(Value: Integer): Boolean;
var
  ptd: PTypeData;
begin
  Assert(IsEnumeration);
  ptd := TypeData;
  Result := Math.InRange(Value, ptd.MinValue, ptd.MaxValue);
end;

class function TEnumeration<T>.EnsureRange(Value: Integer): Integer;
var
  ptd: PTypeData;
begin
  Assert(IsEnumeration);
  ptd := TypeData;
  Result := Math.EnsureRange(Value, ptd.MinValue, ptd.MaxValue);
end;

The ToOrdinal method does what you need, and I'm sure you'll be able to adapt it to your class.

If you don't like using Move in this way, then you can use TValue.

TValue.From<TKey>(Key).AsOrdinal

And @TLama points out that you can avoid calling GetEnumName at all by using

TValue.From<TKey>(Key).ToString

On the face of it, using TValue seems to be more in keeping with the ethos of generics and RTTI. A call to Move relies on the specific implementation details of enumerated types. However, it's quite interesting to step through the debugger and observe quite how much code is involved in executing TValue.From<TKey>(Key).AsOrdinal. That alone is enough to make me hesitate to recommend using TValue.

Yet another way to achieve this is to use TRttiEnumerationType:

TRttiEnumerationType.GetName<TKey>(Key)

The implementation of this is much more efficient than using TValue.ToString, being little more than a call to GetEnumName.

Answer 2

This is an updated version af my class, with the change suggested. Thanks to David and TLama

uses
  TypInfo, Rtti;

type
  TEnumSettings<TKey: record> = class
  private
    Key: TKey;
  public
    constructor Create(aKey: TKey);
    function ToString: string; override;
  end;


{ TEnumSettings<TKey> }

constructor TEnumSettings<TKey>.Create(aKey: TKey);
begin
  if PTypeInfo(System.TypeInfo(TKey)).Kind <> tkEnumeration then
    raise Exception.Create(string(PTypeInfo(System.TypeInfo(TKey)).Name) + ' is not an Enumeration');
  Key := aKey;
end;

function TEnumSettings<TKey>.ToString: string;
begin
  Result := TValue.From<TKey>(Key).ToString;
end;

And a little test example :

Copy the code into OnCreate of a From:

procedure TForm1.FormCreate(Sender: TObject);
begin
  with TEnumSettings<boolean> .Create(True) do
    try
      Caption := ToString;
    finally
      Free;
    end;
end;