Can you use a record as a Key value in TDictionary? I want to find objects based on combination of string, integer and integer.
TUserParKey=record
App:string;
ID:integer;
Nr:integer;
end;
...
var
tmpKey:TUserParKey;
tmpObject:TObject;
begin
tmpObject:= TTObject.Create(1);
tmpKey.App:='1';
tmpKey.ID :=1;
tmpKey.Nr :=1;
DTUserPars.Add(tmpKey,tmpObject)
...
var
tmpKey:TUserParKey;
begin
tmpKey.App:='1';
tmpKey.ID :=1;
tmpKey.Nr :=1;
if not DTUserPars.TryGetValue(tmpKey,Result) then begin
result := TTObject.Create(2);
end;
This returns object 2.
Yes, you can use records as keys in a TDictionary but you should provide your own IEqualityComparer when creating the dictionary because the default one for records just does a dumb binary compare of the record. This fails for a record containing a string because it just compares the pointer of that string which may be different even if the string contains the same value.
Such a comparer would look like this:
type
TUserParKeyComparer = class(TEqualityComparer<TUserParKey>)
function Equals(const Left, Right: TUserParKey): Boolean; override;
function GetHashCode(const Value: TUserParKey): Integer; override;
end;
function TUserParKeyComparer.Equals(const Left, Right: TUserParKey): Boolean;
begin
Result := (Left.App = Right.App) and (Left.ID = Right.ID) and (Left.Nr = Right.Nr);
end;
function TUserParKeyComparer.GetHashCode(const Value: TUserParKey): Integer;
begin
Result := BobJenkinsHash(PChar(Value.App)^, Length(Value.App) * SizeOf(Char), 0);
Result := BobJenkinsHash(Value.ID, SizeOf(Integer), Result);
Result := BobJenkinsHash(Value.Nr, SizeOf(Integer), Result);
end;
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