delphi Using records as key in TDictionary

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;