Question

我有一个使用TDictionary的案例：

var D: TDictionary<string, integer>;
begin
  D := TDictionary<string, integer>.Create(TCustomEqualityComparer.Create());
  try
    D.Add('One', 1);
    D.Add('Two', 2);
    D.Add('Three', 3);

    showmessage(inttostr(D.Items['One']));
    showmessage(inttostr(D.Items['TWO']));
  finally
    D.Free;
  end;
end;

类TCustomEqualityComparer是从Generics Defaults TEqualityComparer (Delphi)复制而在GetHashCode方法上稍作修改：

TCustomEqualityComparer = class(TEqualityComparer<string>)
public
  function Equals(const Left, Right: string): Boolean; override;
  function GetHashCode(const Value: string): Integer; override;
end;

function TCustomEqualityComparer.Equals(const Left, Right: string): Boolean;
begin
  Result := SameText(Left, Right);
end;

function TCustomEqualityComparer.GetHashCode(const Value: string): Integer;
begin
  Result := BobJenkinsHash(Value[1], Length(Value) * SizeOf(Value[1]), 0);
end;

我希望TCustomEqualityComparer能够对键值执行不区分大小写的匹配。例如：

D.Items['TWO']

但是，我收到“找不到项目”的例外情况。我使用的是Delphi 2010版本14.0.3513.24210。

有谁知道我的代码出了什么问题？

Answer 1

uses System.Generics.Collections, System.Generics.Defaults;

var
  D: TDictionary<string, Integer>;
begin
  D := TDictionary<string, Integer>.Create(TIStringComparer.Ordinal); // ‹- this is the trick
  try
    D.Add('One', 1);
    .
    .
  finally
    D.Free;
  end;
end;

Answer 2

对于返回Equals = true的所有值，HashCode必须相同！在将GetHashCode发送到HashFunction之前尝试在GetHashCode中使用Value大写。

Answer 3

感谢。我已经改变了TCustomEqualityComparer.GetHashCode，它就像你说的那样工作：

function TCustomEqualityComparer.Equals(const Left, Right: string): Boolean;
begin
  {$IFDEF UNICODE }
    Result := (Left.ToUpper = Right.ToUpper);
  {$ELSE }
    Result := SameText(Left, Right);
  {$ENDIF }
end;

function TCustomEqualityComparer.GetHashCode(const Value: string): Integer;
var s: string;
begin
  s := {$IFDEF UNICODE } Value.ToUpper {$ELSE } UpperCase(Value) {$ENDIF };
  Result := BobJenkinsHash(s[1], Length(s) * SizeOf(s[1]), 0);
end;

如何为TDictionary创建一个敏感的TEqualityComparer案例？

3 个答案: