在这个问题的答案似乎已经摆脱了讨论性能,而是解决可变值类型的危险。
以防万一你觉得这个有用,这里是一个实现,我扔在一起,做一些类似于你的原始示例使用不可变的值类型包装。
区别在于我的值类型不直接引用它引用的对象;相反,它拥有一个键和引用委托,这些委托使用键(TryGetValueFunc)执行查找或使用键进行创建。(注意:我的原始实现包含了一个IDictionary对象的引用,但我将其更改为一个TryGetValueFunc委托,只是为了使它更灵活一点,尽管这可能更令人困惑,而且我不能100%确定这样做没有造成某种缺陷)。
但是请注意,如果您操作包装器访问的基础数据结构,这仍可能导致意外的行为(取决于您的期望)。
下面是一个完整的工作示例,与控制台程序的使用示例一起:
public delegate bool TryGetValueFunc<TKey, TValue>(TKey key, out TValue value);
public struct KeyedValueWrapper<TKey, TValue>
{
private bool _KeyHasBeenSet;
private TKey _Key;
private TryGetValueFunc<TKey, TValue> _TryGetValue;
private Func<TKey, TValue> _CreateValue;
#region Constructors
public KeyedValueWrapper(TKey key)
{
_Key = key;
_KeyHasBeenSet = true;
_TryGetValue = null;
_CreateValue = null;
}
public KeyedValueWrapper(TKey key, TryGetValueFunc<TKey, TValue> tryGetValue)
{
_Key = key;
_KeyHasBeenSet = true;
_TryGetValue = tryGetValue;
_CreateValue = null;
}
public KeyedValueWrapper(TKey key, Func<TKey, TValue> createValue)
{
_Key = key;
_KeyHasBeenSet = true;
_TryGetValue = null;
_CreateValue = createValue;
}
public KeyedValueWrapper(TKey key, TryGetValueFunc<TKey, TValue> tryGetValue, Func<TKey, TValue> createValue)
{
_Key = key;
_KeyHasBeenSet = true;
_TryGetValue = tryGetValue;
_CreateValue = createValue;
}
public KeyedValueWrapper(TryGetValueFunc<TKey, TValue> tryGetValue)
{
_Key = default(TKey);
_KeyHasBeenSet = false;
_TryGetValue = tryGetValue;
_CreateValue = null;
}
public KeyedValueWrapper(TryGetValueFunc<TKey, TValue> tryGetValue, Func<TKey, TValue> createValue)
{
_Key = default(TKey);
_KeyHasBeenSet = false;
_TryGetValue = tryGetValue;
_CreateValue = createValue;
}
public KeyedValueWrapper(Func<TKey, TValue> createValue)
{
_Key = default(TKey);
_KeyHasBeenSet = false;
_TryGetValue = null;
_CreateValue = createValue;
}
#endregion
#region "Change" methods
public KeyedValueWrapper<TKey, TValue> Change(TKey key)
{
return new KeyedValueWrapper<TKey, TValue>(key, _TryGetValue, _CreateValue);
}
public KeyedValueWrapper<TKey, TValue> Change(TKey key, TryGetValueFunc<TKey, TValue> tryGetValue)
{
return new KeyedValueWrapper<TKey, TValue>(key, tryGetValue, _CreateValue);
}
public KeyedValueWrapper<TKey, TValue> Change(TKey key, Func<TKey, TValue> createValue)
{
return new KeyedValueWrapper<TKey, TValue>(key, _TryGetValue, createValue);
}
public KeyedValueWrapper<TKey, TValue> Change(TKey key, TryGetValueFunc<TKey, TValue> tryGetValue, Func<TKey, TValue> createValue)
{
return new KeyedValueWrapper<TKey, TValue>(key, tryGetValue, createValue);
}
public KeyedValueWrapper<TKey, TValue> Change(TryGetValueFunc<TKey, TValue> tryGetValue)
{
return new KeyedValueWrapper<TKey, TValue>(_Key, tryGetValue, _CreateValue);
}
public KeyedValueWrapper<TKey, TValue> Change(TryGetValueFunc<TKey, TValue> tryGetValue, Func<TKey, TValue> createValue)
{
return new KeyedValueWrapper<TKey, TValue>(_Key, tryGetValue, createValue);
}
public KeyedValueWrapper<TKey, TValue> Change(Func<TKey, TValue> createValue)
{
return new KeyedValueWrapper<TKey, TValue>(_Key, _TryGetValue, createValue);
}
#endregion
public TValue Value
{
get
{
if (!_KeyHasBeenSet)
throw new InvalidOperationException("A key must be specified.");
if (_TryGetValue == null)
throw new InvalidOperationException("A \"try get value\" delegate must be specified.");
// try to find a value in the given dictionary using the given key
TValue value;
if (!_TryGetValue(_Key, out value))
{
if (_CreateValue == null)
throw new InvalidOperationException("A \"create value\" delegate must be specified.");
// if not found, create a value
value = _CreateValue(_Key);
}
// then return that value
return value;
}
}
}
class Foo
{
public string ID { get; set; }
}
class Program
{
static void Main(string[] args)
{
var dictionary = new Dictionary<string, Foo>();
Func<string, Foo> createValue = (key) =>
{
var foo = new Foo { ID = key };
dictionary.Add(key, foo);
return foo;
};
// this wrapper object is not useable, since no key has been specified for it yet
var wrapper = new KeyedValueWrapper<string, Foo>(dictionary.TryGetValue, createValue);
// create wrapper1 based on the wrapper object but changing the key to "ABC"
var wrapper1 = wrapper.Change("ABC");
var wrapper2 = wrapper1;
Foo foo1 = wrapper1.Value;
Foo foo2 = wrapper2.Value;
Console.WriteLine("foo1 and foo2 are equal? {0}", object.ReferenceEquals(foo1, foo2));
// Output: foo1 and foo2 are equal? True
// create wrapper1 based on the wrapper object but changing the key to "BCD"
var wrapper3 = wrapper.Change("BCD");
var wrapper4 = wrapper3;
Foo foo3 = wrapper3.Value;
dictionary = new Dictionary<string, Foo>(); // throw a curve ball by reassigning the dictionary variable
Foo foo4 = wrapper4.Value;
Console.WriteLine("foo3 and foo4 are equal? {0}", object.ReferenceEquals(foo3, foo4));
// Output: foo3 and foo4 are equal? True
Console.WriteLine("foo1 and foo3 are equal? {0}", object.ReferenceEquals(foo1, foo3));
// Output: foo1 and foo3 are equal? False
}
}
使用IDictionary<string, Foo>
代替TryGetValueFunc<string, Foo>
另一种实现。注意反例,我把在使用代码:
public struct KeyedValueWrapper<TKey, TValue>
{
private bool _KeyHasBeenSet;
private TKey _Key;
private IDictionary<TKey, TValue> _Dictionary;
private Func<TKey, TValue> _CreateValue;
#region Constructors
public KeyedValueWrapper(TKey key)
{
_Key = key;
_KeyHasBeenSet = true;
_Dictionary = null;
_CreateValue = null;
}
public KeyedValueWrapper(TKey key, IDictionary<TKey, TValue> dictionary)
{
_Key = key;
_KeyHasBeenSet = true;
_Dictionary = dictionary;
_CreateValue = null;
}
public KeyedValueWrapper(TKey key, Func<TKey, TValue> createValue)
{
_Key = key;
_KeyHasBeenSet = true;
_Dictionary = null;
_CreateValue = createValue;
}
public KeyedValueWrapper(TKey key, IDictionary<TKey, TValue> dictionary, Func<TKey, TValue> createValue)
{
_Key = key;
_KeyHasBeenSet = true;
_Dictionary = dictionary;
_CreateValue = createValue;
}
public KeyedValueWrapper(IDictionary<TKey, TValue> dictionary)
{
_Key = default(TKey);
_KeyHasBeenSet = false;
_Dictionary = dictionary;
_CreateValue = null;
}
public KeyedValueWrapper(IDictionary<TKey, TValue> dictionary, Func<TKey, TValue> createValue)
{
_Key = default(TKey);
_KeyHasBeenSet = false;
_Dictionary = dictionary;
_CreateValue = createValue;
}
public KeyedValueWrapper(Func<TKey, TValue> createValue)
{
_Key = default(TKey);
_KeyHasBeenSet = false;
_Dictionary = null;
_CreateValue = createValue;
}
#endregion
#region "Change" methods
public KeyedValueWrapper<TKey, TValue> Change(TKey key)
{
return new KeyedValueWrapper<TKey, TValue>(key, _Dictionary, _CreateValue);
}
public KeyedValueWrapper<TKey, TValue> Change(TKey key, IDictionary<TKey, TValue> dictionary)
{
return new KeyedValueWrapper<TKey, TValue>(key, dictionary, _CreateValue);
}
public KeyedValueWrapper<TKey, TValue> Change(TKey key, Func<TKey, TValue> createValue)
{
return new KeyedValueWrapper<TKey, TValue>(key, _Dictionary, createValue);
}
public KeyedValueWrapper<TKey, TValue> Change(TKey key, IDictionary<TKey, TValue> dictionary, Func<TKey, TValue> createValue)
{
return new KeyedValueWrapper<TKey, TValue>(key, dictionary, createValue);
}
public KeyedValueWrapper<TKey, TValue> Change(IDictionary<TKey, TValue> dictionary)
{
return new KeyedValueWrapper<TKey, TValue>(_Key, dictionary, _CreateValue);
}
public KeyedValueWrapper<TKey, TValue> Change(IDictionary<TKey, TValue> dictionary, Func<TKey, TValue> createValue)
{
return new KeyedValueWrapper<TKey, TValue>(_Key, dictionary, createValue);
}
public KeyedValueWrapper<TKey, TValue> Change(Func<TKey, TValue> createValue)
{
return new KeyedValueWrapper<TKey, TValue>(_Key, _Dictionary, createValue);
}
#endregion
public TValue Value
{
get
{
if (!_KeyHasBeenSet)
throw new InvalidOperationException("A key must be specified.");
if (_Dictionary == null)
throw new InvalidOperationException("A dictionary must be specified.");
// try to find a value in the given dictionary using the given key
TValue value;
if (!_Dictionary.TryGetValue(_Key, out value))
{
if (_CreateValue == null)
throw new InvalidOperationException("A \"create value\" delegate must be specified.");
// if not found, create a value and add it to the dictionary
value = _CreateValue(_Key);
_Dictionary.Add(_Key, value);
}
// then return that value
return value;
}
}
}
class Foo
{
public string ID { get; set; }
}
class Program
{
static void Main(string[] args)
{
// this wrapper object is not useable, since no key has been specified for it yet
var wrapper = new KeyedValueWrapper<string, Foo>(new Dictionary<string, Foo>(), (key) => new Foo { ID = key });
// create wrapper1 based on the wrapper object but changing the key to "ABC"
var wrapper1 = wrapper.Change("ABC");
var wrapper2 = wrapper1;
Foo foo1 = wrapper1.Value;
Foo foo2 = wrapper2.Value;
Console.WriteLine("foo1 and foo2 are equal? {0}", object.ReferenceEquals(foo1, foo2));
// Output: foo1 and foo2 are equal? True
// create wrapper1 based on the wrapper object but changing the key to "BCD"
var wrapper3 = wrapper.Change("BCD");
var wrapper4 = wrapper3;
Foo foo3 = wrapper3.Value;
Foo foo4 = wrapper4.Value;
Console.WriteLine("foo3 and foo4 are equal? {0}", object.ReferenceEquals(foo3, foo4));
// Output: foo3 and foo4 are equal? True
Console.WriteLine("foo1 and foo3 are equal? {0}", object.ReferenceEquals(foo1, foo3));
// Output: foo1 and foo3 are equal? False
// Counter-example: manipulating the dictionary instance that was provided to the wrapper can disrupt expected behavior
var dictionary = new Dictionary<string, Foo>();
var wrapper5 = wrapper.Change("CDE", dictionary);
var wrapper6 = wrapper5;
Foo foo5 = wrapper5.Value;
dictionary.Clear();
Foo foo6 = wrapper6.Value;
// one might expect this to be true:
Console.WriteLine("foo5 and foo6 are equal? {0}", object.ReferenceEquals(foo5, foo6));
// Output: foo5 and foo6 are equal? False
}
}
这是一日一次,我看到这种类型的问题 – TalentTuner 2010-12-09 15:40:38