C ++和C＃都缺少创建新类型的简便方法，该新类型在语义上与现有类型相同。我发现这样的“ typedefs”对于类型安全的编程至关重要，而真正的遗憾是C＃没有内置它们。void f(string connectionID, string username)到之间的区别void f(ConID connectionID, UserName username)很明显...
（您可以通过提高BOOST_STRONG_TYPEDEF中的C ++实现类似的功能）
使用继承可能很诱人，但这有一些主要限制：
- 它不适用于原始类型
- 派生类型仍然可以转换为原始类型，即我们可以将其发送给接收原始类型的函数，这违背了整个目的
- 我们不能从密封类派生（即许多.NET类是密封的）
在C＃中实现类似功能的唯一方法是在新类中编写我们的类型：
Class SomeType {
public void Method() { .. }
}
sealed Class SomeTypeTypeDef {
public SomeTypeTypeDef(SomeType composed) { this.Composed = composed; }
private SomeType Composed { get; }
public override string ToString() => Composed.ToString();
public override int GetHashCode() => HashCode.Combine(Composed);
public override bool Equals(object obj) => obj is TDerived o && Composed.Equals(o.Composed);
public bool Equals(SomeTypeTypeDefo) => object.Equals(this, o);
// proxy the methods we want
public void Method() => Composed.Method();
}
尽管这可以工作，但对于typedef来说却非常冗长。另外，由于要通过其Composed属性来对类进行序列化，因此在序列化（即Json）方面存在问题。
下面是一个帮助器类，它使用“好奇地重复出现的模板模式”使此过程更加简单：
namespace Typedef {
[JsonConverter(typeof(JsonCompositionConverter))]
public abstract class Composer<TDerived, T> : IEquatable<TDerived> where TDerived : Composer<TDerived, T> {
protected Composer(T composed) { this.Composed = composed; }
protected Composer(TDerived d) { this.Composed = d.Composed; }
protected T Composed { get; }
public override string ToString() => Composed.ToString();
public override int GetHashCode() => HashCode.Combine(Composed);
public override bool Equals(object obj) => obj is Composer<TDerived, T> o && Composed.Equals(o.Composed);
public bool Equals(TDerived o) => object.Equals(this, o);
}
class JsonCompositionConverter : JsonConverter {
static FieldInfo GetCompositorField(Type t) {
var fields = t.BaseType.GetFields(BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.FlattenHierarchy);
if (fields.Length!=1) throw new JsonSerializationException();
return fields[0];
}
public override bool CanConvert(Type t) {
var fields = t.GetFields(BindingFlags.Instance | BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.FlattenHierarchy);
return fields.Length == 1;
}
// assumes Compositor<T> has either a constructor accepting T or an empty constructor
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) {
while (reader.TokenType == JsonToken.Comment && reader.Read()) { };
if (reader.TokenType == JsonToken.Null) return null;
var compositorField = GetCompositorField(objectType);
var compositorType = compositorField.FieldType;
var compositorValue = serializer.Deserialize(reader, compositorType);
var ctorT = objectType.GetConstructor(new Type[] { compositorType });
if (!(ctorT is null)) return Activator.CreateInstance(objectType, compositorValue);
var ctorEmpty = objectType.GetConstructor(new Type[] { });
if (ctorEmpty is null) throw new JsonSerializationException();
var res = Activator.CreateInstance(objectType);
compositorField.SetValue(res, compositorValue);
return res;
}
public override void WriteJson(JsonWriter writer, object o, JsonSerializer serializer) {
var compositorField = GetCompositorField(o.GetType());
var value = compositorField.GetValue(o);
serializer.Serialize(writer, value);
}
}
}
使用Composer，上面的类变得很简单：
sealed Class SomeTypeTypeDef : Composer<SomeTypeTypeDef, SomeType> {
public SomeTypeTypeDef(SomeType composed) : base(composed) {}
// proxy the methods we want
public void Method() => Composed.Method();
}
此外，SomeTypeTypeDef它将以相同的方式序列化到Json SomeType。
希望这可以帮助 ！