C++ 和 C# 都缺少创建 new 类型的简单方法，该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来说是完全必要的，而且真的很遗憾 c# 没有内置它们。 void f(string connectionID, string username) 和 void f(ConID connectionID, UserName username) 的区别很明显……
（您可以在 C++ 中通过 BOOST_STRONG_TYPEDEF 中的 boost 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到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将我们的类型组合到一个新类中：
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();
}
虽然这会起作用，但对于 typedef 来说是非常冗长的。
此外，我们在序列化（即对 Json）方面存在问题，因为我们想通过其 Composed 属性对类进行序列化。
下面是一个帮助类，它使用“Curiously Recurring Template Pattern”来简化它：
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);
}
}
}
有了 Composer，上面的类就变得简单了：
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 将以与 SomeType 相同的方式序列化为 Json。
希望这会有所帮助！