你可以通过使用标记接口和扩展方法来获取语法。
先决条件：接口需要定义后续由扩展方法使用的契约。基本上，接口为能够“实现”一个特征定义了契约；理想情况下，添加接口的类应该已经具有接口的所有成员，以便
不需要进行额外的实现。
public class Client {
public double Weight { get; }
public double Height { get; }
}
public interface TClientWeight {
double Weight { get; }
}
public interface TClientHeight {
double Height { get; }
}
public class ClientA: Client, TClientWeight { }
public class ClientB: Client, TClientHeight { }
public class ClientC: Client, TClientWeight, TClientHeight { }
public static class TClientWeightMethods {
public static bool IsHeavierThan(this TClientWeight client, double weight) {
return client.Weight > weight;
}
}
public static class TClientHeightMethods {
public static bool IsTallerThan(this TClientHeight client, double height) {
return client.Height > height;
}
}
使用方法如下：
var ca = new ClientA();
ca.IsHeavierThan(10); // OK
ca.IsTallerThan(10); // compiler error
编辑：有人提出了如何存储附加数据的问题。这也可以通过进行一些额外的编码来解决：
public interface IDynamicObject {
bool TryGetAttribute(string key, out object value);
void SetAttribute(string key, object value);
}
public class DynamicObject: IDynamicObject {
private readonly Dictionary<string, object> data = new Dictionary<string, object>(StringComparer.Ordinal);
bool IDynamicObject.TryGetAttribute(string key, out object value) {
return data.TryGet(key, out value);
}
void IDynamicObject.SetAttribute(string key, object value) {
data[key] = value;
}
}
然后，如果“trait interface”继承自IDynamicObject，trait方法可以添加和检索数据：
public class Client: DynamicObject { }
public interface TClientWeight, IDynamicObject {
double Weight { get; }
}
public class ClientA: Client, TClientWeight { }
public static class TClientWeightMethods {
public static bool HasWeightChanged(this TClientWeight client) {
object oldWeight;
bool result = client.TryGetAttribute("oldWeight", out oldWeight) && client.Weight.Equals(oldWeight);
client.SetAttribute("oldWeight", client.Weight);
return result;
}
}
注意：通过实现
IDynamicMetaObjectProvider，对象甚至可以通过DLR公开动态数据，当使用
dynamic关键字时，访问其他属性将变得透明。
