錯誤「用戶定義的轉換必須轉換爲封閉類型或從封閉類型轉換」正是這個意思。如果你有一個轉換操作符
class MyClass {
public static explicit operator xxx(string s) { // details }
public static implicit operator string(xxx x) { // details }
}
然後xxx
必須MyClass
。這就是「轉換必須轉換爲封閉類型或從封閉類型轉換」的含義。這裏的封閉類型是MyClass
。
的ECMA334 C#規範的相關章節17.9.4:
A conversion operator converts from a source type, indicated by the parameter type of the conversion operator, to a target type, indicated by the return type of the conversion operator. A class or struct is permitted to declare a conversion from a source type S to a target type T only if all of the following are true, where S0 and T0 are the types that result from removing the trailing ? modifiers, if any, from S and T:
S0 and T0 are different types.
Either S0 or T0 is the class or struct type in which the operator declaration takes place.
Neither S0 nor T0 is an interface-type.
Excluding user-defined conversions, a conversion does not exist from S to T or from T to S.
因此,這裏是你的代碼:
public static explicit operator List<Model.objA>(List<Entity.objA> entities) {
List<Model.objA> objs= new List<Model.objA>();
foreach (Entity.objA entity in entities) {
objs.Add((Model.objA)entity);
}
return claims;
}
的問題是,這個被定義爲一個轉換操作符它必須位於List<Model.objA>
或List<Entity.objA>
類中,但當然由於您無權更改這些類型,因此您無法這樣做。
您可以使用Enumerable.Select
投影到另一種類型或List<T>.ConvertAll
。例如:
public static class ListExtensions {
public static List<Model.objA> ConvertToModel(this List<Entity.objA> entities) {
return entities.ConvertAll(e => (Model.objA)e);
}
}