如何为 Variant 值创建类型安全的包装器
我正在使用 OPC Server 控件,该控件将数据标签存储为变体类型,由 < 描述强>System.Runtime.InteropServices.VarEnum。这些类型包括以下类型:VT_BSTR(字符串)、VT_I2(短)和 VT_I4(长)。
所有这些值都由服务器存储为对象,然后在获取它们时必须转换为正确的值。
我知道我可以执行如下操作:
object tagValue = (object)"testing"; //this would be returned from a function rather than created this way!!
var typedVariant = new TypedVariant<string>(tagValue);
string actualString = typedVariant.Value;
其中 TypedVariant 是一个通用类,如下所示:
class TypedVariant<T> where T : class, struct
{
public TypedVariant(object variant)
{
Value = variant as T;
}
public T Value { private set; get; }
public static implicit operator TypedVariant<T> (T m)
{
// code to convert from TypedVariant<T> to T
return new TypedVariant<T>(m);
}
public static implicit operator T (TypedVariant<T> m)
{
// code to convert from T to TypedVariant<T>
return m.Value;
}
}
但是有什么方法可以在运行时完成这一切,即如下所示:
TypedVariant<> typedVariant = TypedVariant.Create(VarEnum.VT_BSTR, tagValue);
//typedVariant should now be of type TypedVariant<string>
显然此代码不会编译,但可以可以这样做吗?
更新:根据@Konamiman的建议,我已经更改了类以允许隐式转换。因此,您现在可以编写此代码,并且它都是类型安全的,因此您不能将变体存储为与创建它所用的类型不同的类型。
object objectStr = (object)"testing"; //created this way just for testing
TypedVariant<string> typedVariant = (string)objectStr;
string actualString = typedVariant;
I'm working with a OPC Server control that stores data tags as variant types, described by System.Runtime.InteropServices.VarEnum. These types include the following, VT_BSTR (string), VT_I2 (short) and VT_I4 (long).
All these values are stored by the server as objects and then I have to cast to the correct value when I fetch them.
I know that I can do something like the following:
object tagValue = (object)"testing"; //this would be returned from a function rather than created this way!!
var typedVariant = new TypedVariant<string>(tagValue);
string actualString = typedVariant.Value;
Where TypedVariant is a generic class something like this:
class TypedVariant<T> where T : class, struct
{
public TypedVariant(object variant)
{
Value = variant as T;
}
public T Value { private set; get; }
public static implicit operator TypedVariant<T> (T m)
{
// code to convert from TypedVariant<T> to T
return new TypedVariant<T>(m);
}
public static implicit operator T (TypedVariant<T> m)
{
// code to convert from T to TypedVariant<T>
return m.Value;
}
}
But is there any way I can do it all at runtime, i.e. something like the following:
TypedVariant<> typedVariant = TypedVariant.Create(VarEnum.VT_BSTR, tagValue);
//typedVariant should now be of type TypedVariant<string>
Obviously this code won't compile, but can it be done like this?
Update: as per the suggestion from @Konamiman, I've changed the class to allow implicit casting. So you can now write this code and it's all typesafe, so you can't store the variant in a different type than the one it was created with.
object objectStr = (object)"testing"; //created this way just for testing
TypedVariant<string> typedVariant = (string)objectStr;
string actualString = typedVariant;
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我认为 TypedVariant 包装器是一个好主意,您可以扩展它以覆盖来自/到包装类型的隐式转换运算符,然后您将能够更透明地使用它,例如:
参见此处:MSDN 上的“隐式”关键字参考
I think that the
TypedVariant
wrapper is a good idea, you could extend it to override the implicit conversion operator from/to the wrapped type and then you would be able to use it more transparently, such as:See here: "implicit" keyword reference at MSDN