在c＃中将一个任意类型转换为另一个类型的通用方法

Question

I want to do something like this:

public static TResult MyCast<TSource, TResult>(TSource item)
{
    return (TResult)item;
}

Without restrictions on TSource or TResult and avoiding unnecessary boxing if possible. Edit : I want to stress out, that I want a simple casting of types, not elaborate type conversion here. It would be perfectly ok to fail at casting, say string to int .

Is there any sane way to do this using CLR 2.0?

Edit : this is a simplified version, so it's pretty useless, yes. But consider casting generic collections, such as this:

public static Dictionary<string, TResult> CastValues<TSource, TResult>(this Dictionary<string, TSource> dictionary)

After some discussions with my co-workers, it seems like there's no simple way to implement such a feature (if at all possible), so I'm stuck with code bloat of several very simple methods for different situations (ie up- and downcast of reference types and casting of some value types) :(

Too bad I can't use .NET 4.0 with all it's dynamic et al goodness.

Answer 1

How would
x = MyCast<SourceType, ResultType>(y)
be any more useful than
x = (ResultType)y ?

Answer 2

This is straightforward when TSource and TResult are both reference types.

If one or the other are value types, how do you want it to work? Value types can't inherit from each other, so it's not a matter of doing an up- or down-cast. You might expect numeric conversions between, say, int and double , but you'd have to code these yourself: .NET doesn't treat them as typecasts. And conversion between, say, DateTime and string involves more intelligence (what format? which culture? etc.).

If you're just handling reference types then this method can be a one-liner. If you want to handle value types as well then you'll need to write special case code for the various combinations.

Edit: Convert.ChangeType does a reasonable job at encapsulating the various conversions between value types. However you mentioned you're keen not to introduce boxing: Convert.ChangeType isn't generic and it takes an object .

Answer 3

I think that the problem you are trying to solve is the same as the problem that you cannot cast a collection of one type to a collection of another type. eg

class Obj1
{}
class Obj2:Obj1
{}
List<Obj2> srcList = GetList();
List<Obj1> castedList=(List<Obj2>) srcList;//this line wont compile

I have not done much at actually looking at the CLR code However on the asuumption that it is like C++ what you would have here is actually different values stored in the collection. In other words srcList would contain a list of pointers to object 2's interface in castedList you would have a pointer to the the interface of the object 1's within object 2.

In order to resolve this you would need to have your casting function iterate through each of the items within the collection. However in order to be able to iterate through the items the list would have to implement some sort of enumeration interface. So the enumeration interface would need to be a constraint on the casting function.

So the answer would therefore be no.

However if you were prepared to implement this with restrictions on the in types you could have:

static class ListCast<TSource,TResult,TItemType>
where TSource:IEnumerable<TItemType>
where TResult:IList<TItemType>,new()
{
static TResult Cast(TSource list)
{
TResult castedList=newTResult();
foreach(TtemType item in list)
{
castedList.Add(TItemType)item);
}

return castedList;
}
}

Answer 4

you can just do this:

public static TResult MyCast<TSource, TResult>(TSource item)
{
    return (TResult)((object)item);
}

Would love to hear how this could be bad.