你能从c＃中的泛型中获得特定类型的行为吗？

Question

I think it should be pretty clear what I want to do - get an explicit specialization for int and string , and in C++ this is trivial with explicit specialization - is it possible to get this same behavior in C#? Assume I have a good reason for doing this and this is a trivial example of something broader in my program.

static class ReturnConstant<T>
{
    public static T FiveOrHello()
    {
        if(typeof(T) == typeof(int))
        {
            return 5;
        }
        else if (typeof(T) == typeof(string))
        {
            return "Hello!";
        }
        else
        {
            throw new NotImplementedException("OH NO");
        }
    }
}

Edit:

Here's the equivalent, perfectly legit c++ code:

template <typename T>
T giveConst();
template <>
int giveConst<int>() { return 5; }
template <>
std::string giveConst<std::string>() { return "Hello"; }

Answer 1

The only thing wrong with your implementation is that the C# compiler can't verify the type cast to T.

But you can work around that like this:

static class ReturnConstant<T>
{
    public static T FiveOrHello()
    {
        if (typeof(T) == typeof(int))
        {
            return (T)(object)5;
        }
        else if (typeof(T) == typeof(string))
        {
            return (T)(object)"Hello!";
        }
        else
        {
            throw new NotImplementedException("OH NO");
        }
    }
}

Answer 2

No, it's impossible. Generic T for class ReturnConstant<T> means, that ReturnConstant<int> and ReturnConstant<string> are different types. And you can't return different types in common.

Answer 3

A concept you may be interested in is a discriminated union; it would allow you to return either an int or a string with a single type, but you'd have to check at runtime to work out which one actually got returned:

sadly the concept doesn't currently exist in a way that can be efficiently implemented, requiring some language support, which is not yet added.

https://github.com/dotnet/csharplang/issues/113