初始化泛型类中的泛型参数

Question

Alright, I have a generic class. The basics are this:

public class GenericPrimitiveContainer<T> : IGetAndSetGenericValue
{
    private T _value;

    public T Value
    {
        get
        {
            return _value;
        }
        set
        {
            _value = value;
        }
    }
}

Fine enough. I also have a SetValue method in that class which takes an object and sets it to the value which uses this code:

PropertyInfo pi = this.GetType().GetProperty("Value");
            pi.SetValue(this, o, null);

That's not all it does because it also checks the type of the object and the type of Value and compares them. If they are the same, o (the object) is assigned to Value. If they're not the same, a conversion is applied. I won't go into that because that's not where the problem is (famous last words I'm sure).

The problem is if Value is of type string. As I said before, I compare the types to see if they are the same. This is as follows ('o' being the object passed in):

Type t1 = Value.GetType();
        Type t2 = o.GetType();
        if (t1 == t2) ...

If T is int, no problem. If T is String, 'Value' is just 'null'. I can't do 'GetType' on it even to see if it's a String because it's just null.

I have tried, as a test, just getting rid of the checks and testing the set method in a situation where I know a string will be passed to the method. Value was still initially null, but it worked out okay and Value was assigned.

Now I know string is not a primitive and so will work a little differently to an int, but I'm not sure how to overcome this problem. I considered initialising _value to default(T), which didn't work. I also added a constructor to the class which did the same thing. That also didn't work.

I have also tried constraining the class with a 'where t : new()' but that doesn't work because String is not a "non-abstract type with a public parameterless constructor in order to use it as parameter 'T'".

So hopefully someone wiser can help me out on this one.

Answer 1

Your problem is that Value.GetType() doesn't do what you want. Here's a really short complete example:

using System;

    static class Generic
    {
        public static string WhatIsT<T>(T value)
        {
            return $"T is {value.GetType().FullName}";
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            int i = 5;
            Console.WriteLine(Generic.WhatIsT(i));

            string s = "hello";
            Console.WriteLine(Generic.WhatIsT(s));

            s = null;
            Console.WriteLine(Generic.WhatIsT(s));

            Console.ReadLine();
        }
    }

The first and second calls to WhatIsT will be fine, but there will be a null reference exception on the third.

If you really really really really need to know the exact name of the generic type you've been closed over - and please heed the caveats in the comments that this almost certainly isn't the right thing to be doing - simply use typeof(T) , like this:

            return $"T is {typeof(T).FullName}";

Result:

T is System.Int32
T is System.String
T is System.String

Remember, GetType needs an object. typeof just needs a compile time name, which includes type parameters.