Question

I have a generic function that is constrained to struct. My inputs are boxed (“objects”). Is it possible to unbox the value at runtime to avoid having to check for each possible type and do the casts manually?

See the above example:

   public struct MyStruct
    {
        public int Value;
    }

    public void Foo<T>(T test)
        where T : struct
    {
        // do stuff
    }

    public void TestFunc()
    {
        object o = new MyStruct() { Value = 100 }; // o is always a value type

        Foo(o);
    }

In the example, I know that o must be a struct (however, it does not need to be MyStruct …). Is there a way to call Foo without tons of boilerplate code to check for every possible struct type?

Thank you.

Answer 1

.NET Generics are implemented in a manner that allows value types as a generic type parameter without incurring any boxing/unboxing overhead. Because your’re casting to object before calling Foo you don’t take advantage of that, in fact you’re not even taking advantage of generics at all.

The whole point of using generics in the first place is to replace the “object-idiom”. I think you’re missing the concept here.
Whatever type T happens to be, it is available at run-time and because you constrained it to struct guaranteed to be a struct type.

Your TestFunc could be written like this without problem:

public void TestFunc()
{
    MyStruct o = new MyStruct() { Value = 100 }; // o is always a value type

    Foo<MyStruct>(o);
}

Looking at Foo, it would look like this in your example:

public void Foo<T>(T test)
    where T : struct
{
    T copy = test; // T == MyStruct
}

EDIT:

Ok, since the OP clarified what he wants to call the generic method but doesn’t know the type of his struct (it’s just object). The easiest way to call your generic method with the correct type parameter is to use a little reflection.

public void TestFunc()
{
    object o = new DateTime();

    MethodInfo method = this.GetType().GetMethod("Foo");
    MethodInfo generic = method.MakeGenericMethod(o.GetType());
    generic.Invoke(this, new object[] {o});


}
public void Foo<T>(T test)
    where T : struct
{
    T copy = test; // T == DateTime
}