HashSet.Add(int) doesn't accept the parameter of boxed int

Question

I have the following function. The function will always be passed the parameter of k which can be converted to keyType.

void F(object k, Type keyType) // keyType is from runtime. cannot be generic
{
    object kvalue = Convert.ChangeType(k, keyType); //boxed int as expected
    dynamic hs = Activator.CreateInstance(typeof(HashSet<>).MakeGenericType(keyType)); 
    hs.Add(kvalue); // Runtime error
    //....
}

// Setup for testing, not real usage.
object o = 10m; // decimal
var t = typeof(int); // will get the type at runtime. 

// Call the function
F(o, t);

However, the above code will get the runtime error of:

Microsoft.CSharp.RuntimeBinder.RuntimeBinderException: 'The best overloaded method match for 'System.Collections.Generic.HashSet<int>.Add(int)' has some invalid arguments'

Changing

dynamic hs = Activator.CreateInstance(typeof(HashSet<>).MakeGenericType(keyType));

to

//dynamic hs = Activator.CreateInstance(typeof(HashSet<dynamic>));
var hs = new HashSet<dynamic>();

can resolve the problem. Is there any other way?

Why do you need Activator.CreateInstance to create a new Hashset? — OneCricketeer, Mar 15 '20 at 23:49
@jdweng It will not compile. keyType is an instance of `Type`. — ca9163d9, Mar 15 '20 at 23:50
I've edited title to make it clear what you trying to add to the HashSet ("object {int}" is very confusing term for "boxed int"). I would recommend re-wording the title rather than simply reverting if you disagree with my change. — Alexei Levenkov, Mar 16 '20 at 00:41
@ca9163d9 the example you've provided is somewhat misleading since you explicitly pass `typeof(int)` to `F()`, implying that you know the type at compile-time. Which conflicts with your comments stating that the type is only known at runtime — devNull, Mar 16 '20 at 01:49
@devNull, I've updated the question. I gave the code for testing. — ca9163d9, Mar 16 '20 at 02:37

weichch · Accepted Answer · 2020-03-16T00:00:39.757

Convert.ChangeType returns an instance of object. There's no Add(object) method defined on HashSet<int>, only Add(int).

Try the following using expression:

void F(object k, Type keyType)
{
    var hsType = typeof(HashSet<>).MakeGenericType(keyType);
    object hs = Activator.CreateInstance(hsType);
    var kvalue = Convert.ChangeType(k, keyType);

    Expression.Lambda<Action>(
        Expression.Call(
            Expression.Constant(hs, hsType),
            "Add",
            Type.EmptyTypes,
            Expression.Constant(kvalue, keyType))
        ).Compile()();
}

HashSet.Add(int) doesn't accept the parameter of boxed int

1 Answers1