Question

The following code executes without error and prints “In some“, which means that the statement

m[0].Invoke(o, args);

invokes the function some which is a member of the foo class, on object o and affects its public variable i. But when we uncomment the last line of the code and try to compile it, it produces an error. Why??

using System;
using System.Reflection;

class foo
{
    public int i;
    public foo(int ii = 0)
    {
        i = ii;
    }
    public void some(int ii)
    {
        i = ii;
        Console.WriteLine("In some ");
    }
}

class main
{
    static public void Main()
    {
        foo f = new foo();
        object o = new foo();

        Type t = typeof(foo);


        object[] args = new object[1];
        args[0] = 9;
        MethodInfo[] m = t.GetMethods();
        m[0].Invoke(o, args);
        //Console.WriteLine(o.i);
    }
}

Answer 1

But when we uncomment the last line of the code and try to compile it, it produces an error. Why??

Because you’ve declared o as System.Object, which doesn’t have a variable i defined on it as far as the compiler is concerned. You’d need to either cast it to the known type, or use reflection to retrieve this value.

For example:

// You can cast here, since you know the type
foo oAsFoo = o as foo;
Console.WriteLine(oAsFoo.i);

Alternatively, use reflection to get the value:

FieldInfo field = t.GetField("i");
Console.WriteLine(field.GetValue(o));