Question

I have a problem using a class of made of structures.

Here’s the basic definition:

using System;

struct Real
{
    public double real;

    public Real(double real)
    { 
        this.real = real;
    }
}

class Record
{
    public Real r;
    public Record(double r)
    {
        this.r = new Real(r);
    }
    public void Test(double origval, double newval)
    {
        if (this.r.real == newval)
            Console.WriteLine("r = newval-test passed\n");
        else if (this.r.real == origval)
            Console.WriteLine("r = origval-test failed\n");
        else
            Console.WriteLine("r = neither-test failed\n");
    }
}

When I create a non-dynamic (static?) Record, setting the Real works.
When I create a dynamic Record, setting the real doesn’t work.
When I create a dynamic Record, replacing the real works.

And here’s the test program

class Program
{
    static void Main(string[] args)
    {
        double origval = 8.0;
        double newval = 5.0;

        // THIS WORKS - create fixed type Record, print, change value, print
        Record record1 = new Record(origval);
        record1.r.real = newval;        // change value  ***
        record1.Test(origval, newval);

        // THIS DOESN'T WORK.  change value is not making any change!
        dynamic dynrecord2 = new Record(origval);
        dynrecord2.r.real = newval;     // change value
        dynrecord2.Test(origval, newval);

        // THIS WORKS - create dynamic type Record, print, change value, print
        dynamic dynrecord3 = new Record(origval);
        dynamic r = dynrecord3.r;       // copy out value
        r.real = newval;                // change copy
        dynrecord3.r = r;               // copy in modified value
        dynrecord3.Test(origval, newval);

    }
}

And here’s the output:
r = newval-test passed
r = origval-test failed
r = newval-test passed

When I change the struct Real to class Real, all three cases work.

So what’s going on?
Thanks,
Max

Answer 1

dynamic is really a fancy word for object as far as the core CLI is concerned, so you are mutating a boxed copy. This is prone to craziness. Mutating a struct in the first place is really, really prone to error. I would simply make the struct immutable – otherwise you are going to get this over and over.