I want to write a method that takes a field name and a new value for it

Question

What is the problem here? For some reason, the method EditInfo does not change the values of variables.

struct Address
{
    public string id;
    public string addressline;
    public string postalcode;

    public Address(string[] data)
    {
        this.id = data[0];
        this.addressline = data[1];
        this.postalcode = data[2];
    }

    public void EditInfo(string field, string newdata)
    {
        Type myType = typeof(Address);
        myType.GetField(field).SetValue(this, newdata);
    }
}

Any reason you are using a `struct` and not a `class`. Mutating structs often results in surprises — Flydog57, Feb 11 '21 at 17:54
Can you provide an example of you attempting to use "EditInfo"? Also, can you be any more specific apart from "does not work", maybe a stacktrace if possible? — Frontear, Feb 11 '21 at 18:13
I'm curious, in this context, what is "vector"? Is it System.Numerics.Vector. That seems designed for use with numeric quantities (its one-line description is _Represents a single vector of a specified numeric type that is suitable for low-level optimization of parallel algorithms._) — Flydog57, Feb 11 '21 at 18:14
@Frontear: Surprisingly, in this case "doesn't work" is a pretty fair description. Create an `Address` with the constructor, call `EditInfo` on the `Address` you have and nothing seems to happen. John Wu's answer explains why. — Flydog57, Feb 11 '21 at 18:16

score 2 · Accepted Answer · answered Feb 11 '21 at 18:06

It's tricky to update structs via reflection because you're usually working with a boxed value, which is immediately lost. See this answer for a little more info.

The simplest fix in your case is to change your struct to a class, and then your original code will work.

But if you insist on having a struct, you'll need to use the undocumented __makeref and a typed reference update using SetValueDirect, like this:

public void EditInfo(string field, string newdata)
{
    Type myType = typeof(Address);
    var fieldInfo = myType.GetField(field);
    var reference = __makeref(this);
    fieldInfo.SetValueDirect(reference, newdata);
}

score 0 · Answer 2 · answered Feb 11 '21 at 18:34

One solution is to avoid Reflection altogether (of course the real solution is to avoid using a mutable value type). Replacing EditInfo with this seems to work:

public void EditInfo(string field, string newdata)
{
    switch (field)
    {
        case nameof(id):
            id = newdata;
            break;
        case nameof(addressline):
            addressline = newdata;
            break;
        case nameof(postalcode):
            postalcode = newdata;
            break;
        default:
            throw new Exception("oops");
    }
}

You are doing a field lookup by name, so this is likely about the same amount of work, but without the abstraction and boxing/unboxing overhead of Reflection.

Just an idea

I want to write a method that takes a field name and a new value for it

2 Answers2