Access fields of a Struct in an Object with Reflection

Question

I'm trying to use reflection (ultimately on unknown at compile time) object which include struct. I've got as far as TypedReference.MakeTypedReference but I've hit a wall.

Here's my Class and Struct

public class MyObject
{
    public int Id;
    public Money Amount; 
}

public struct Money
{
    public int Vaule;
    public string Code;
}

And here is how I am trying to set "Code" of "Amount" in MyObject using reflection. As I mention above, I'm looking for a solution which does not know about these types at compile time (that would be too easy!)

Here's the code I have so far (I've used [0], [1] to make the code simpler)

var obj = new MyObject() { Id = 1 };
obj.Amount.Vaule = 10;
obj.Amount.Code = "ABC";

FieldInfo[] objFields = obj.GetType().GetFields();
FieldInfo[] moneyFields = objFields[1].GetValue(obj).GetType().GetFields();

List<FieldInfo> fields = new List<FieldInfo>() { objFields[1] };
fields.AddRange( moneyFields );

TypedReference typeRef = TypedReference.MakeTypedReference( 
                           objFields[1].GetValue( obj ), fields.ToArray() );

moneyFields[1].SetValueDirect( typeRef, "XXX" );

The TypedReference.MakeTypedReference blows up with; "FieldInfo does not match the target Type." Likewise if I just pass objFields[1]. And if pass just moneyFields I get "TypedReferences cannot be redefined as primitives."

Why? Let's say I'm creating Random test fixtures and want to populate class fields with random data :)

Hiroshima

Are you saying that at compile time you don't know if its using MyObject or Money, so it needs to detect which object is being used and read/write the property accordingly? – Scrim

How about using AutoFixture and getting out of the reflection Rube Goldberg machines business? (Though wrt struct, see #12931463) (have not run into any need for MakeTypedReference professionally and have to say something as I happened to actually be watching the tweet stream for once, sry!) – Turbofan

@RubenBartelink - the "why" was simply for reference. Would defo' use AutoFixture if that was what I was doing. – Hiroshima

@activwerx It's purely using reflection on unknown objects and structs. Marc's answer below is the perfect answer for this situation. – Hiroshima

@IanQuigley I'm guessing I definitely wouldn't like to work on the real problem... – Turbofan

@RubenBartelink the whole thing was a BAD idea, and am following the advice of Marc Gravell and many others. Poco's all the way :) I only dug into it and posted here because I wanted to know the answer academically. – Hiroshima

Possible duplicate of Is there a way to set properties on struct instances using reflection? – Panhandle

Answer 1

Frankly, there's no need whatsoever for TypedReference here - just a boxed struct should work fine:

    var amountField = obj.GetType().GetField("Amount");
    object money = amountField.GetValue(obj);
    var codeField = money.GetType().GetField("Code");
    codeField.SetValue(money, "XXX");
    amountField.SetValue(obj, money);

However! I will advise you of a few things:

• public fields instead of properties are not usually a good idea; that will often bite you later
• mutable structs (i.e. structs that can be changed after creation) are almost never a good idea, and will bite even more often, and bite harder
• combining mutable structs and public fields compounds it, but making it very problematic to change later

Capacious

@Ian just please note the bullet points... this is not a great idea – Capacious

@MarcGravell Can you please explain (or give a link where I can learn from) why boxing is needed for using reflection on struct. Other than boxing rest of the steps are same as a regular reflection. – Cheiron

@Sнаđошƒаӽ because all the reflection APIs take object as their input. – Capacious

@Marc Isn't struct an object too? Inheritance chain is Object -> ValueType -> struct right? – Cheiron

@Sнаđошƒаӽ it can behave as an object, via boxing. Until then: no, it isn't an object in the "heap" sense or the OO sense. – Capacious