C# returning public field from method

Question

I'm writing scripts for Unity3D in C# and have a little helper method in one of my classes called constraintForType that returns a public field for a certain type like so:

public Constraint topConstraint = Constraint.falseConstraint;
public Constraint bottomConstraint = Constraint.falseConstraint;

enum ConstraintType {
    Top,
    Bottom
}

Constraint constraintForType(ConstraintType type) {
    switch(type) {
    case ConstraintType.Top:
        return topConstraint;
    case ConstraintType.Bottom:
        return bottomConstraint;
    }

    return Constraint.falseConstraint;
}

I'm calling this method from the Update loop but it seems that the constraint that I'm returning is not the same as the public declaration (maybe a copy?).

So in the Update loop this would work:

void Update() {

    topConstraint.constant = // Calculated value
}

But this doesn't:

void Update() {

    Constraint con = constraintForType(ConstraintType.Top);
    con.constant = // Calculated value
}

I thought maybe the method was returning a copy of the public field so I changed it to:

void constraintForType(ConstraintType type, ref Constraint con) {

    switch(type) {
    case ConstraintType.Top:
        con = topConstraint;
        break;
    case ConstraintType.Bottom:
        con = bottomConstraint;
        break;
    }
}

So I then call this in the Update:

Constraint con = Constraint.falseConstraint;
constraintForType(ConstraintType.Top, ref con);

This still doesn't work though.

What is going on here?

EDIT:

Constraint is a struct.

I'm trying to avoid calling a large `switch` block more than once. Tidy code you know. I've only put two here but the switch is significantly bigger in the actual Script. — Rob Sanders, Apr 13 '16 at 09:34
It is a static property in the `Constraint` struct that returns a placeholder `Constraint` so I know something went wrong. And yes I thought there must be only one instance which is why I'm confused. — Rob Sanders, Apr 13 '16 at 09:36
You set both topConstraint and bottomConstraint to Constraint.falseConstraint so unless you change them somewhere outside of the code you show, constraintForType always returns Constraint.falseConstraint. — diidu, Apr 13 '16 at 09:41
They're edited in the Unity interface inspector. The struct is serialisable. How can I return the right struct then? `Constraint` is my type. — Rob Sanders, Apr 13 '16 at 09:41
You should never, ever, write mutable structs. Make it a class. — Lasse V. Karlsen, Apr 13 '16 at 09:45

score 1 · Accepted Answer · answered Apr 13 '16 at 09:42

1

Change your Constraint -struct to class. Structs are value types that are (usually) immutable.

answered Apr 13 '16 at 09:42

Simo Erkinheimo

1,347
9
17

Thank you I've done that. I thought struct might be more efficient. – Rob Sanders Apr 13 '16 at 09:45

score 1 · Answer 2 · edited May 23 '17 at 10:28

You are right, your first method is returning a copy of the stuct. However, your second version is making a copy as well:

void constraintForType(ConstraintType type, ref Constraint con) {

    switch(type) {
    case ConstraintType.Top:
        con = topConstraint; // struct is copied here
        break;
    case ConstraintType.Bottom:
        con = bottomConstraint; // struct is copied here
        break;
    }
}

One way to make this work in your case would be to assign inside the helper function

void constraintForType(ConstraintType type, Foo constant) {

    switch(type) {
    case ConstraintType.Top:
        topConstraint.constant = constant; 
        break;
    case ConstraintType.Bottom:
        bottomConstraint.constant = constant; 
        break;
    }
}

Of course, if you do not have a particular reason to use a mutable struct, use a class instead. Mutable structs have some difficulties.

Probably for the best, see http://stackoverflow.com/questions/441309/why-are-mutable-structs-evil — HugoRune, Apr 13 '16 at 09:51

C# returning public field from method

2 Answers2