Understanding the “Struct” in Unity ParticleSystem

Question

The code below is working, \"Particle\" is an instance of class \"ParticleSystem\".

\"Particle.emission\" is a get-only property return struct \"Par

Answer 1

I noticed this behavior too but I found out what's happening after digging deeper with .NET Reflector.

The complete example of your code with the latest Unity version:

ParticleSystem particle = GetComponent<ParticleSystem>();
ParticleSystem.EmissionModule em = particle.emission;
em.rate = new ParticleSystem.MinMaxCurve(5);

Things to bear in mind:

ParticleSystem is a class.

EmissionModule is a struct.

To change the Particle's emission rate in Unity 5 and above, you must get the ParticleSystem.emission then store it in a temporary EmissionModule(struct) then you can modify it's rate variable

How does this work?

When you do:

ParticleSystem particle = GetComponent<ParticleSystem>(); 

or create/instantiate new ParticleSystem or attach one through the Editor, Unity will create new EmissionModule instance. EmissionModule has a an internal constructor that takes ParticleSystem as a parameter. Unity will immediately pass the current ParticleSystem instance to this EmissionModule constructor and that instance is stored in a temporary variable in the EmissionModule struct for later use.

It looks something like this:

private ParticleSystem tempParticleSystem;
internal EmissionModule(ParticleSystem particleInstance)
{
    this.tempParticleSystem = particleInstance;
}

When you do:

ParticleSystem.EmissionModule em = particle.emission;

Unity will create new instance of EmissionModule from the current particle (particle) and return it. That will contain that ParticleSystem (tempParticleSystem) reference that was saved. Remember that ParticleSystem is a class, so the reference is still there. The emission property only have the get accessor. No set accessor. Because of this, it is a read only property.

The emission property looks something like this:

public EmissionModule emission
{
    get
    {
        return new EmissionModule(this);
    }
}

Finally, when the you do:

em.rate = ....

or change the emission rate, that saved reference is used to change the Particle rate in the native side of Unity which is written in C++.

public ParticleSystem.MinMaxCurve rate
{
    get
    {
        ParticleSystem.MinMaxCurve curve = new ParticleSystem.MinMaxCurve();
        getParticleRate(this.tempParticleSystem, ref curve);
        return curve;
    }
    set
    {
        setParticleRate(this.tempParticleSystem, ref value);
    }
}

To simplify this, we can call this a result of a class (ParticleSystem) inside a struct (EmissionModule).