Unity - I can't change the "colorOverLifetime" property of the "Particle System" via script

Question

The goal is to remove the particles via script with fade effect. But I'm not just interested in an alternative, I want to understand why my code doesn't work.

The code below is modified to show only the important parts. I can't know before what time the particles will be dissolved, so I can't set the parameter from the editor.

StarsConfig.Stars.colorOverLifetime.color.gradient.SetKeys This is the important point (see the log image). The gradient does not take my new values but alphaKey is correct.

Why ? Thanks

private bool FadeStarts = false;

void Update()
{
    if (FadeStarts) return;
    StartCoroutine(fadeStars(false));
}


private IEnumerator fadeStars(bool show)
{

    FadeStarts = true;

    ParticleSystem.ColorOverLifetimeModule c = new ParticleSystem.ColorOverLifetimeModule();

    c = StarsConfig.Stars.colorOverLifetime;

    Gradient g = c.color.gradient;

    Debug.Log("OFF " + c.color.gradient.alphaKeys[0].alpha + " --> " + g.alphaKeys[0].alpha);

    GradientAlphaKey[] alphaKey = new GradientAlphaKey[2];
    alphaKey[0] = c.color.gradient.alphaKeys[0];
    alphaKey[1] = c.color.gradient.alphaKeys[1];


    while (c.color.gradient.alphaKeys[0].alpha > 0f) {

        alphaKey[0].alpha -= StarsConfig.VelocityAlpha * Time.deltaTime;
        alphaKey[1].alpha -= StarsConfig.VelocityAlpha * Time.deltaTime;

        StarsConfig.Stars.colorOverLifetime.color.gradient.SetKeys(c.color.gradient.colorKeys, alphaKey);

        Debug.Log("g " + alphaKey[0].alpha + " --> " + StarsConfig.Stars.colorOverLifetime.color.gradient.alphaKeys[0].alpha);

        /*
        StarsConfig.Stars.colorOverLifetime.color.gradient.SetKeys(
            new GradientColorKey[] { new GradientColorKey(Color.white, 0f),
                                    new GradientColorKey(Color.white, 1.0f) },
            new GradientAlphaKey[] { new GradientAlphaKey(StarsConfig.Stars.colorOverLifetime.color.gradient.alphaKeys[0].alpha - (StarsConfig.VelocityAlpha * Time.deltaTime), 0f),
                                    new GradientAlphaKey(StarsConfig.Stars.colorOverLifetime.color.gradient.alphaKeys[1].alpha - (StarsConfig.VelocityAlpha * Time.deltaTime), 1f) });

        */

        yield return 0;
    }

    StarsConfig.Stars.Stop(true, ParticleSystemStopBehavior.StopEmittingAndClear);


    FadeStarts = false;
}

UPDATE

Ed_Marty's suggestion is correct, and the solution is there (thanks!). But I would like to understand what is the difference between these two codes.

This code works

var col = StarsConfig.Stars.colorOverLifetime;
while (col.color.gradient.alphaKeys[0].alpha > 0f) {

    float alpha = StarsConfig.Stars.colorOverLifetime.color.gradient.alphaKeys[0].alpha - (StarsConfig.VelocityAlpha * Time.deltaTime);

    Gradient grad = new Gradient();
    grad.SetKeys(
        new GradientColorKey[] {
            new GradientColorKey(Color.white, 0.0f),
            new GradientColorKey(Color.white, 1.0f) }, 
        new GradientAlphaKey[] {
            new GradientAlphaKey(alpha, 0.0f),
            new GradientAlphaKey(alpha, 1.0f) });

    col.color = grad;

    yield return 0;
}

And this doesn't work

while (StarsConfig.Stars.colorOverLifetime.color.gradient.alphaKeys[0].alpha > 0f) {

    float alpha = StarsConfig.Stars.colorOverLifetime.color.gradient.alphaKeys[0].alpha - (StarsConfig.VelocityAlpha * Time.deltaTime);

    StarsConfig.Stars.colorOverLifetime.color.gradient.SetKeys(
        new GradientColorKey[] { new GradientColorKey(Color.white, 0f),
                                 new GradientColorKey(Color.white, 1.0f) },
        new GradientAlphaKey[] { new GradientAlphaKey(alpha, 0f),
                                 new GradientAlphaKey(alpha, 1f) });

    yield return 0;
}

Answer 1

ColorOverLifetimeModule.color is a property getter that returns a copy of the MinMaxGradient struct's values.

So your first earlier code StarsConfig.Stars.colorOverLifetime.color.gradient.SetKeys(...) takes this copy, and modified the copy... then discards it. Since you were modifying the copy, your particle system's original values remain unchanged.

The magic line in the new code is col.color = grad, which takes the modified gradient you've made and hands it back to the module to update its internal state.