Why cant i assign a variable to ParticleSystem.MainModule?

In following this tutorial:

        ParticleSystem.MainModule psMain = particleLauncher.main;
        psMain.startColor = particleColorGradient.Evaluate (Random.Range (0f, 1f));

and I have the same code written down, but when I want to declare the variable psMain to ParticleSystem.MainModule it doesnt work.

You dont need to assign it back.

Thank you so much