How to change object material more than one element in run-time?

Hello sir, ma'am. I'm a newbie.

I'm here asking for a solution to change an object material in c# script

Here is the simple code I created:

Code (CSharp):

1.     public GameObject MainObject;
2.     public Material MaterialYouNeed;
3.     void Start()
4.     {
5.  
6.     }
7.  
8.     void Update()
9.     {
10.         if (Input.GetKeyDown("i"))
11.             {
12.             MainObject.GetComponent<Renderer>().material = MaterialYouNeed;
13.            
14.         }
15.     }

It works fine for a simple object, but the problem is some objects have more than one material on it.




As you can see here this object got 3, and I just don't know how to call each one of them in c#

Thank you.