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In Solidity, is it possible to change states via function calls in other contracts?

For example, let's consider the code below, where we have three assumptions:

  1. The function test2 is defined in the contract C2.
  2. The contract object c2 of the type C2 is created at somewhere within C1.
  3. We deployed C1 as a root contract.

Then, is it possible for the function call statement c2.test2() to change the values of variables in C1 (e.g., x1, x2, x3, etc)?

Contract C1 {
  uint x1;
  uint x2;
  uint x3;

  ...

  function test1 () public{
     /* can c2.test2() change the values of variables */
     /*   within C1? (e.g., x1,x2,x3, etc) */
     c2.test2 (); 
  }  
}
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A contract's state variables can only be changed by that contract. But of course the contract can expose a method that other contracts can call, e.g.:

contract C1 {
    uint256 foo;
    function setFoo(uint256 value) external {
        foo = value;
    }
}

Now anyone (and any contract) can call setFoo to change the value of C1's foo state variable.

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