拡張メソッド作り方　メモ ref: https://qiita.com/satotin/items/b643cc6765009b75f20e

file0.txt

using UnityEngine;

public class ExtensionTest : MonoBehaviour {

    public void OnExtensionTest()
    {
        var test = new Test()
        {
            a = 1,
            b = "abc"
        };

        test.AddValue();

        Debug.Log("a = " + test.a.ToString());
        Debug.Log("b = " + test.b);
        Debug.Log("c = " + test.c);
        Debug.Log("d = " + test.d);
    }
}

file1.txt

public class Test {

    public int a;

    public string b;

    public bool c = false;

    public float d = 0.0f;
}

file2.txt

public static class TestExtension  {

    public static void AddValue(this Test test)
    {
        test.c = true;
        test.d = 123.4f;
    }
}