c # – How do I convert an interface into a MonoBehavior in Unity?

In my iTest.doStuff () method, I try to convert the testInterf _ti interface into a MonoBehavior. This gives me "error CS0246: The namespace type or name could not be found & # 39; MonoBehavior & # 39;", but in the same file, my iTest class has no problem inheriting from MonoBehavior.

//testInterface.cs

using UnityEngine;
using System;
using System.Collections;
using System.Collections.Generic;

public interface testInterf{//this is my interface
    string Name {get;}
    Sprite mySprite {get;}
    void func();
}

public class testInterfEventArgs : EventArgs {
    public testInterf i;

    public testInterfEventArgs(testInterf _i){
        i=_i;
    }
}

//iTest.cs

using UnityEngine;
using System;
using System.Collections;
using System.Collections.Generic;

public class iTest : MonoBehaviour{
    public void doStuff(testInterf _ti){//this is the implementation
        Collider collider = (_ti as MonoBehavior).GetComponent();//ERROR
    }
}