How to make an object that walks forward until it hits a wall?

I want a character that simply walks forward until it hits a wall, then it does a 180 degree spin and repeats the action.
Making him walk forward is easy, how do I program him hitting the wall?
My current code:

public class Enemy : MonoBehaviour
{

    public float speed = 5;
    public Vector3 userDirection = Vector3.forward;   
    // Update is called once per frame
    void Update()
    {
        transform.position += userDirection * speed * Time.deltaTime;
    }
}

>Solution :

You can use raycast to detect walls and avoid them.

Raycast in Unity is a Physics function that projects a Ray into the scene, returning a boolean value if a target was successfully hit

The following code is a simple demenstration of raycast. Distance determine how far you ray is casted and layermask determine which layers ray should detect. Therefore, you must put your walls in a Layer and sit this variable equal to that:

    public float distance;
    public LayerMask wallLayerMask;
    public Vector3 userDirection = Vector3.forward;   

    void Update()
    {
       if (Physics.Raycast(transform.position, userDirection, distance, wallLayerMask))
        {
            // rotate, possibly:
            // userDirection *= -1
        }
      transform.position += userDirection * speed * Time.deltaTime;
    }