error CS0115: no suitable method found to override

Read the Console line first

The message names the class and method marked override. Compare its name, parameters and return type with the method in the base class.

Common causes

1. Overriding a Unity message on MonoBehaviour

   Unity finds Start, Update and the other messages by name; MonoBehaviour does not declare them, so there is nothing to override.

   Example

   C# Copy

   public class Enemy : MonoBehaviour
   {
       protected override void Start() { } // CS0115
   }

   4 lines

2. The base method is not virtual

   Your own base class declares void Start() without virtual or abstract, so derived classes cannot override it.

3. The signature differs

   The name, parameter types or return type differ from the base method, for example OnTriggerEnter(Collider2D) against a base OnTriggerEnter(Collider), or a typo in the name.

The fix

public class Enemy : MonoBehaviour
{
    protected virtual void Start()
    {
        // shared setup
    }
}

public class Boss : Enemy
{
    protected override void Start()
    {
        base.Start();
        // boss-only setup
    }
}

Stop it coming back

Remove override from Unity message methods on classes that inherit MonoBehaviour directly. When several components share setup, declare the method protected virtual in your base class and call base.Start() from the override.

Questions

Do I need base.Start()?

Only when the base class's Start does work the derived class also needs. Calling it is the usual choice; forgetting it is a common source of half-initialized objects.

Should Unity messages be public?

No. Unity calls them whatever their access level. Keep them private, or protected virtual when subclasses override them.

What about the 'hides inherited member' warning?

That is CS0114: a derived class declares a method with the same name as a virtual one without override. Add override if you meant to replace it, or new if you meant to hide it.

Scripts that avoid it

• Health SystemReusable health component with damage, healing, invincibility frames, and events.
• Enemy Chase AIEnemy AI that detects the player within radius, chases with line-of-sight, and returns to patrol when player escapes.

Related errors

• NullReferenceException: Object reference not set to an instance of an object
• MissingReferenceException: The object has been destroyed but you are still trying to access it
• IndexOutOfRangeException: Index was outside the bounds of the array
• ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection
• error CS0246: The type or namespace name could not be found

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