Extension helpers that either return an existing component or add one if it doesn’t exist.

GetOrAddComponentExtensions.cs

//  Extension helpers that either return an existing component
//  or add one if it doesn’t exist—zero allocations.
//
//  Usage examples:
//
//      var rb = gameObject.GetOrAddComponent<Rigidbody>();
//      var cam = this.GetOrAddComponentInChildren<Camera>(includeInactive: true);
//

using UnityEngine;
using System.Runtime.CompilerServices;

namespace Common.Extensions
{
    public static class GetOrAddComponentExtensions
    {
        // ---------- GameObject ------------------------------------------------

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static T GetOrAddComponent<T>(this GameObject go) where T : Component
        {
#if UNITY_2022_2_OR_NEWER
            if (go.TryGetComponent(out T component)) return component;
#else
            T component = go.GetComponent<T>();
            if (component) return component;
#endif
            return go.AddComponent<T>();
        }

        /// <summary>
        /// Returns the first <typeparamref name="T"/> in children, or adds one
        /// to the root GameObject if none exists.  Optionally includes inactive children.
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static T GetOrAddComponentInChildren<T>(
            this GameObject go, bool includeInactive = false) where T : Component
        {
            var component = go.GetComponentInChildren<T>(includeInactive);
            if (component) return component;
            return go.AddComponent<T>();
        }

        // ---------- Component (MonoBehaviour / Transform) ---------------------

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static T GetOrAddComponent<T>(this Component self) where T : Component =>
            self.gameObject.GetOrAddComponent<T>();

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static T GetOrAddComponentInChildren<T>(
            this Component self, bool includeInactive = false) where T : Component =>
            self.gameObject.GetOrAddComponentInChildren<T>(includeInactive);
    }
}