GameObjectExtensions.cs

using System;
using UnityEngine;
using System.Linq;

public static class GameObjectExtensions
{
    /// <summary>
    /// Returns all monobehaviours (casted to T)
    /// </summary>
    /// <typeparam name="T">interface type</typeparam>
    /// <param name="gObj"></param>
    /// <returns></returns>
    public static T[] GetInterfaces<T>(this GameObject gObj)
    {
        if (!typeof(T).IsInterface) throw new SystemException("Specified type is not an interface!");
        var mObjs = gObj.GetComponents<MonoBehaviour>();

        return (from a in mObjs where a.GetType().GetInterfaces().Any(k => k == typeof(T)) select (T)(object)a).ToArray();
    }

    /// <summary>
    /// Returns the first monobehaviour that is of the interface type (casted to T)
    /// </summary>
    /// <typeparam name="T">Interface type</typeparam>
    /// <param name="gObj"></param>
    /// <returns></returns>
    public static T GetInterface<T>(this GameObject gObj)
    {
        if (!typeof(T).IsInterface) throw new SystemException("Specified type is not an interface!");
        return gObj.GetInterfaces<T>().FirstOrDefault();
    }

    /// <summary>
    /// Returns the first instance of the monobehaviour that is of the interface type T (casted to T)
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="gObj"></param>
    /// <returns></returns>
    public static T GetInterfaceInChildren<T>(this GameObject gObj)
    {
        if (!typeof(T).IsInterface) throw new SystemException("Specified type is not an interface!");
        return gObj.GetInterfacesInChildren<T>().FirstOrDefault();
    }

    /// <summary>
    /// Gets all monobehaviours in children that implement the interface of type T (casted to T)
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <param name="gObj"></param>
    /// <returns></returns>
    public static T[] GetInterfacesInChildren<T>(this GameObject gObj)
    {
        if (!typeof(T).IsInterface) throw new SystemException("Specified type is not an interface!");

        var mObjs = gObj.GetComponentsInChildren<MonoBehaviour>();

        return (from a in mObjs where a.GetType().GetInterfaces().Any(k => k == typeof(T)) select (T)(object)a).ToArray();
    }
}