public class Actor : ENTITY
{
        /*create entity player*/
        private static ENTITY m_actor = null; 
        ...
        public Actor()
        {
           // m_actor = new ENTITY();  //CRASH
           // m_actor = (ENTITY)EngVar.player; //CRASH
           //m_actor = (ENTITY)EngFun.ent_create(null, new Vector(0, 0, 0), null); //CRASH
           ...
        }
        ...
}

public class Var
    {
        private NativeVar HiddenVar = new NativeVar();
        private IntPtr VarIntPtr = IntPtr.Zero;

        private int mValue
        {
            get
            {
                if (VarIntPtr != IntPtr.Zero)
                {
                    unsafe
                    {
                        HiddenVar = *((NativeVar*)VarIntPtr);
                    }
                }
                return HiddenVar.mValue;
            }
            set
            {
                HiddenVar.mValue = value;
                if (VarIntPtr != IntPtr.Zero)  //update structure in unmanaged memory if the var was already used as IntPtr
                {
                    Marshal.StructureToPtr(HiddenVar, VarIntPtr, false); //CRASH HERE
                }
            }
        }

public class Actor : ENTITY
    {
        private static ENTITY m_actor = null;

        public Actor()
        {
            m_actor = new ENTITY();  //CRASH
            m_actor = (ENTITY)EngVar.player; //CRASH
            m_actor = (ENTITY)EngFun.ent_create(null, new Vector(0, 0, 0), null); //CRASH
        }
    }

Actor testactor = new Actor();

Actor m_actor = new Actor(models + "cbabe.mdl", new Vector(0, 0, 16), Actor.ActorAction);