Why am I getting this error: "Keyword 'this' is not available in current context."

Question

 using UnityEngine;
 using System.Collections;
 
 public class PlayerLife : MonoBehaviour 
 {
       public LifeState lifeState = new LifeState(this);
 
       public class LifeState
       {
         private PlayerLife playerLife;
         private bool currentState;
         public bool alive
         {
             get {return currentState;} 
             set 
             {
                 if (value != currentState)
                 {
                     currentState = value;
                     if (value) playerLife.SpawnPlayer();
                     else playerLife.KillPlayer();
                 }
             }
         }
 
         public LifeState(PlayerLife container) 
         {
             playerLife = container;
             currentState = false;
         }
     }
 
     void SpawnPlayer() {}
     void KillPlayer () {}

}

Answer 1

Best Answer

Answer by tigertrussell · Mar 12, 2015 at 02:35 PM

You need to do that in an Awake() or Start() method; not where you are making your properties declarations.

 public class PlayerLife : MonoBehaviour 
 {
        public LifeState lifeState;
 
        void Awake() {
             lifeState = new LifeState(this);
        }
  
        public class LifeState {...}

because the this keyword is relative to an instance of the class, not a property of the class itself.

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maxnorth · Mar 12, 2015 at 02:43 PM 0

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You're right, that worked. I guess I still don't entirely understand why, though. Aren't the initializers called when the class is instantiated? Or are only constructors called upon instantiation, and field initializers have to be statically compiled?

maxnorth · Mar 12, 2015 at 02:51 PM 0

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I found a useful page on this issue saying pretty much the same thing. Apparently it's also not allowed to initialize a variable by referencing a previously initialized field of the same class either. Didn't know that.

https://msdn.microsoft.com/en-us/library/aa645759%28v=vs.71%29.aspx

tigertrussell · Mar 12, 2015 at 04:10 PM 0

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The "real" reason is that under the hood the compiler is trying to get a generic description of each class for purposes of allocating memory and understanding what to expect from other code later on down the line. When the compiler looks at your classes, the first thing it checks are the properties and the signatures of the functions to get this generic "description" of the class. At that time, you can choose to initialize a "default" value, but only if it is constant (i.e. the compiler can easily initialize it). At that point in time, things like the this keyword are meaningless to the compiler, and there is no context under which you can refer to pre-existing code (because it hasn't compiled it yet, it's just trying to get a picture of the class).

There's some mild oversimplification and psuedoscience in that explanation, but that's the gist of it. $$anonymous$$aybe someone else in the community who knows a bit more about the lower-level stuff will chime in?

edit: oh so I guess the answer to your question is "yes" field initializers are statically compiled; constructors are called upon initialization.

Why am I getting this error: "Keyword 'this' is not available in current context."

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