Which one to use for, while or something else?

Hello, I need to cycle through a array of "keys" when I try to pick up or drop a key and add or remove a key from the correct spot but I can't seem to get this to work. Code:

 public void AddKey(KeyObject key)
 {
     for (int i = 0; i < keys.Length; i++)
     {
         if (keys[i] != null) break;
         if (keys[i] == key) return;

         if (keys[i] == null)
         {
             keys[i] = key;
             return;
         }
     }
 }

 public void DropKey(KeyObject key)
 {
     for (int i = 0; i < keys.Length; i++)
     {
         if (keys[i] != key) break;

         if (keys[i] == key)
         {
             keys[i] = null;
             return;
         }
     }
 }