- add GBaseKey class that is only settable once

GBase/GBaseKey.cs

@@ -0,0 +1,30 @@
+// Author: Gockner, Simon
+// Created: 2020-11-13
+// Copyright(c) 2020 SimonG. All Rights Reserved.
+
+using System;
+
+namespace GBase
+{
+    public class GBaseKey
+    {
+        private int _key = -1;
+        private bool _isSet;
+
+        public int Key
+        {
+            get => _key;
+            internal set
+            {
+                if (_isSet)
+                    throw new InvalidOperationException("Key is already set.");
+
+                _key = value;
+                _isSet = true;
+            }
+        }
+
+        public override string ToString() => $"{Key}";
+        public static implicit operator int(GBaseKey key) => key.Key;
+    }
+}