List of available skies is now generated from reflection information.

ScriptableRenderPipeline/Core/CoreRP/Editor/Volume/VolumeComponentEditor.cs

             using (new EditorGUILayout.HorizontalScope())
             {
                 // Override checkbox
-                var overrideRect = GUILayoutUtility.GetRect(17f, 17f, GUILayout.ExpandWidth(false));
-                overrideRect.yMin += 4f;
-                overrideRect.xMin += EditorGUI.indentLevel * 15f;
-                DrawOverrideCheckbox(overrideRect, property.overrideState);
+                DrawOverrideCheckbox(property);
 
                 // Property
                 using (new EditorGUI.DisabledScope(!property.overrideState.boolValue))
             }
         }
 
-        void DrawOverrideCheckbox(Rect rect, SerializedProperty property)
+        protected void DrawOverrideCheckbox(SerializedDataParameter property)
+            var overrideRect = GUILayoutUtility.GetRect(17f, 17f, GUILayout.ExpandWidth(false));
+            overrideRect.yMin += 4f;
+            overrideRect.xMin += EditorGUI.indentLevel * 15f;
+
-            property.boolValue = GUI.Toggle(rect, property.boolValue, CoreEditorUtils.GetContent("|Override this setting for this volume."), CoreEditorStyles.smallTickbox);
+            property.overrideState.boolValue = GUI.Toggle(overrideRect, property.overrideState.boolValue, CoreEditorUtils.GetContent("|Override this setting for this volume."), CoreEditorStyles.smallTickbox);
             GUI.color = oldColor;
         }
     }

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/HDRenderPipelineMenuItems.cs

             volume.isGlobal = true;
             volume.Add<HDShadowSettings>(true);
             var visualEnv = volume.Add<VisualEnvironment>(true);
-            visualEnv.skyType.value = SkyType.ProceduralSky;
+            visualEnv.skyType.value = SkySettings.GetUniqueID<ProceduralSky>();
             visualEnv.fogType.value = FogType.Exponential;
             volume.Add<ProceduralSky>(true);
             volume.Add<ExponentialFog>(true);

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Sky/HDRISky/HDRISky.cs

 namespace UnityEngine.Experimental.Rendering.HDPipeline
 {
-    [DisallowMultipleComponent]
+    [SkyUniqueID((int)SkyType.HDRISky)]
     public class HDRISky : SkySettings
     {
         [Tooltip("Cubemap used to render the sky.")]

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Sky/ProceduralSky/ProceduralSky.cs

 namespace UnityEngine.Experimental.Rendering.HDPipeline
 {
-    [DisallowMultipleComponent]
+    [SkyUniqueID((int)SkyType.ProceduralSky)]
     public class ProceduralSky : SkySettings
     {
         public ClampedFloatParameter sunSize = new ClampedFloatParameter(0.04f, 0.0f, 1.0f);

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Sky/SkyManager.cs

-using UnityEngine.Rendering;
-using System;
+using System;
+using System.Linq;
+using System.Reflection;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using UnityEngine.Rendering;
 
 namespace UnityEngine.Experimental.Rendering.HDPipeline
 {
         VolumeStack         m_LightingOverrideVolumeStack;
         LayerMask           m_LightingOverrideLayerMask = -1;
 
+        static Dictionary<int, Type> m_SkyTypesDict = null;
+        public static Dictionary<int, Type> skyTypesDict { get { if (m_SkyTypesDict == null) UpdateSkyTypes(); return m_SkyTypesDict; } }
+
-            SkySettings result;
-            switch (visualEnv.skyType.value)
+            int skyID = visualEnv.skyType;
+            Type skyType;
+            if(m_SkyTypesDict.TryGetValue(skyID, out skyType))
+            {
+                return (SkySettings)stack.GetComponent(skyType);
+            }
+            else
+            {
+                return null;
+            }
+        }
+
+        static void UpdateSkyTypes()
+        {
+            if(m_SkyTypesDict == null)
-                case SkyType.HDRISky:
+                m_SkyTypesDict = new Dictionary<int, Type>();
+
+                var skyTypes = CoreUtils.GetAllAssemblyTypes().Where(t => t.IsSubclassOf(typeof(SkySettings)) && !t.IsAbstract);
+                foreach(Type skyType in skyTypes)
+                {
+                    var uniqueIDs = skyType.GetCustomAttributes(typeof(SkyUniqueID), false);
+                    if(uniqueIDs.Length == 0)
-                        result = stack.GetComponent<HDRISky>();
-                        break;
+                        Debug.LogWarningFormat("Missing attribute SkyUniqueID on class {0}. Class won't be registered as an available sky.", skyType);
-                case SkyType.ProceduralSky:
+                    else
-                        result = stack.GetComponent<ProceduralSky>();
-                        break;
+                        int uniqueID = ((SkyUniqueID)uniqueIDs[0]).uniqueID;
+                        if(uniqueID == 0)
+                        {
+                            Debug.LogWarningFormat("0 is a reserved SkyUniqueID and is used in class {0}. Class won't be registered as an available sky.", skyType);
+                            continue;
+                        }
+
+                        Type value;
+                        if(m_SkyTypesDict.TryGetValue(uniqueID, out value))
+                        {
+                            Debug.LogWarningFormat("SkyUniqueID {0} used in class {1} is already used in class {2}. Class won't be registered as an available sky.", uniqueID, skyType, value);
+                            continue;
+                        }
+
+                        m_SkyTypesDict.Add(uniqueID, skyType);
-                default:
-                    result = null;
-                    break;
+                }
-
-            return result;
         }
 
         void UpdateCurrentSkySettings(HDCamera camera)
                 m_NeedUpdateRealtimeEnv = false;
             }
 
+            UpdateSkyTypes();
             UpdateCurrentSkySettings(camera);
 
             m_NeedUpdateBakingSky = m_BakingSkyRenderingContext.UpdateEnvironment(m_BakingSky, camera, sunLight, m_UpdateRequired, cmd);

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Sky/SkySettings.cs

 
 namespace UnityEngine.Experimental.Rendering.HDPipeline
 {
-    [Serializable]
-    public sealed class SkyResolutionParameter : VolumeParameter<SkyResolution>
+    // This class is used to associate a unique ID to a sky class.
+    // This is needed to be able to automatically register sky classes and avoid collisions and refactoring class names causing data compatibility issues.
+    [AttributeUsage(AttributeTargets.Class, AllowMultiple = false)]
+    public sealed class SkyUniqueID : Attribute
-        public SkyResolutionParameter(SkyResolution val, bool overrideState = false)
-            : base(val, overrideState)
-        {
+        public readonly int uniqueID;
+        public SkyUniqueID(int uniqueID)
+        {
+            this.uniqueID = uniqueID;
         }
     }
 
 
         }
 
+        public static int GetUniqueID<T>()
+        {
+            var uniqueIDs = typeof(T).GetCustomAttributes(typeof(SkyUniqueID), false);
+            if (uniqueIDs.Length == 0)
+                return -1;
+            else
+                return ((SkyUniqueID)uniqueIDs[0]).uniqueID;
+
+        }
 
         public abstract SkyRenderer CreateRenderer();
     }

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Sky/VisualEnvironment.cs

 
 namespace UnityEngine.Experimental.Rendering.HDPipeline
 {
+    // This enum is just here to centralize UniqueID values for skies provided with HDRP
+    public enum SkyType
+    {
+        HDRISky = 1,
+        ProceduralSky = 2
+    }
+
-        public SkyTypeParameter skyType = new SkyTypeParameter(SkyType.None);
+        public IntParameter skyType = new IntParameter(0);
         public FogTypeParameter fogType = new FogTypeParameter(FogType.None);
 
         public void PushFogShaderParameters(CommandBuffer cmd, FrameSettings frameSettings)
                         break;
                     }
             }
-        }
-    }
-
-    // TODO instead of hardcoding this, we need to generate the information from the existing sky currently implemented.
-    public enum SkyType
-    {
-        None,
-        HDRISky,
-        ProceduralSky
-    }
-    
-    [Serializable]
-    public sealed class SkyTypeParameter : VolumeParameter<SkyType>
-    {
-        public SkyTypeParameter(SkyType value, bool overrideState = false)
-            : base(value, overrideState)
-        {
         }
     }
 }

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/Sky/VisualEnvironmentEditor.cs

+using System;
+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+using UnityEditor;
+using UnityEngine.Experimental.Rendering.HDPipeline;
+using UnityEditor.Experimental.Rendering;
+
+namespace UnityEditor.Experimental.Rendering.HDPipeline
+{
+    [VolumeComponentEditor(typeof(VisualEnvironment))]
+    public class VisualEnvironmentEditor : VolumeComponentEditor
+    {
+        SerializedDataParameter m_SkyType;
+        SerializedDataParameter m_FogType;
+
+        List<GUIContent> m_SkyClassNames = null;
+        List<int> m_SkyUniqueIDs = null;
+
+        public override void OnEnable()
+        {
+            base.OnEnable();
+            var o = new PropertyFetcher<VisualEnvironment>(serializedObject);
+
+            m_SkyType = Unpack(o.Find(x => x.skyType));
+            m_FogType = Unpack(o.Find(x => x.fogType));
+        }
+
+        void UpdateSkyIntPopupData()
+        {
+            if(m_SkyClassNames == null)
+            {
+                m_SkyClassNames = new List<GUIContent>();
+                m_SkyUniqueIDs = new List<int>();
+
+                // Add special "None" case.
+                m_SkyClassNames.Add(new GUIContent("None"));
+                m_SkyUniqueIDs.Add(0);
+
+                var skyTypesDict = SkyManager.skyTypesDict;
+
+                foreach (KeyValuePair<int, Type> kvp in skyTypesDict)
+                {
+                    m_SkyClassNames.Add(new GUIContent(kvp.Value.Name.ToString()));
+                    m_SkyUniqueIDs.Add(kvp.Key);
+                }
+            }
+        }
+
+        public override void OnInspectorGUI()
+        {
+            UpdateSkyIntPopupData();
+
+            using (new EditorGUILayout.HorizontalScope())
+            {
+                DrawOverrideCheckbox(m_SkyType);
+                using (new EditorGUI.DisabledScope(!m_SkyType.overrideState.boolValue))
+                {
+                    EditorGUILayout.IntPopup(m_SkyType.value, m_SkyClassNames.ToArray(), m_SkyUniqueIDs.ToArray(), new GUIContent("Sky Type"));
+                }
+            }
+            PropertyField(m_FogType);
+        }
+    }
+}