GetAssetPath returning incomplete path for default materials

As you might know you can pack multiple assets into a single "asset file". That is the case here. The individual assets do not have seperate asset files. The actual asset file is located here:

 C:\Program Files\Unity\Editor\Data\Resources\unity_builtin_extra

It contains many assets. If you need to find one particular asset you have to use a method like AssetDatabase.LoadAllAssetsAtPath and find your desired asset. Keep in mind that all UnityEngine.Objects can have a "name". Though not all internal assets have one.

edit

Here's an example script which allows you to get a list of all assets stored in the unity_builtin_extra asset file:

 //BuiltInAssetsExtra.cs
 using UnityEngine;
 
 public class BuiltInAssetsExtra : MonoBehaviour
 {
     public UnityEngine.Object[] objs;
     [ContextMenu("GetAllExtraAssets")]
     void GetAllExtraAssets()
     {
 #if UNITY_EDITOR
         objs = UnityEditor.AssetDatabase.LoadAllAssetsAtPath("Resources/unity_builtin_extra");
 #endif
     }
 }

Attach it to an object and execute the method from the context menu.

second edit

I haven't really tested this, but it probably should work:

 //AssetDatabaseHelper.cs
 using UnityEditor;
 using System.Linq;

 public class AssetDatabaseHelper
 {
     public static T LoadAssetFromUniqueAssetPath<T>(string aAssetPath) where T : UnityEngine.Object
     {
         if (aAssetPath.Contains("::"))
         {
             string[] parts = aAssetPath.Split(new string[] { "::" },System.StringSplitOptions.RemoveEmptyEntries);
             aAssetPath = parts[0];
             if (parts.Length > 1)
             {
                 string assetName = parts[1];
                 System.Type t = typeof(T);
                 var assets = AssetDatabase.LoadAllAssetsAtPath(aAssetPath)
                     .Where(i => t.IsAssignableFrom(i.GetType())).Cast<T>();
                 var obj = assets.Where(i => i.name == assetName).FirstOrDefault();
                 if (obj == null)
                 {
                     int id;
                     if (int.TryParse(parts[1], out id))
                         obj = assets.Where(i => i.GetInstanceID() == id).FirstOrDefault();
                 }
                 if (obj != null)
                     return obj;
             }
         }
         return AssetDatabase.LoadAssetAtPath<T>(aAssetPath);
     }
     public static string GetUniqueAssetPath(UnityEngine.Object aObj)
     {
         string path = AssetDatabase.GetAssetPath(aObj);
         if (!string.IsNullOrEmpty(aObj.name))
             path += "::" + aObj.name;
         else
             path += "::" + aObj.GetInstanceID();
         return path;
     }
 }

This allows you to create a unique "assetpath" which includes the actual asset name or instance ID after a double colon "::". The "LoadAssetFromUniqueAssetPath" method will then do the reverse. The type check should allow inheritance. So LoadAssetFromUniqueAssetPath<Object>(somePath) will work with any type (in case you don't know the type at compile time).

Again, not tested yet.

Ok i quickly tested both methods and it works with the built-in "Default-Material". It gives you a path like this:

 "Resources/unity_builtin_extra::Default-Material"

Which does create a proper reference to the Default-Material. I also works with an actual assetpath. If you use GetUniqueAssetPath on a prefab reference you get this:

 "Assets/MyPrefab.prefab::MyPrefab"

"LoadAllAssetsAtPath" also works on single assets.