GuidInspector.cs

using UnityEditor;

[CustomEditor( typeof( Guid ) )]

class GuidInspector : Editor
{
	void OnEnable()
	{
		Guid guid = (Guid)target;
		
		if ( guid.guid == System.Guid.Empty )
		{
			guid.Generate();
			EditorUtility.SetDirty( target );
		}
	}
	
	public override void OnInspectorGUI()
	{
		Guid guid = (Guid)target;
		
		EditorGUILayout.SelectableLabel( guid.guid.ToString() );
	}
}