ソースコード

using UnityEditor;

[InitializeOnLoad]
public static class InspectorHeaderGUI
{
    static InspectorHeaderGUI()
    {
        Editor.finishedDefaultHeaderGUI -= OnGUI;
        Editor.finishedDefaultHeaderGUI += OnGUI;
    }

    private static void OnGUI( Editor editor )
    {
        if ( !EditorUtility.IsPersistent( editor.target ) )
        {
            return;
        }

        var assetPath   = AssetDatabase.GetAssetPath( editor.target );
        var guid        = AssetDatabase.AssetPathToGUID( assetPath );
        var totalRect   = EditorGUILayout.GetControlRect();
        var controlRect = EditorGUI.PrefixLabel( totalRect, EditorGUIUtility.TrTempContent( "GUID" ) );

        if ( 1 < editor.targets.Length )
        {
            var label = EditorGUIUtility.TrTempContent( "[Multiple objects selected]" );
            EditorGUI.LabelField( controlRect, label );
        }
        else
        {
            EditorGUI.SelectableLabel( controlRect, guid );
        }
    }
}