添加项目文件。

Szmedi.Toolkit.Revit/RvAssists/HasActiveDocument.cs

@@ -0,0 +1,13 @@
+using Autodesk.Revit.DB;
+using Autodesk.Revit.UI;
+
+namespace Szmedi.Toolkit.RvAssists
+{
+    public class HasActiveDocument : IExternalCommandAvailability
+    {
+        public bool IsCommandAvailable(UIApplication applicationData, CategorySet selectedCategories)
+        {
+            return applicationData.ActiveUIDocument != null;
+        }
+    }
+}