From e3295595a0d87aeaf0ff7bf78792881013b5ca28 Mon Sep 17 00:00:00 2001
From: "David R. Williamson" <drwill@microsoft.com>
Date: Mon, 12 Jul 2021 15:15:13 -0700
Subject: [PATCH] Document InvalidOperation and ArgumentInvalid (#2089)

---
 iothub/service/src/Common/Exceptions/ErrorCode.cs | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/iothub/service/src/Common/Exceptions/ErrorCode.cs b/iothub/service/src/Common/Exceptions/ErrorCode.cs
index d3ab52d1e5..838e8c3bf7 100644
--- a/iothub/service/src/Common/Exceptions/ErrorCode.cs
+++ b/iothub/service/src/Common/Exceptions/ErrorCode.cs
@@ -35,8 +35,21 @@ public enum ErrorCode
         [EditorBrowsable(EditorBrowsableState.Never)]
         DeviceInvalidResultCount = 400002,
 
+        /// <summary>
+        /// The client has requested an operation that the hub recognizes as invalid. Check the error message
+        /// for more information about what is invalid.
+        /// </summary>
+        // Note: although infrequent, this does appear in logs for "Amqp Message.Properties.To must contain the device identifier".
+        // and perhaps other cases.
         InvalidOperation = 400003,
+
+        /// <summary>
+        /// Something in the request payload is invalid. Check the error message for more information about what
+        /// is invalid.
+        /// </summary>
+        // Note: one example found in logs is for invalid characters in a twin property name.
         ArgumentInvalid = 400004,
+
         ArgumentNull = 400005,
         IotHubFormatError = 400006,