From a0c22f935d16eadc2fc00c2a7e361f4b522566de Mon Sep 17 00:00:00 2001
From: Richard Webb <richard.webb@helpsystems.com>
Date: Sun, 13 Oct 2024 15:40:04 +0100
Subject: [PATCH] Tweak error handling

---
 .../OLEProperties/OLEPropertiesContainer.cs           | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/sources/OpenMcdf.Extensions/OLEProperties/OLEPropertiesContainer.cs b/sources/OpenMcdf.Extensions/OLEProperties/OLEPropertiesContainer.cs
index 6b17947d..253f71bd 100644
--- a/sources/OpenMcdf.Extensions/OLEProperties/OLEPropertiesContainer.cs
+++ b/sources/OpenMcdf.Extensions/OLEProperties/OLEPropertiesContainer.cs
@@ -180,11 +180,16 @@ public void AddProperty(OLEProperty property)
         /// </summary>
         /// <param name="vtPropertyType">The type of property to create.</param>
         /// <param name="name">The name of the new property.</param>
-        /// <returns>The new property, of null if this container can't contain user defined properties.</returns>
+        /// <returns>The new property.</returns>
+        /// <exception cref="InvalidOperationException">If UserDefinedProperties aren't allowed for this container.</exception>
+        /// <exception cref="ArgumentException">If a property with the name <paramref name="name"/> already exists."/></exception>
         public OLEProperty AddUserDefinedProperty(VTPropertyType vtPropertyType, string name)
         {
+            // @@TBD@@ If this is a DocumentSummaryInfo container, we could forward the add on to that.
             if (this.ContainerType != ContainerType.UserDefinedProperties)
-                return null;
+            {
+                throw new InvalidOperationException($"UserDefinedProperties are not allowed in containers of type {this.ContainerType}");
+            }
 
             // As per https://learn.microsoft.com/en-us/openspecs/windows_protocols/MS-OLEPS/4177a4bc-5547-49fe-a4d9-4767350fd9cf
             // the property names have to be unique, and are case insensitive.
@@ -192,7 +197,7 @@ public OLEProperty AddUserDefinedProperty(VTPropertyType vtPropertyType, string
             {
                 throw new ArgumentException($"User defined property names must be unique and {name} already exists", nameof(name));
             }
-    
+
             // Work out a property identifier - must be > 1 and unique as per 
             // https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-oleps/333959a3-a999-4eca-8627-48a224e63e77
             uint identifier = 2;