1 files changed, 1 insertions, 1 deletions

diff --git a/src/WixToolset.Extensibility/BaseExtensionFactory.cs b/src/WixToolset.Extensibility/BaseExtensionFactory.cs
index 2e21b51b..8b6dc566 100644
--- a/src/WixToolset.Extensibility/BaseExtensionFactory.cs
+++ b/src/WixToolset.Extensibility/BaseExtensionFactory.cs
@@ -15,7 +15,7 @@ namespace WixToolset.Extensibility
         /// <summary>
         /// The extension types of the WiX extension.
         /// </summary>
-        protected abstract IEnumerable<Type> ExtensionTypes { get; }
+        protected abstract IReadOnlyCollection<Type> ExtensionTypes { get; }
 
         /// <summary>
         /// See <see cref="IExtensionFactory.TryCreateExtension(Type, out object)"/>