1 files changed, 2 insertions, 2 deletions

diff --git a/src/WixToolset.Core/PreprocessContext.cs b/src/WixToolset.Core/PreprocessContext.cs
index 0b735a7c..986045ff 100644
--- a/src/WixToolset.Core/PreprocessContext.cs
+++ b/src/WixToolset.Core/PreprocessContext.cs
@@ -18,11 +18,11 @@ namespace WixToolset.Core
 
         public IServiceProvider ServiceProvider { get; }
 
-        public IEnumerable<IPreprocessorExtension> Extensions { get; set; }
+        public IReadOnlyCollection<IPreprocessorExtension> Extensions { get; set; }
 
         public Platform Platform { get; set; }
 
-        public IEnumerable<string> IncludeSearchPaths { get; set; }
+        public IReadOnlyCollection<string> IncludeSearchPaths { get; set; }
 
         public string SourcePath { get; set; }