Prefer IReadOnlyCollection<> or IReadOnlyList<> over IEnumerable<>

Part of wixtoolset/issues#6422

22 files changed, 148 insertions, 48 deletions

diff --git a/src/WixToolset.Extensibility/BaseBurnBackendBinderExtension.cs b/src/WixToolset.Extensibility/BaseBurnBackendBinderExtension.cs
index 3afc8678..da570af0 100644
--- a/src/WixToolset.Extensibility/BaseBurnBackendBinderExtension.cs
+++ b/src/WixToolset.Extensibility/BaseBurnBackendBinderExtension.cs
@@ -33,7 +33,7 @@ namespace WixToolset.Extensibility
         /// <summary>
         /// Optional symbol definitions.
         /// </summary>
-        protected virtual IEnumerable<IntermediateSymbolDefinition> SymbolDefinitions => Enumerable.Empty<IntermediateSymbolDefinition>();
+        protected virtual IReadOnlyCollection<IntermediateSymbolDefinition> SymbolDefinitions => Array.Empty<IntermediateSymbolDefinition>();
 
         /// <summary>
         /// See <see cref="IBurnBackendBinderExtension.PreBackendBind(IBindContext)"/>
diff --git a/src/WixToolset.Extensibility/BaseExtensionCommandLine.cs b/src/WixToolset.Extensibility/BaseExtensionCommandLine.cs
index 8a336e1b..c716ac7e 100644
--- a/src/WixToolset.Extensibility/BaseExtensionCommandLine.cs
+++ b/src/WixToolset.Extensibility/BaseExtensionCommandLine.cs
@@ -2,8 +2,8 @@
 
 namespace WixToolset.Extensibility
 {
+    using System;
     using System.Collections.Generic;
-    using System.Linq;
     using WixToolset.Extensibility.Data;
     using WixToolset.Extensibility.Services;
 
@@ -15,7 +15,7 @@ namespace WixToolset.Extensibility
         /// <summary>
         /// See <see cref="IExtensionCommandLine.CommandLineSwitches" />
         /// </summary>
-        public virtual IEnumerable<ExtensionCommandLineSwitch> CommandLineSwitches => Enumerable.Empty<ExtensionCommandLineSwitch>();
+        public virtual IReadOnlyCollection<ExtensionCommandLineSwitch> CommandLineSwitches => Array.Empty<ExtensionCommandLineSwitch>();
 
         /// <summary>
         /// See <see cref="IExtensionCommandLine.PostParse" />
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)"/>
diff --git a/src/WixToolset.Extensibility/BaseLibrarianExtension.cs b/src/WixToolset.Extensibility/BaseLibrarianExtension.cs
index 1f231ba6..cbc9e4ba 100644
--- a/src/WixToolset.Extensibility/BaseLibrarianExtension.cs
+++ b/src/WixToolset.Extensibility/BaseLibrarianExtension.cs
@@ -59,7 +59,7 @@ namespace WixToolset.Extensibility
         /// <param name="path">Optional resolved path to file.</param>
         /// <param name="checkedPaths">Optional collection of paths checked for the file.</param>
         /// <returns>Resolved file result.</returns>
-        protected IResolveFileResult CreateResolveFileResult(string path = null, IEnumerable<string> checkedPaths = null)
+        protected IResolveFileResult CreateResolveFileResult(string path = null, IReadOnlyCollection<string> checkedPaths = null)
         {
             var result = this.Context.ServiceProvider.GetService<IResolveFileResult>();
             result.Path = path;
diff --git a/src/WixToolset.Extensibility/BaseWindowsInstallerBackendBinderExtension.cs b/src/WixToolset.Extensibility/BaseWindowsInstallerBackendBinderExtension.cs
index 93d8a1df..ffff186b 100644
--- a/src/WixToolset.Extensibility/BaseWindowsInstallerBackendBinderExtension.cs
+++ b/src/WixToolset.Extensibility/BaseWindowsInstallerBackendBinderExtension.cs
@@ -2,6 +2,7 @@
 
 namespace WixToolset.Extensibility
 {
+    using System;
     using System.Collections.Generic;
     using System.Linq;
     using WixToolset.Data;
@@ -33,7 +34,7 @@ namespace WixToolset.Extensibility
         /// <summary>
         /// Optional table definitions.
         /// </summary>
-        public virtual IEnumerable<TableDefinition> TableDefinitions => Enumerable.Empty<TableDefinition>();
+        public virtual IReadOnlyCollection<TableDefinition> TableDefinitions => Array.Empty<TableDefinition>();
 
         /// <summary>
         /// Creates a resolved cabinet result.
diff --git a/src/WixToolset.Extensibility/Data/IBindContext.cs b/src/WixToolset.Extensibility/Data/IBindContext.cs
index f1977c01..d0c65683 100644
--- a/src/WixToolset.Extensibility/Data/IBindContext.cs
+++ b/src/WixToolset.Extensibility/Data/IBindContext.cs
@@ -35,27 +35,27 @@ namespace WixToolset.Extensibility.Data
         /// <summary>
         /// Delayed fields that need to be resolved again.
         /// </summary>
-        IEnumerable<IDelayedField> DelayedFields { get; set; }
+        IReadOnlyCollection<IDelayedField> DelayedFields { get; set; }
 
         /// <summary>
         /// Embedded files to extract.
         /// </summary>
-        IEnumerable<IExpectedExtractFile> ExpectedEmbeddedFiles { get; set; }
+        IReadOnlyCollection<IExpectedExtractFile> ExpectedEmbeddedFiles { get; set; }
 
         /// <summary>
         /// Binder extensions.
         /// </summary>
-        IEnumerable<IBinderExtension> Extensions { get; set; }
+        IReadOnlyCollection<IBinderExtension> Extensions { get; set; }
 
         /// <summary>
         /// File system extensions.
         /// </summary>
-        IEnumerable<IFileSystemExtension> FileSystemExtensions { get; set; }
+        IReadOnlyCollection<IFileSystemExtension> FileSystemExtensions { get; set; }
 
         /// <summary>
         /// Set of ICEs to execute.
         /// </summary>
-        IEnumerable<string> Ices { get; set; }
+        IReadOnlyCollection<string> Ices { get; set; }
 
         /// <summary>
         /// Intermedaite folder.
@@ -100,7 +100,7 @@ namespace WixToolset.Extensibility.Data
         /// <summary>
         /// Set of ICEs to skip.
         /// </summary>
-        IEnumerable<string> SuppressIces { get; set; }
+        IReadOnlyCollection<string> SuppressIces { get; set; }
 
         /// <summary>
         /// Skip all ICEs.
diff --git a/src/WixToolset.Extensibility/Data/IBindResult.cs b/src/WixToolset.Extensibility/Data/IBindResult.cs
index 1654f24d..7fc52bd7 100644
--- a/src/WixToolset.Extensibility/Data/IBindResult.cs
+++ b/src/WixToolset.Extensibility/Data/IBindResult.cs
@@ -6,13 +6,24 @@ namespace WixToolset.Extensibility.Data
     using System.Collections.Generic;
     using WixToolset.Data;
 
-#pragma warning disable 1591 // TODO: add documentation
+    /// <summary>
+    /// Result of bind operation.
+    /// </summary>
     public interface IBindResult : IDisposable
     {
-        IEnumerable<IFileTransfer> FileTransfers { get; set; }
+        /// <summary>
+        /// Collection of file transfers to complete.
+        /// </summary>
+        IReadOnlyCollection<IFileTransfer> FileTransfers { get; set; }
 
-        IEnumerable<ITrackedFile> TrackedFiles { get; set; }
+        /// <summary>
+        /// Collection of files tracked during binding.
+        /// </summary>
+        IReadOnlyCollection<ITrackedFile> TrackedFiles { get; set; }
 
+        /// <summary>
+        /// Ouput of binding.
+        /// </summary>
         WixOutput Wixout { get; set; }
     }
 }
diff --git a/src/WixToolset.Extensibility/Data/ICompileContext.cs b/src/WixToolset.Extensibility/Data/ICompileContext.cs
index a86fee1a..2ee8f2a1 100644
--- a/src/WixToolset.Extensibility/Data/ICompileContext.cs
+++ b/src/WixToolset.Extensibility/Data/ICompileContext.cs
@@ -26,7 +26,7 @@ namespace WixToolset.Extensibility.Data
         /// <summary>
         /// Set of extensions provided to the compiler.
         /// </summary>
-        IEnumerable<ICompilerExtension> Extensions { get; set; }
+        IReadOnlyCollection<ICompilerExtension> Extensions { get; set; }
 
         /// <summary>
         /// Gets or sets the platform which the compiler will use when defaulting 64-bit attributes and elements.
diff --git a/src/WixToolset.Extensibility/Data/IDecompileContext.cs b/src/WixToolset.Extensibility/Data/IDecompileContext.cs
index 63ed27d5..fe7d0465 100644
--- a/src/WixToolset.Extensibility/Data/IDecompileContext.cs
+++ b/src/WixToolset.Extensibility/Data/IDecompileContext.cs
@@ -16,7 +16,7 @@ namespace WixToolset.Extensibility.Data
 
         OutputType DecompileType { get; set; }
 
-        IEnumerable<IDecompilerExtension> Extensions { get; set; }
+        IReadOnlyCollection<IDecompilerExtension> Extensions { get; set; }
 
         string ExtractFolder { get; set; }
 
diff --git a/src/WixToolset.Extensibility/Data/IDecompileResult.cs b/src/WixToolset.Extensibility/Data/IDecompileResult.cs
index ad4d0841..cffd0976 100644
--- a/src/WixToolset.Extensibility/Data/IDecompileResult.cs
+++ b/src/WixToolset.Extensibility/Data/IDecompileResult.cs
@@ -11,7 +11,7 @@ namespace WixToolset.Extensibility.Data
     {
         XDocument Document { get; set; }
 
-        IEnumerable<string> ExtractedFilePaths { get; set; }
+        IReadOnlyCollection<string> ExtractedFilePaths { get; set; }
 
         Platform? Platform { get; set; }
     }
diff --git a/src/WixToolset.Extensibility/Data/ILayoutContext.cs b/src/WixToolset.Extensibility/Data/ILayoutContext.cs
index 3d49b877..b11b4d13 100644
--- a/src/WixToolset.Extensibility/Data/ILayoutContext.cs
+++ b/src/WixToolset.Extensibility/Data/ILayoutContext.cs
@@ -19,17 +19,17 @@ namespace WixToolset.Extensibility.Data
         /// <summary>
         /// Extensions for use during layout.
         /// </summary>
-        IEnumerable<ILayoutExtension> Extensions { get; set; }
+        IReadOnlyCollection<ILayoutExtension> Extensions { get; set; }
 
         /// <summary>
         /// Set of tracked of files created during processing to be cleaned up.
         /// </summary>
-        IEnumerable<ITrackedFile> TrackedFiles { get; set; }
+        IReadOnlyCollection<ITrackedFile> TrackedFiles { get; set; }
 
         /// <summary>
         /// Set of files to transfer.
         /// </summary>
-        IEnumerable<IFileTransfer> FileTransfers { get; set; }
+        IReadOnlyCollection<IFileTransfer> FileTransfers { get; set; }
 
         /// <summary>
         /// File to capture list of content files.
diff --git a/src/WixToolset.Extensibility/Data/ILibraryContext.cs b/src/WixToolset.Extensibility/Data/ILibraryContext.cs
index d6359ffc..208b6f73 100644
--- a/src/WixToolset.Extensibility/Data/ILibraryContext.cs
+++ b/src/WixToolset.Extensibility/Data/ILibraryContext.cs
@@ -7,23 +7,49 @@ namespace WixToolset.Extensibility.Data
     using System.Threading;
     using WixToolset.Data;
 
-#pragma warning disable 1591 // TODO: add documentation
+    /// <summary>
+    /// Context provided during library creation operations.
+    /// </summary>
     public interface ILibraryContext
     {
+        /// <summary>
+        /// Service provider.
+        /// </summary>
         IServiceProvider ServiceProvider { get; }
 
+        /// <summary>
+        /// Indicates whether files should be bound into the library.
+        /// </summary>
         bool BindFiles { get; set; }
 
-        IEnumerable<IBindPath> BindPaths { get; set; }
+        /// <summary>
+        /// Collection of bindpaths used to bind files.
+        /// </summary>
+        IReadOnlyCollection<IBindPath> BindPaths { get; set; }
 
-        IEnumerable<ILibrarianExtension> Extensions { get; set; }
+        /// <summary>
+        /// Collection of extensions used during creation of library.
+        /// </summary>
+        IReadOnlyCollection<ILibrarianExtension> Extensions { get; set; }
 
+        /// <summary>
+        /// Identifier of the library.
+        /// </summary>
         string LibraryId { get; set; }
 
-        IEnumerable<Localization> Localizations { get; set; }
+        /// <summary>
+        /// Collection of localization files to use in the library.
+        /// </summary>
+        IReadOnlyCollection<Localization> Localizations { get; set; }
 
-        IEnumerable<Intermediate> Intermediates { get; set; }
+        /// <summary>
+        /// Collection of intermediates to include in the library.
+        /// </summary>
+        IReadOnlyCollection<Intermediate> Intermediates { get; set; }
 
+        /// <summary>
+        /// Cancellation token.
+        /// </summary>
         CancellationToken CancellationToken { get; set; }
     }
 }
diff --git a/src/WixToolset.Extensibility/Data/ILinkContext.cs b/src/WixToolset.Extensibility/Data/ILinkContext.cs
index 7524d18c..d56866f7 100644
--- a/src/WixToolset.Extensibility/Data/ILinkContext.cs
+++ b/src/WixToolset.Extensibility/Data/ILinkContext.cs
@@ -7,21 +7,44 @@ namespace WixToolset.Extensibility.Data
     using System.Threading;
     using WixToolset.Data;
 
-#pragma warning disable 1591 // TODO: add documentation
+    /// <summary>
+    /// Context provided during linking.
+    /// </summary>
     public interface ILinkContext
     {
+        /// <summary>
+        /// Service provider.
+        /// </summary>
         IServiceProvider ServiceProvider { get; }
 
-        IEnumerable<ILinkerExtension> Extensions { get; set; }
+        /// <summary>
+        /// Collection of extensions to use during linking.
+        /// </summary>
+        IReadOnlyCollection<ILinkerExtension> Extensions { get; set; }
 
-        IEnumerable<IExtensionData> ExtensionData { get; set; }
+        /// <summary>
+        /// Collection of extension data to use during linking.
+        /// </summary>
+        IReadOnlyCollection<IExtensionData> ExtensionData { get; set; }
 
+        /// <summary>
+        /// Expected output type.
+        /// </summary>
         OutputType ExpectedOutputType { get; set; }
 
-        IEnumerable<Intermediate> Intermediates { get; set; }
+        /// <summary>
+        /// Collection of intermediates to link.
+        /// </summary>
+        IReadOnlyCollection<Intermediate> Intermediates { get; set; }
 
+        /// <summary>
+        /// Symbol definition creator used to load extension data.
+        /// </summary>
         ISymbolDefinitionCreator SymbolDefinitionCreator { get; set; }
 
+        /// <summary>
+        /// Cancellation token.
+        /// </summary>
         CancellationToken CancellationToken { get; set; }
     }
 }
diff --git a/src/WixToolset.Extensibility/Data/IPreprocessContext.cs b/src/WixToolset.Extensibility/Data/IPreprocessContext.cs
index c6bdfe3a..69057c33 100644
--- a/src/WixToolset.Extensibility/Data/IPreprocessContext.cs
+++ b/src/WixToolset.Extensibility/Data/IPreprocessContext.cs
@@ -7,14 +7,25 @@ namespace WixToolset.Extensibility.Data
     using System.Threading;
     using WixToolset.Data;
 
-#pragma warning disable 1591 // TODO: add documentation
+    /// <summary>
+    /// Preprocessor context.
+    /// </summary>
     public interface IPreprocessContext
     {
+        /// <summary>
+        /// Service provider.
+        /// </summary>
         IServiceProvider ServiceProvider { get; }
 
-        IEnumerable<IPreprocessorExtension> Extensions { get; set; }
+        /// <summary>
+        /// Collection of extensions to use during preprocessing.
+        /// </summary>
+        IReadOnlyCollection<IPreprocessorExtension> Extensions { get; set; }
 
-        IEnumerable<string> IncludeSearchPaths { get; set; }
+        /// <summary>
+        /// Collection of search paths to find include files.
+        /// </summary>
+        IReadOnlyCollection<string> IncludeSearchPaths { get; set; }
 
         /// <summary>
         /// Gets the platform which the compiler will use when defaulting 64-bit attributes and elements.
@@ -22,12 +33,24 @@ namespace WixToolset.Extensibility.Data
         /// <value>The platform which the compiler will use when defaulting 64-bit attributes and elements.</value>
         Platform Platform { get; set; }
 
+        /// <summary>
+        /// Path to the source file being preprocessed.
+        /// </summary>
         string SourcePath { get; set; }
 
+        /// <summary>
+        /// Collection of name/value pairs used as preprocessor variables.
+        /// </summary>
         IDictionary<string, string> Variables { get; set; }
 
+        /// <summary>
+        /// Current source line number of the preprocessor.
+        /// </summary>
         SourceLineNumber CurrentSourceLineNumber { get; set; }
 
+        /// <summary>
+        /// Cancellation token.
+        /// </summary>
         CancellationToken CancellationToken { get; set; }
     }
 }
diff --git a/src/WixToolset.Extensibility/Data/IPreprocessResult.cs b/src/WixToolset.Extensibility/Data/IPreprocessResult.cs
index d46c8147..af224c1e 100644
--- a/src/WixToolset.Extensibility/Data/IPreprocessResult.cs
+++ b/src/WixToolset.Extensibility/Data/IPreprocessResult.cs
@@ -5,11 +5,19 @@ namespace WixToolset.Extensibility.Data
     using System.Collections.Generic;
     using System.Xml.Linq;
 
-#pragma warning disable 1591 // TODO: add documentation
+    /// <summary>
+    /// Result of preprocessing.
+    /// </summary>
     public interface IPreprocessResult
     {
+        /// <summary>
+        /// Document result of preprocessor.
+        /// </summary>
         XDocument Document { get; set; }
 
-        IEnumerable<IIncludedFile> IncludedFiles { get; set; }
+        /// <summary>
+        /// Collection of files included during preprocessing.
+        /// </summary>
+        IReadOnlyCollection<IIncludedFile> IncludedFiles { get; set; }
     }
 }
diff --git a/src/WixToolset.Extensibility/Data/IResolveContext.cs b/src/WixToolset.Extensibility/Data/IResolveContext.cs
index 6ab5b545..63159ccb 100644
--- a/src/WixToolset.Extensibility/Data/IResolveContext.cs
+++ b/src/WixToolset.Extensibility/Data/IResolveContext.cs
@@ -20,22 +20,22 @@ namespace WixToolset.Extensibility.Data
         /// <summary>
         /// Bind paths used during resolution.
         /// </summary>
-        IEnumerable<IBindPath> BindPaths { get; set; }
+        IReadOnlyCollection<IBindPath> BindPaths { get; set; }
 
         /// <summary>
         /// Resolve extensions.
         /// </summary>
-        IEnumerable<IResolverExtension> Extensions { get; set; }
+        IReadOnlyCollection<IResolverExtension> Extensions { get; set; }
 
         /// <summary>
         /// Extension data.
         /// </summary>
-        IEnumerable<IExtensionData> ExtensionData { get; set; }
+        IReadOnlyCollection<IExtensionData> ExtensionData { get; set; }
 
         /// <summary>
         /// List of cultures to filter the localizations.
         /// </summary>
-        IEnumerable<string> FilterCultures { get; set; }
+        IReadOnlyCollection<string> FilterCultures { get; set; }
 
         /// <summary>
         /// Intermediate folder.
@@ -50,7 +50,7 @@ namespace WixToolset.Extensibility.Data
         /// <summary>
         /// Localizations used to resolve.
         /// </summary>
-        IEnumerable<Localization> Localizations { get; set; }
+        IReadOnlyCollection<Localization> Localizations { get; set; }
 
         /// <summary>
         /// Indicates whether to allow localization and bind variables to remain unresolved.
diff --git a/src/WixToolset.Extensibility/Data/IResolveFileResult.cs b/src/WixToolset.Extensibility/Data/IResolveFileResult.cs
index c8dca81a..2f0df96c 100644
--- a/src/WixToolset.Extensibility/Data/IResolveFileResult.cs
+++ b/src/WixToolset.Extensibility/Data/IResolveFileResult.cs
@@ -4,11 +4,19 @@ namespace WixToolset.Extensibility.Data
 {
     using System.Collections.Generic;
 
-#pragma warning disable 1591 // TODO: add documentation
+    /// <summary>
+    /// Result of resolving a file.
+    /// </summary>
     public interface IResolveFileResult
     {
-        IEnumerable<string> CheckedPaths { get; set; }
+        /// <summary>
+        /// Collection of paths checked to find file.
+        /// </summary>
+        IReadOnlyCollection<string> CheckedPaths { get; set; }
 
+        /// <summary>
+        /// Path to found file, if found.
+        /// </summary>
         string Path { get; set; }
     }
 }
diff --git a/src/WixToolset.Extensibility/Data/IResolveResult.cs b/src/WixToolset.Extensibility/Data/IResolveResult.cs
index abffb415..0c5e0ccf 100644
--- a/src/WixToolset.Extensibility/Data/IResolveResult.cs
+++ b/src/WixToolset.Extensibility/Data/IResolveResult.cs
@@ -28,12 +28,12 @@ namespace WixToolset.Extensibility.Data
         /// <summary>
         /// Fields still requiring resolution.
         /// </summary>
-        IEnumerable<IDelayedField> DelayedFields { get; set; }
+        IReadOnlyCollection<IDelayedField> DelayedFields { get; set; }
 
         /// <summary>
         /// Files to extract from embedded .wixlibs.
         /// </summary>
-        IEnumerable<IExpectedExtractFile> ExpectedEmbeddedFiles { get; set; }
+        IReadOnlyCollection<IExpectedExtractFile> ExpectedEmbeddedFiles { get; set; }
 
         /// <summary>
         /// Resolved intermediate.
diff --git a/src/WixToolset.Extensibility/IExtensionCommandLine.cs b/src/WixToolset.Extensibility/IExtensionCommandLine.cs
index 2c7d0a7e..f7b19955 100644
--- a/src/WixToolset.Extensibility/IExtensionCommandLine.cs
+++ b/src/WixToolset.Extensibility/IExtensionCommandLine.cs
@@ -15,7 +15,7 @@ namespace WixToolset.Extensibility
         /// Gets the supported command line types for this extension.
         /// </summary>
         /// <value>The supported command line types for this extension.</value>
-        IEnumerable<ExtensionCommandLineSwitch> CommandLineSwitches { get; }
+        IReadOnlyCollection<ExtensionCommandLineSwitch> CommandLineSwitches { get; }
 
         /// <summary>
         /// Called before the command-line is parsed.
diff --git a/src/WixToolset.Extensibility/IWindowsInstallerBackendBinderExtension.cs b/src/WixToolset.Extensibility/IWindowsInstallerBackendBinderExtension.cs
index 12a38b9a..067745c2 100644
--- a/src/WixToolset.Extensibility/IWindowsInstallerBackendBinderExtension.cs
+++ b/src/WixToolset.Extensibility/IWindowsInstallerBackendBinderExtension.cs
@@ -16,7 +16,7 @@ namespace WixToolset.Extensibility
         /// <summary>
         /// Table definitions provided by the extension.
         /// </summary>
-        IEnumerable<TableDefinition> TableDefinitions { get; }
+        IReadOnlyCollection<TableDefinition> TableDefinitions { get; }
 
         /// <summary>
         /// Called before binding occurs.
diff --git a/src/WixToolset.Extensibility/Services/IBackendHelper.cs b/src/WixToolset.Extensibility/Services/IBackendHelper.cs
index ce3ddc89..5c4d9a68 100644
--- a/src/WixToolset.Extensibility/Services/IBackendHelper.cs
+++ b/src/WixToolset.Extensibility/Services/IBackendHelper.cs
@@ -72,7 +72,7 @@ namespace WixToolset.Extensibility.Services
         /// </summary>
         /// <param name="embeddedFiles">Embedded files to extract.</param>
         /// <returns><c>ITrackedFile</c> for each embedded file extracted.</returns>
-        IEnumerable<ITrackedFile> ExtractEmbeddedFiles(IEnumerable<IExpectedExtractFile> embeddedFiles);
+        IReadOnlyList<ITrackedFile> ExtractEmbeddedFiles(IEnumerable<IExpectedExtractFile> embeddedFiles);
 
         /// <summary>
         /// Generate an identifier by hashing data from the row.
diff --git a/src/WixToolset.Extensibility/Services/IExtensionManager.cs b/src/WixToolset.Extensibility/Services/IExtensionManager.cs
index a8087941..8e49c38d 100644
--- a/src/WixToolset.Extensibility/Services/IExtensionManager.cs
+++ b/src/WixToolset.Extensibility/Services/IExtensionManager.cs
@@ -37,6 +37,6 @@ namespace WixToolset.Extensibility.Services
         /// </summary>
         /// <typeparam name="T">Type of extension to get.</typeparam>
         /// <returns>Extensions of the specified type.</returns>
-        IEnumerable<T> GetServices<T>() where T : class;
+        IReadOnlyCollection<T> GetServices<T>() where T : class;
     }
 }