Minor code refactoring to TestHost library (#23692)

* Make fields readonly

Make a number of fields that aren't changed readonly.

* Use range to trim string

Use a range to trim the last character of the paths.

* Use nameof()

Use nameof() for exceptions rather than literals.

* Use Task.CompletedTask

Use Task.CompletedTask instead of Task.FromResult().

* Use Array.Empty()

Use Array.Empty() instead of allocating an empty array.

* Remove unused parameter

Remove unused CancellationToken parameter.

* Fix compilation error

Fix compilation error.

* Apply suggestions from code review

Add comment explaining range syntax.

src/Hosting/TestHost/src/AsyncStreamWrapper.cs

@@ -10,8 +10,8 @@ namespace Microsoft.AspNetCore.TestHost
 {
     internal class AsyncStreamWrapper : Stream
     {
-        private Stream _inner;
-        private Func<bool> _allowSynchronousIO;
+        private readonly Stream _inner;
+        private readonly Func<bool> _allowSynchronousIO;
 
         internal AsyncStreamWrapper(Stream inner, Func<bool> allowSynchronousIO)
         {

src/Hosting/TestHost/src/ClientHandler.cs

@@ -39,7 +39,7 @@ namespace Microsoft.AspNetCore.TestHost
             // PathString.StartsWithSegments that we use below requires the base path to not end in a slash.
             if (pathBase.HasValue && pathBase.Value.EndsWith("/"))
             {
-                pathBase = new PathString(pathBase.Value.Substring(0, pathBase.Value.Length - 1));
+                pathBase = new PathString(pathBase.Value[..^1]); // All but the last character
             }
             _pathBase = pathBase;
         }

src/Hosting/TestHost/src/HttpContextBuilder.cs

@@ -26,7 +26,7 @@ namespace Microsoft.AspNetCore.TestHost
         private bool _pipelineFinished;
         private bool _returningResponse;
         private object _testContext;
-        private Pipe _requestPipe;
+        private readonly Pipe _requestPipe;
 
         private Action<HttpContext> _responseReadCompleteCallback;
         private Task _sendRequestStreamTask;

src/Hosting/TestHost/src/RequestBuilder.cs

@@ -64,7 +64,7 @@ namespace Microsoft.AspNetCore.TestHost
                 if (!_req.Content.Headers.TryAddWithoutValidation(name, value))
                 {
                     // TODO: throw new ArgumentException(string.Format(CultureInfo.CurrentCulture, Resources.InvalidHeaderName, name), "name");
-                    throw new ArgumentException("Invalid header name: " + name, "name");
+                    throw new ArgumentException("Invalid header name: " + name, nameof(name));
                 }
             }
             return this;

src/Hosting/TestHost/src/ResponseBodyReaderStream.cs

@@ -105,15 +105,15 @@ namespace Microsoft.AspNetCore.TestHost
         {
             if (buffer == null)
             {
-                throw new ArgumentNullException("buffer");
+                throw new ArgumentNullException(nameof(buffer));
             }
             if (offset < 0 || offset > buffer.Length)
             {
-                throw new ArgumentOutOfRangeException("offset", offset, string.Empty);
+                throw new ArgumentOutOfRangeException(nameof(offset), offset, string.Empty);
             }
             if (count <= 0 || count > buffer.Length - offset)
             {
-                throw new ArgumentOutOfRangeException("count", count, string.Empty);
+                throw new ArgumentOutOfRangeException(nameof(count), count, string.Empty);
             }
         }
 

src/Hosting/TestHost/src/ResponseFeature.cs

@@ -16,8 +16,8 @@ namespace Microsoft.AspNetCore.TestHost
         private readonly HeaderDictionary _headers = new HeaderDictionary();
         private readonly Action<Exception> _abort;
 
-        private Func<Task> _responseStartingAsync = () => Task.FromResult(true);
-        private Func<Task> _responseCompletedAsync = () => Task.FromResult(true);
+        private Func<Task> _responseStartingAsync = () => Task.CompletedTask;
+        private Func<Task> _responseCompletedAsync = () => Task.CompletedTask;
         private int _statusCode;
         private string _reasonPhrase;
 

src/Hosting/TestHost/src/TestServer.cs

@@ -150,7 +150,7 @@ namespace Microsoft.AspNetCore.TestHost
                 var pathBase = PathString.FromUriComponent(BaseAddress);
                 if (pathBase.HasValue && pathBase.Value.EndsWith("/"))
                 {
-                    pathBase = new PathString(pathBase.Value.Substring(0, pathBase.Value.Length - 1));
+                    pathBase = new PathString(pathBase.Value[..^1]); // All but the last character.
                 }
                 request.PathBase = pathBase;
             });

src/Hosting/TestHost/src/TestWebSocket.cs

@@ -12,8 +12,8 @@ namespace Microsoft.AspNetCore.TestHost
 {
     internal class TestWebSocket : WebSocket
     {
-        private ReceiverSenderBuffer _receiveBuffer;
-        private ReceiverSenderBuffer _sendBuffer;
+        private readonly ReceiverSenderBuffer _receiveBuffer;
+        private readonly ReceiverSenderBuffer _sendBuffer;
         private readonly string _subProtocol;
         private WebSocketState _state;
         private WebSocketCloseStatus? _closeStatus;
@@ -165,7 +165,7 @@ namespace Microsoft.AspNetCore.TestHost
                 throw new ArgumentOutOfRangeException(nameof(messageType), messageType, string.Empty);
             }
 
-            var message = new Message(buffer, messageType, endOfMessage, cancellationToken);
+            var message = new Message(buffer, messageType, endOfMessage);
             return _sendBuffer.SendAsync(message, cancellationToken);
         }
 
@@ -225,7 +225,7 @@ namespace Microsoft.AspNetCore.TestHost
 
         private class Message
         {
-            public Message(ArraySegment<byte> buffer, WebSocketMessageType messageType, bool endOfMessage, CancellationToken token)
+            public Message(ArraySegment<byte> buffer, WebSocketMessageType messageType, bool endOfMessage)
             {
                 Buffer = buffer;
                 CloseStatus = null;
@@ -236,7 +236,7 @@ namespace Microsoft.AspNetCore.TestHost
 
             public Message(WebSocketCloseStatus? closeStatus, string closeStatusDescription)
             {
-                Buffer = new ArraySegment<byte>(new byte[0]);
+                Buffer = new ArraySegment<byte>(Array.Empty<byte>());
                 CloseStatus = closeStatus;
                 CloseStatusDescription = closeStatusDescription;
                 MessageType = WebSocketMessageType.Close;
@@ -255,8 +255,8 @@ namespace Microsoft.AspNetCore.TestHost
             private bool _receiverClosed;
             private bool _senderClosed;
             private bool _disposed;
-            private SemaphoreSlim _sem;
-            private Queue<Message> _messageQueue;
+            private readonly SemaphoreSlim _sem;
+            private readonly Queue<Message> _messageQueue;
             
             public ReceiverSenderBuffer()
             {

src/Hosting/TestHost/src/WebSocketClient.cs

@@ -27,7 +27,7 @@ namespace Microsoft.AspNetCore.TestHost
             // PathString.StartsWithSegments that we use below requires the base path to not end in a slash.
             if (pathBase.HasValue && pathBase.Value.EndsWith("/"))
             {
-                pathBase = new PathString(pathBase.Value.Substring(0, pathBase.Value.Length - 1));
+                pathBase = new PathString(pathBase.Value[..^1]); // All but the last character.
             }
             _pathBase = pathBase;