Undocumented NullReferenceException is Raised by ClientWebSocket.ReceiveAsync #47582
Description
The ReceiveAsync and SendAsync methods of System.Net.WebSockets.ClientWebSocket (System.Net.WebSockets.Client.dll) can raise Null Reference Exceptions. This is not in the documentation.
Should an Invalid Operation Exception be raised instead?
It can happen when ReceiveAsync or SendAsync are called after an unsuccessful call to OpenAsync.
It happens with .net 5.0.2 (current release version on x64 Windows 20H2), but also happens with 5.0.1.
Sample Code:
using System;
using System.Net.WebSockets;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
#nullable enable
namespace HelloWebSocket
{
class Program
{
static async Task Main(string[] args)
{
using (var webSocket = new ClientWebSocket())
{
var encoding = UTF8Encoding.UTF8;
var address = "wss://echo.websocket.NON-EXISTENT/"; // <-- Bad Address (should be: echo.websocket.org)
var uri = new Uri(address);
try
{
await webSocket.ConnectAsync(uri, CancellationToken.None);
}
catch (WebSocketException)
{
Console.WriteLine($"EXCEPTION while connecting to: {address}");
Console.WriteLine("But carrying on regardless ...");
}
var rcvBuff = new byte[100];
var rcvTask = webSocket.ReceiveAsync(rcvBuff, CancellationToken.None); // <-- Ln 32 NullReference Exception
var sndText = "Hello, WebSocket!";
var sndBuff = encoding.GetBytes(sndText);
await webSocket.SendAsync(sndBuff, WebSocketMessageType.Text, true, CancellationToken.None);
var rcvMessage = await rcvTask;
switch (rcvMessage.MessageType)
{
case WebSocketMessageType.Close:
Console.WriteLine("Received Close");
break;
case WebSocketMessageType.Binary:
Console.WriteLine("Received Binary");
break;
case WebSocketMessageType.Text:
var rcvText = encoding.GetString(rcvBuff, 0, rcvMessage.Count);
Console.WriteLine($"Received Text: {rcvText}");
break;
}
}
}
}
}Sample output:
...
Loaded 'C:\Program Files\dotnet\shared\Microsoft.NETCore.App\5.0.2\System.IO.FileSystem.dll'. Cannot find or open the PDB file.
Loaded 'C:\Program Files\dotnet\shared\Microsoft.NETCore.App\5.0.2\System.Collections.Immutable.dll'. Cannot find or open the PDB file.
Exception thrown: 'System.Net.Sockets.SocketException' in System.Private.CoreLib.dll
Exception thrown: 'System.Net.Sockets.SocketException' in System.Net.Sockets.dll
Exception thrown: 'System.Net.Sockets.SocketException' in System.Private.CoreLib.dll
Exception thrown: 'System.Net.Sockets.SocketException' in System.Net.Http.dll
Exception thrown: 'System.Net.Sockets.SocketException' in System.Private.CoreLib.dll
Exception thrown: 'System.Net.Http.HttpRequestException' in System.Net.Http.dll
Exception thrown: 'System.Net.Http.HttpRequestException' in System.Private.CoreLib.dll
Exception thrown: 'System.Net.Http.HttpRequestException' in System.Net.Http.dll
Exception thrown: 'System.Net.Http.HttpRequestException' in System.Private.CoreLib.dll
Exception thrown: 'System.Net.WebSockets.WebSocketException' in System.Net.WebSockets.Client.dll
Exception thrown: 'System.Net.WebSockets.WebSocketException' in System.Private.CoreLib.dll
Loaded 'C:\Program Files\dotnet\shared\Microsoft.NETCore.App\5.0.2\System.Text.Encoding.Extensions.dll'. Cannot find or open the PDB file.
EXCEPTION while connecting to: wss://echo.websocket.NON-EXISTENT/
But carrying on regardless ...
Exception thrown: 'System.NullReferenceException' in System.Net.WebSockets.Client.dll
Exception thrown: 'System.NullReferenceException' in System.Private.CoreLib.dll
Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
at System.Net.WebSockets.ClientWebSocket.ReceiveAsync(ArraySegment`1 buffer, CancellationToken cancellationToken)
at HelloWebSocket.Program.Main(String[] args) in C:\Stuff\Code\bug\Program.cs:line 32
at HelloWebSocket.Program.<Main>(String[] args)
The program '[3772] bug.dll' has exited with code 0 (0x0).