System.Net.Sockets.SocketException

A SocketException is thrown when an operation on a network socket fails. It is the exception under most network errors, including the ones you see from HttpClient, SQL connections and message queues, where it is typically the inner exception.

The error code is the most useful part. The SocketErrorCode property gives it as an enum, and the message includes the number, such as (10061).

Statistics

10

6

Common causes

The error code tells you which of these it is. The most common ones are below.

using var client = new TcpClient("localhost", 1);   // SocketException (10061): connection refused

for (var attempt = 1; attempt <= 3; attempt++)
{
    try
    {
        using var client = new TcpClient("localhost", 1);
        break;
    }
    catch (SocketException e) when (e.SocketErrorCode == SocketError.ConnectionRefused && attempt < 3)
    {
        await Task.Delay(TimeSpan.FromMilliseconds(100 * attempt));
    }
    catch (SocketException)
    {
        break;
    }
}

How to fix it and prevent it

try
{
    using var client = new TcpClient("localhost", 1);
}
catch (SocketException e)
{
    var advice = e.SocketErrorCode switch
    {
        SocketError.ConnectionRefused => "Nothing is listening on the port.",
        SocketError.TimedOut => "The machine did not answer.",
        SocketError.HostNotFound => "The host name is not known.",
        _ => "Network error: " + e.SocketErrorCode
    };

    Console.WriteLine(advice);
}

How to read the stack trace

The first line has the error code and the address. The frames show which part of your code opened the connection:

System.Net.Sockets.SocketException (10061): No connection could be made because the target machine actively refused it. [::ffff:127.0.0.1]:5001
   at System.Net.Sockets.Socket.DoConnect(EndPoint endPointSnapshot, SocketAddress socketAddress)
   at System.Net.Sockets.Socket.Connect(EndPoint remoteEP)
   at System.Net.Sockets.Socket.Connect(IPAddress[] addresses, Int32 port)
--- End of stack trace from previous location ---
   at System.Net.Sockets.Socket.Connect(IPAddress[] addresses, Int32 port)
   at System.Net.Sockets.Socket.Connect(String host, Int32 port)
   at System.Net.Sockets.TcpClient.Connect(String hostname, Int32 port)
   at System.Net.Sockets.TcpClient..ctor(String hostname, Int32 port)
   at Shop.Services.SocketProbe.Connect(Int32 port) in C:\src\Shop\Services\SocketProbe.cs:line 10
   at Program.<Main>$(String[] args) in C:\src\Shop\Program.cs:line 4

The code is 10061, so the connection was actively refused by 127.0.0.1 on port 5001. The constructor of TcpClient connected on line 10 of SocketProbe.cs. Check that the service runs and that it listens on that address and port.

Should you catch it?

Yes, around code that talks to the network. Use a filter on SocketErrorCode to catch the ones you can handle, and let others pass.

When you use HttpClient, catch HttpRequestException instead. The SocketException is its InnerException.

Related exceptions

• HttpRequestException wraps it when you use HttpClient.
• WebException wraps it when you use the older WebRequest API.
• TimeoutException is thrown when a time limit set in code is reached.

Frequently asked questions

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Further reading

YouTube videos

Answers from Stack Overflow

const uint IOC_IN = 0x80000000U;
const uint IOC_VENDOR = 0x18000000U;

/// <summary>
/// Controls whether UDP PORT_UNREACHABLE messages are reported. 
/// </summary>
const int SIO_UDP_CONNRESET = unchecked((int)(IOC_IN | IOC_VENDOR | 12));

var udpClient = new UdpClient(9001);
udpClient.Client.IOControl(SIO_UDP_CONNRESET, new byte[] { 0x00 }, null);

Socket serverSocket =
    new Socket(ipAddress.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
                        ↑

Source: Stack Overflow