..The Underlying Connection Was Closed: an Unexpected Error Occurred on a Receive

..The underlying connection was closed: An unexpected error occurred on a receive

The underlying connection was closed: An unexpected error occurred on a receive.

This problem occurs when the server or another network device
unexpectedly closes an existing Transmission Control Protocol (TCP)
connection. This problem may occur when a time-out value on the
server or on the network device is set too low. To resolve this
problem, see resolutions A, D, E, F, and O. The problem can also
occur if the server resets the connection unexpectedly, such as if an
unhandled exception crashes the server process. Analyze the server
logs to see if this may be the issue.

Resolution

To resolve this problem, make sure that you are using the most recent version of the .NET Framework.

Add a method to the class to override the GetWebRequest method. This change lets you access the HttpWebRequest object. If you are using Microsoft Visual C#, the new method must be similar to the following.

class MyTestService:TestService.TestService
{
    protected override WebRequest GetWebRequest(Uri uri)
    {
        HttpWebRequest webRequest = (HttpWebRequest) base.GetWebRequest(uri);
        //Setting KeepAlive to false
        webRequest.KeepAlive = false;
        return webRequest;
    }
}

Excerpt from KB915599: You receive one or more error messages when you try to make an HTTP request in an application that is built on the .NET Framework 1.1 Service Pack 1.

C# HttpWebRequest The underlying connection was closed: An unexpected error occurred on a send

In 4.0 version of the .Net framework the ServicePointManager.SecurityProtocol only offered two options to set:

• Ssl3: Secure Socket Layer (SSL) 3.0 security protocol.
• Tls: Transport Layer Security (TLS) 1.0 security protocol

In the next release of the framework the SecurityProtocolType enumerator got extended with the newer Tls protocols, so if your application can use th 4.5 version you can also use:

• Tls11: Specifies the Transport Layer Security (TLS) 1.1 security protocol
• Tls12: Specifies the Transport Layer Security (TLS) 1.2 security protocol.

So if you are on .Net 4.5 change your line

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls;

to

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

so that the ServicePointManager will create streams that support Tls12 connections.

Do notice that the enumeration values can be used as flags so you can combine multiple protocols with a logical OR

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | 
                                       SecurityProtocolType.Tls11 |
                                       SecurityProtocolType.Tls12;

Note
Try to keep the number of protocols you support as low as possible and up-to-date with today security standards. Ssll3 is no longer deemed secure and the usage of Tls1.0 SecurityProtocolType.Tls is in decline.

asp.NET 4.8 WebRequest errors with The underlying connection was closed: An unexpected error occurred on a send.

As mentioned in the comments, this is often an issue with the internal DNS or a HOSTS file entry. Either the server is resolving the site's domain to the wrong internal address, or it's trying to access the site's public address and confusing the router.

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