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

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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.

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