C# System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send

Setting the HttpWebRequest.KeepAlive to false didn't work for me.

Since I was accessing a HTTPS page I had to set the Service Point Security Protocol to Tls12.

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;

Notice that there are other SecurityProtocolTypes:

SecurityProtocolType.Ssl3 
SecurityProtocolType.Tls
SecurityProtocolType.Tls11

So if the Tls12 doesn't work for you, try the three remaining options.

Also notice you can set multiple protocols. This is preferable on most cases.

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

Edit: Since this is a choice of security standards it's obviously best to go with the latest (TLS 1.2 as of writing this), and not just doing what works. In fact, SSL3 has been officially prohibited from use since 2015 and TLS 1.0 and TLS 1.1 will likely be prohibited soon as well. source: @aske-b

I was getting the same error, using RestSharp with .NET 4.5. I tested the same URL with cURL and it worked fine. After a long time debugging I found that setting the SecurityProtocol fixed the issue.

See: "The underlying connection was closed: An unexpected error occurred on a send." With SSL Certificate