The underlying connection was closed Error

The underlying connection was closed: Unable to connect to the remote server. so I searched and found that many people encountered this exception, but there were no standard solutions. There are generally several. 1. If a proxy is used, the setting is correct. 2. If firewall is used, disable it. 3. Disable Automatically Detect Settings in the Lan Settings. 4. reinstall the. Net Framework. 5. if crawling is used, you can set the KeepAlive attribute of HttpWebRequest to false. The KeepAlive attribute of HttpWebRequest is true by default, which corresponds to my online programs. The options 1, 2, 3, and 4 can be excluded, this error is reported when I capture it. After 5th methods are used, the exception disappears.