c# – HttpWebRequest：请求已中止：无法创建SSL / TLS安全通道

The request was aborted: Could not create SSL/TLS secure channel.

Description: An unhandled exception occurred during the execution of
the current web request. Please review the stack trace for more
information about the error and where it originated in the code.

Exception Details: System.Net.WebException: The request was aborted:
Could not create SSL/TLS secure channel.

Source Error:

Line 203: Line 204: //Retrieve the Response returned from the
NVP API call to PayPal. Line 205: HttpWebResponse objResponse =
(HttpWebResponse)objRequest.GetResponse(); Line 206: string
result; Line 207: using (StreamReader sr = new
StreamReader(objResponse.GetResponseStream()))

Source File: C:Userswillemdocumentsvisual studio
2015ProjectsWingtipToysWingtipToysLogicPayPalFunctions.cs
Line: 205

在我的错误发生的方法下面

public string HttpCall(string NvpRequest)
{
    string url = pEndPointURL;

    string strPost = NvpRequest + "&" + buildCredentialsNVPString();
    strPost = strPost + "&BUTTONSOURCE=" + HttpUtility.UrlEncode(BNCode);

    HttpWebRequest objRequest = (HttpWebRequest)WebRequest.Create(url);
    objRequest.Timeout = Timeout;
    objRequest.Method = "POST";
    //objRequest.ContentLength = strPost.Length;

    try
    {
        using (StreamWriter myWriter = new StreamWriter(objRequest.GetRequestStream()))
        {
            myWriter.Write(strPost);
        }
    }
    catch (Exception)
    {
        // No logging for this tutorial.
    }

    //Retrieve the Response returned from the NVP API call to PayPal.
    HttpWebResponse objResponse = (HttpWebResponse)objRequest.GetResponse();
    string result;
    using (StreamReader sr = new StreamReader(objResponse.GetResponseStream()))
    {
        result = sr.ReadToEnd();
    }

    return result;
}