在 C# 中使用带有 SSL (https) 的 Web 服务

我想在 C# 中使用一个 ssl 安全的 Web 服务。请求如下所示：

<soapenv:Envelope
  xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
  xmlns:biw="http://tempuri.org/ws_returnTable"
  xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd"
  >
  <soapenv:Header>
    <wsse:Security>
      <wsse:UsernameToken>
        <wsse:Username>sasdemo</wsse:Username>
        <wsse:Password>sasch.111</wsse:Password>
      </wsse:UsernameToken>
    </wsse:Security>
  </soapenv:Header>
   <soapenv:Body>
      <biw:meansclass />
   </soapenv:Body>
</soapenv:Envelope> 

WSDL 中的端点声明如下所示：

<wsdl:service name="meansclass">
  <wsdl:port binding="tns:meansclassSoapBinding" name="meansclassPort">
    <soap:address location="https://sas.ssz.intra.stzh.ch:8443/SASBIWS/services/Shared%20Data/ws/meansclass" /> 
  </wsdl:port>
</wsdl:service>

app.config 看起来像这样：

<bindings>
    <basicHttpBinding>
        <binding name="meansclassSoapBinding" closeTimeout="00:01:00"
            openTimeout="00:01:00" receiveTimeout="00:10:00" sendTimeout="00:01:00"
            allowCookies="false" bypassProxyOnLocal="false" hostNameComparisonMode="StrongWildcard"
            maxBufferSize="65536" maxBufferPoolSize="524288" maxReceivedMessageSize="65536"
            messageEncoding="Text" textEncoding="utf-8" transferMode="Buffered"
            useDefaultWebProxy="true">
            <readerQuotas maxDepth="32" maxStringContentLength="8192" maxArrayLength="16384"
                maxBytesPerRead="4096" maxNameTableCharCount="16384" />
            <security mode="Transport">
                <transport clientCredentialType="None" proxyCredentialType="None"
                    realm="" />
                <message clientCredentialType="UserName" algorithmSuite="Default" />
            </security>
        </binding>
    </basicHttpBinding>
</bindings>
<client>
    <endpoint address="https://localhost:8443/SASBIWS/services/Shared%20Data/ws/meansclass"
        binding="basicHttpBinding" bindingConfiguration="meansclassSoapBinding"
        contract="ServiceReference1.meansclassPortType" name="meansclassPort" />
</client>

我尝试使用该 C# 代码访问 Web 服务：

var service = new meansclassPortTypeClient();
service.ClientCredentials.UserName.UserName = "username";
service.ClientCredentials.UserName.Password = "password";
var test = service.meansclass();

但是总是有一个由 meansclass(( 抛出的异常。在德语中，它说：

类型为"客户端身份验证"的异常 (...(没有安全上下文。

用谷歌搜索了很多，但没有发现我做错了什么。如何制作这样的"安全上下文"？还是怎么了？