LDAP服务器不可用异常

Question

我尝试在C＃Web应用程序中实现LDAP身份验证。

我尝试使用以下代码。

try
{
    using (LdapConnection conn = new LdapConnection(this.Server))
    {
        string uname = userName;
        if (!string.IsNullOrEmpty(this.UsernamePrepend))
        {
            uname = string.Concat(this.UsernamePrepend, userName);
        }
        NetworkCredential cred = new NetworkCredential(uname, password, null);
        conn.SessionOptions.SecureSocketLayer = true;
        conn.SessionOptions.VerifyServerCertificate = (LdapConnection con, X509Certificate cer) => true;
        conn.SessionOptions.ProtocolVersion = 3;
        conn.AuthType = AuthType.Basic;
        conn.Bind(cred);
    }
}
catch (LdapException ldapException)
{
    LdapException ex = ldapException;
    if (!ex.ErrorCode.Equals(49))
    {
        this.LogError(ex, userName);
        throw ex;
    }
}
flag = true;

每次运行它时，都会进入catch块，异常LDAP服务器不可用。

我错过了什么吗？