为什么serverSelected为null？

Question

查看模型

let stringArray = ["88612345678", "98765432", "55587654321"]
let result = stringArray.map{String($0.characters.suffix(8))}

print(result)  // "["12345678", "98765432", "87654321"]\n"

DAL

string _serverSelected;
        public string serverSelected
        {
            get { return _serverSelected; }
            set
            {
                if (_serverSelected != value)
                    _serverSelected = value;

                RaisePropertyChanged("serverSelected");              
            }
        }

        public BackUpViewModel()
        {

             BackUpContext servObj = new BackUpContext();                      
            _DBNameList = servObj.GetDBNames(serverSelected);
        }

的app.config

public List<string> GetDBNames(string sqlServerName)
        {
    List<string> lstDBName = new List<string>();         


                SqlConnectionStringBuilder connection = new SqlConnectionStringBuilder();
                connection.DataSource = sqlServerName;
                connection.IntegratedSecurity = true;
                String strCon = connection.ToString();
                SqlConnection sqlConn = new SqlConnection(strCon);
                sqlConn.Open();

                SqlCommand cmd = new SqlCommand("select * from sys.databases", sqlConn);
                SqlDataReader dr = cmd.ExecuteReader();
                while (dr.Read())
                {
                    lstDBName.Add(dr[0].ToString());
                }
                sqlConn.Close();


                return lstDBName;
}

<configuration> <connectionStrings> <add name="CostAllocationEntities2" connectionString="Data Source=pc210090\SQLExpress;Persist Security Info=True;User ID=sa;Password=password1$" providerName="System.Data.SqlClient"/> </connectionStrings> <appSettings> <add key="ServerName1" value="PC210090\SQLEXPRESS"/> <add key="ServerName2" value="s2"/> <add key="ServerName3" value="s3"/> <add key="BackupDestinationPath" value="D:\BackUpDatabase\"/> <add key="RestoreDestinationPath" value="D:\RestoreDatabase\"/> </appSettings> </configuration>  这个serverSelected总是传递为null，这不允许我的代码构建，因此我无法从服务器列表中选择服务器。