为什么serverSelected为null？

ViewModel：

ViewModel :

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);
        }

DAL：

DAL :

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;
}

AppConfig：

AppConfig :

<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>

_DBNameList = servObj.GetDBNames（serverSelected）;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。

_DBNameList = servObj.GetDBNames(serverSelected); this serverSelected is always passing as null which is not allowing my code to build and i am not able to select server from server list because of this.