类里的一个代码

public DataTable FillDataTable(String query)
{

DataTable dt = new DataTable();
using (conn = new SqlConnection(ConnStr))
{
SqlCommand cmd = new SqlCommand();
cmd.Connection = conn;
cmd.CommandText = query;
SqlDataAdapter ada = new SqlDataAdapter();
ada.SelectCommand = cmd;
ada.Fill(dt);   
}

return dt;
}

报错时指向代码 ada.Fill(dt)；显示system.InvalidOperationException: ConnectionString 属性尚未初始化

<connectionStrings>
<add name="ApplicationServices"
connectionString="data source=.\SQLEXPRESS;Integrated Security=SSPI;AttachDBFilename=|DataDirectory|\pho.mdf;User Instance=true"
providerName="System.Data.SqlClient" />

</connectionStrings>

这一段应该怎么修改？ add name="ApplicationServices" 是什么意思？