Visual studio中创建的类,ConfigurationException未包含ConnectionStrings定义,怎么改?

namespace WindowsFormsApplication1
{
class DbConn
{
static public string connString =
System.Configuration.ConfigurationException.ConnectionStrings["ConnectionString"].ConnectionString;
static public SqlConnection GetConn()
{
SqlConnection conn = new SqlConnection(connString);
if (conn.State != ConnectionState.Open)
{
conn.Open();
}
return conn;
}

}

}

ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString