为什么会显示实例失败，如何解决？

Question

using System.Data.SqlClient;
using System.Configuration;

namespace Logon
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}
ManagerForm mf = new ManagerForm();
UserForm uf = new UserForm();
bool testMyUser()
{
DBManager db = new DBManager();
SqlParameter[] parames=new SqlParameter[3];
parames[0] = new SqlParameter("@name",txtName.Text);
parames[1] = new SqlParameter("@password",txtPassword.Text);
parames[2] = new SqlParameter("@role",cmbRole.Text);
DataTable dt = db.SelectMyUser("select * from MyUser where Name=@name and Password=@password and Role=@role",parames);
if(dt.Rows.Count>0)
{
return true;
}
else 
{
return false;
}
}

private void btnLog_Click(object sender, EventArgs e)
{
if(cmbRole.Text=="Manager")
{
if(testMyUser())
{
mf.Show();
}
else 
{
MessageBox.Show("用户名或密码错误！");
}
}  
}
class DBManager
{
public static readonly string connectionString = ConfigurationManager.ConnectionStrings["connectionString"].ConnectionString;
SqlConnection cn = new SqlConnection(connectionString);
public DBManager()
{
cn.Open();运行时，此处显示cn实例失败
}
public int RunMyUser(string strSql, SqlParameter[] parames)
{
SqlCommand cm = new SqlCommand();
cm.Connection = cn;
cm.CommandType = CommandType.Text;
cm.CommandText = strSql;
cm.Parameters.Clear();
if (parames != null)
{
foreach (SqlParameter parame in parames)
{
cm.Parameters.Add(parame);
}
}
int i = cm.ExecuteNonQuery();
return i;
}
public DataTable SelectMyUser(string strSql, SqlParameter[] parames)
{
SqlCommand cm = new SqlCommand();
cm.Connection = cn;
cm.CommandType = CommandType.Text;
cm.CommandText = strSql;
cm.Parameters.Clear();
if (parames != null)
{
foreach (SqlParameter parame in parames)
{
cm.Parameters.Add(parame);
}
}
SqlDataAdapter da = new SqlDataAdapter();
da.SelectCommand = cm;
DataSet ds = new DataSet();
da.Fill(ds);
DataTable dt = ds.Tables[0];
return dt;
}
}
}

Answer 1

这说明数据库连接失败了改成try{cn.Open();}catch(SqlException ex){MessageBox.Show(ex.ToString());}看看错误信息，看看是数据库连接字符串的问题，还是其他问题。

Answer 2

cn.Open();把这句话注释掉