web.config中的connectionstring错误：System.InvalidOperationException：实例故障

Question

I am developing an app using Visual Studio Community Edition with C# and SQL Server Express 2014 for the database.

When I define the connection string in Web.config file as shown below:

<connectionStrings>
    <add name="connect" 
         connectionString="data source=PTW\SQLEXPRESS; Database=Mydb; integrated security=SSPI" 
         providerName="System.Data.SqlClient"/>
  </connectionStrings>

And retrieve it with the following statement:

string cs = ConfigurationManager.ConnectionStrings["connect"].ConnectionString;

I get an error:

System.InvalidOperationException: Instance failure.

But when I define the connection string inside a form, I get no error and a connection will be established.

How can I resolve this problem?

here's what i have in the web.config file:

<configuration>
  <connectionStrings>
    <add name="connect" 
         connectionString="data source=PTW\SQLEXPRESS; Database=Mydb; integrated security=SSPI" 
         providerName="System.Data.SqlClient"/>
  </connectionStrings>
  <system.web>
    <compilation debug="true" targetFramework="4.5.2"/>
    <httpRuntime targetFramework="4.5.2"/>
  </system.web>
  <system.codedom>
    <compilers>
      <compiler language="c#;cs;csharp" extension=".cs"
        type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.CSharpCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
        warningLevel="4" compilerOptions="/langversion:6 /nowarn:1659;1699;1701"/>
      <compiler language="vb;vbs;visualbasic;vbscript" extension=".vb"
        type="Microsoft.CodeDom.Providers.DotNetCompilerPlatform.VBCodeProvider, Microsoft.CodeDom.Providers.DotNetCompilerPlatform, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
        warningLevel="4" compilerOptions="/langversion:14 /nowarn:41008 /define:_MYTYPE=\&quot;Web\&quot; /optionInfer+"/>
    </compilers>
  </system.codedom>
</configuration>

and this what i have inside the form :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;

namespace GlobalCS
{
    public partial class Connect : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            string cs = ConfigurationManager.ConnectionStrings["connect"].ConnectionString;
            using (SqlConnection con = new SqlConnection(cs))
            {
                con.Open();
                Label1.Text = "Connected";
            }
        }
    }
}

Answer 1

问题出在SQL Server方面，刚刚从Microsoft（hotfixv4.microsoft.com）安装了SQL Server更新（4），仅此而已，完成。