title: An exception of type 'System.Data.SqlClient.SqlException' Instance failure
body: <p>I'm a beginner with C# and I'm working with a web service. When I tried to test my service, this error message occurs:</p>
  
  <blockquote>
    <p>An exception of type 'System.InvalidOperationException' occurred in System.Data.dll but was not handled in user code</p>
    
    <p>Additional information: Instance failure.</p>
  </blockquote>
  
  <p>Here is my connection string in web configuration:</p>
  
  <pre><code>&lt;connectionStrings&gt;
      &lt;add name="connection" 
           connectionString="Data Source=DESKTOP-CIRUCVV\\SQLEXPRESS;Initial Catalog=CustomerManagerment;Integrated Security=True;"/&gt;
  &lt;/connectionStrings&gt;
  </code></pre>
  
  <p>Here is my code inside the web service class</p>
  
  <pre><code>using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Web;
  using System.Web.Services;
  using System.Data.SqlClient;
  using System.Data;
  using System.Web.Configuration;
  
  namespace WebService
  {
      /// &lt;summary&gt;
      /// Summary description for CustomerService
      /// &lt;/summary&gt;
      [WebService(Namespace = "http://tempuri.org/")]
      [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
      [System.ComponentModel.ToolboxItem(false)]
      // To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
      // [System.Web.Script.Services.ScriptService]
      public class CustomerService : System.Web.Services.WebService
      {
          static string strcon = WebConfigurationManager.ConnectionStrings["connection"].ConnectionString;
  
          public CustomerService() { }
  
          [WebMethod(Description = "Show Customers")]
          public DataSet showCustomer() 
          {
              SqlConnection con = new SqlConnection(strcon);
              string Sql = "select * from Customers";
  
              SqlDataAdapter da = new SqlDataAdapter(Sql, con);
              DataSet ds = new DataSet();
              da.Fill(ds);
  
              return ds;
          }
      }
  }
  </code></pre>