The ConnectionString Property Has Not Been Initialized

Hi all

      This is the code in my web.config file.

       <appSettings>
    <add key="ConnectionString" value="server=127.0.0.1;database=testdb;uid=sa;pwd=sa"/>
     </appSettings>

      When I'm connecting to the sqlserver database,the page shows that The ConnectionString property has not been initialized.

      The code that I used to get the "ConnectionString" as belows

using System.Configuration;

protected static string connectionString =ConfigurationSettings.AppSettings["ConnectionString"]; 

  public static object GetSingle(string SQLString,params SqlParameter[] cmdParms)
  {
   using (SqlConnection connection = new SqlConnection(connectionString))
   {
    using (SqlCommand cmd = new SqlCommand())
    {
     try
     {
                       
      PrepareCommand(cmd, connection, null,SQLString, cmdParms);
      object obj = cmd.ExecuteScalar();
      cmd.Parameters.Clear();
      if((Object.Equals(obj,null))||(Object.Equals(obj,System.DBNull.Value)))
      {     
       return null;
      }
      else
      {
       return obj;
      }    
     }
     catch(System.Data.SqlClient.SqlException e)
     {    
      throw new Exception(e.Message);
     }     
    }
   }
  }

 

These code works well on my machine.When running on my colleague's machine ,the page show that exception.

  Any ideal?