connecting to SQL Server from c#

Question

i am reposting this question because alot of people here advised me to repost the error messages and the code for the DB_connection,the error is popping in the DB_Access class on this code conn = DB_conection.GetConnection()`;,i am working on a simple database project in c sharp and ms sql sever 2008 but im having an error upon compiling the program its poping up this message below

System.TypeInitializationException was unhandled

  Message=The type initializer for 'StudentsInformationSystem.DB_conection' threw an exception.
  Source=StudentsInformationSystem
  TypeName=StudentsInformationSystem.DB_conection
  StackTrace:
       at StudentsInformationSystem.DB_conection.GetConnection()
       at StudentsInformationSystem.DB_Access..ctor() in C:\Users\soft\Documents\Visual Studio 2010\Projects\StudentsInformationSystem\StudentsInformationSystem\DB_Access.cs:line 16
       at StudentsInformationSystem.frmNewStudent..ctor() in C:\Users\soft\Documents\Visual Studio 2010\Projects\StudentsInformationSystem\StudentsInformationSystem\frmNewStudent.cs:line 14
       at StudentsInformationSystem.Form1.btnAddNewStudent_Click(Object sender, EventArgs e) in C:\Users\soft\Documents\Visual Studio 2010\Projects\StudentsInformationSystem\StudentsInformationSystem\Form1.cs:line 31
       at System.Windows.Forms.Control.OnClick(EventArgs e)
       at System.Windows.Forms.Button.OnClick(EventArgs e)
       at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
       at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
       at System.Windows.Forms.Control.WndProc(Message& m)
       at System.Windows.Forms.ButtonBase.WndProc(Message& m)
       at System.Windows.Forms.Button.WndProc(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
       at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
       at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
       at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.Run(Form mainForm)
       at StudentsInformationSystem.Program.Main() in C:\Users\soft\Documents\Visual Studio 2010\Projects\StudentsInformationSystem\StudentsInformationSystem\Program.cs:line 18
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: System.NullReferenceException
       Message=Object reference not set to an instance of an object.
       Source=StudentsInformationSystem
       StackTrace:
            at StudentsInformationSystem.DB_conection..cctor() in C:\Users\soft\Documents\Visual Studio 2010\Projects\StudentsInformationSystem\StudentsInformationSystem\DB_conection.cs:line 15
       InnerException:

and this my DB_connection class

 class DB_conection
    {
        public static SqlConnection NewCon;
        public static string constr = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;

        public static SqlConnection GetConnection() {

            NewCon = new SqlConnection(constr);
            return NewCon;

        }

    }

and this is my code for the DB_Access

namespace StudentsInformationSystem
{
    class DB_Access
    {
        SqlConnection conn;
        public DB_Access() {

            conn = DB_conection.GetConnection();

        }
        public void add_student(string regNo,string fname, string lname, string phoneNo){

            if (conn.State == ConnectionState.Closed) {

                conn.Open();
            }
            SqlCommand newCmd = conn.CreateCommand();
            newCmd.Connection = conn;
            newCmd.CommandType = CommandType.Text;
            newCmd.CommandText = "insert into student values('" + regNo + "','" + fname + "','" + lname + "','" + phoneNo + "')";
            newCmd.ExecuteNonQuery();

        }
    }
}

Answer 1

You have a NullReferenceException in the static constructor of your DB_conection [sic] class. The only possible place I can spot where that happens is in this line (line-breaks added for readability):

public static string constr = 
    ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;

In particular, it looks as if the ConnnectionStrings[string] indexer property returns null. Typically it does that, when the connection string as specified by the name (here ConString) cannot be found.

Make sure your application configuration file (named app.config in Visual Studio) contains a definition for the connection string named "ConString" in the connectionStrings element.

Example:

<connectionStrings>
  <add name="ConString" connectionString="..."/>
</connectionStrings>

Of course, you would have to replace the "..." with the actual connection string.

Note that you have a couple of other issues in your code, like imminent SQL Injection, missing Dispose calls (or using blocks) on at least the SqlCommand objects, etc.