System.invalidoperationexception: collection was modified; enumeration operation may not execute. At system.throwhelper.throwinvalidoperationexception(exceptionresource resource)

System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections.Generic.List`1.Enumerator.MoveNextRare()

<pre>  List<string> AppList = new List<string>();

  public void loadApplicationDetail()
{
AppList.Clear();
                var Selectlistapp = Dbl.ReturnDataset("select Address,Aort from tblor_Details");
                if (Selectlistapp.Tables[0].Rows.Count > 0)
                {
                    for (var i = 0; i < Selectlistapp.Tables[0].Rows.Count; i++)
                    {
                        string Address = Selectlistapp.Tables[0].Rows[i]["Address"].ToString();
                        string Aort = Selectlistapp.Tables[0].Rows[i]["Aort"].ToString();
                        AppList.Add("http://" + Address + ":" + Aort + "/");
                    }
                    foreach (var app in AppList) //Error throws here in log file
                    {
                        bool sappvalue;
                        ApplicationMonitorScript appmonitor = new ApplicationMonitorScript();

                        sappvalue = appmonitor.UrlIsValid(app);
                        if (sappvalue == false)
                        {
                            MessageBox.Show("Stopped");
                        }
                        else
                        {
                             MessageBox.Show("start");
                        }
                    }
                }

}

List<string> AppList = new List<string>()

  public void loadApplicationDetail()
{
AppList.Clear();
                var Selectlistapp = Dbl.ReturnDataset("select Address,Aort from tblor_Details");
                if (Selectlistapp.Tables[0].Rows.Count > 0)
                {
                    for (var i = 0; i < Selectlistapp.Tables[0].Rows.Count; i++)
                    {
                        string Address = Selectlistapp.Tables[0].Rows[i]["Address"].ToString();
                        string Aort = Selectlistapp.Tables[0].Rows[i]["Aort"].ToString();
                        AppList.Add("http://" + Address + ":" + Aort + "/");
                    }
                    foreach (var app in AppList) //Error throws here in log file
                    {
                        bool sappvalue;
                        ApplicationMonitorScript appmonitor = new ApplicationMonitorScript();

                        sappvalue = appmonitor.UrlIsValid(app);
                        if (sappvalue == false)
                        {
                            MessageBox.Show("Stopped");
                        }
                        else
                        {
                             MessageBox.Show("start");
                        }
                    }
                }

}