在C＃中的数据表中添加行

Question

I have created a data table in my Application start method and I am trying to add rows to this data table from my session start method. When I try to add rows, it gives an error on the data table that

the name does not exists in the current context.

I am a beginner to programming. Any help is appreciated.

Here is my code:

// application begins
void Application_Start(Object s, EventArgs e)
{
    DataTable dt = new DataTable();
    dt.Columns.Add(new DataColumn("session_id", System.Type.GetType("System.String")));
    dt.Columns.Add(new DataColumn("username",   System.Type.GetType("System.String")));
    dt.Columns.Add(new DataColumn("login_time", System.Type.GetType("System.DateTime")));
    dt.Columns.Add(new DataColumn("ip_address", System.Type.GetType("System.String")));

    Application["visitorTable"] = dt;
}

// browser's first visit to the page, (session starts)
void Session_Start(Object s, EventArgs e)
{
    {
        Application.Lock();

        DataRow dr = dt.NewRow();

        dr["session_id"] = (System.String)HttpContext.Current.Session.SessionID; // session id
        dr["ip_address"] = Request.ServerVariables["SERVER_NAME"]; //ip-address

        dt.Rows.Add(dr);

        dt = (Database)Application["visitorTable"];

        Application["visitorTable"] = dt;

        Application.UnLock();
    }
}

Answer 1

You should specify the table name when creating a new DataTabe...

    DataTable dt = new DataTable("VisitorTable");
    dt.Columns.Add(new DataColumn("session_id", System.Type.GetType("System.String")));
    dt.Columns.Add(new DataColumn("username",   System.Type.GetType("System.String")));
    dt.Columns.Add(new DataColumn("login_time", System.Type.GetType("System.DateTime")));
    dt.Columns.Add(new DataColumn("ip_address", System.Type.GetType("System.String")));

    Application["visitorTable"] = dt;

Answer 2

    // you create it like this 
    Application["visitorTable"] = dt;

    // and you read it like this
    dt = (Database)Application["VisitorTable"];

Notice the issue?

Also you don't need to rely on reflection to get a type's description, a simple compile time typeof(string) and typeof(DateTime) will take care of it.

Answer 3

when your are executing following code

DataTable dt = new DataTable();
    dt.Columns.Add(new DataColumn("session_id", System.Type.GetType("System.String")));
    dt.Columns.Add(new DataColumn("username",   System.Type.GetType("System.String")));
    dt.Columns.Add(new DataColumn("login_time", System.Type.GetType("System.DateTime")));
    dt.Columns.Add(new DataColumn("ip_address", System.Type.GetType("System.String")));

have you checked whether your dt added with column name specified ? or dt is generated with columns Column1,colum2,column3 if those are generated with Column1,colum2,column3 ths why you ll be facing above error , rename column with your name of dt

Answer 4

Check this

Dynamically create DataTable and bind to GridView in ASP.Net

it might be help you

 DataTable dt; void Application_Start(Object s, EventArgs e) { dt= new DataTable(); dt.Columns.Add(new DataColumn("session_id", System.Type.GetType("System.String"))); dt.Columns.Add(new DataColumn("username", System.Type.GetType("System.String"))); dt.Columns.Add(new DataColumn("login_time", System.Type.GetType("System.DateTime"))); dt.Columns.Add(new DataColumn("ip_address", System.Type.GetType("System.String"))); Application["visitorTable"] = dt; }