用户单击ASP.NET Web表单上的按钮与计时器运行调用相同按钮单击事件的方法之间的区别？

Question

I have created an ASP.NET web form (page?) with a button that when clicked, will open up an Access database that runs an autoexec whose purpose is to run a delete/append sequence so I have data that I need in a table. The code in the button on click event is supposed to also refresh the web page to display the updated data. This works for the most part when a user physically clicks the button. However, when I create a timer object to run a method that calls the click event I get an error: System.NullReferenceException: 'Object reference not set to an instance of an object.' System.Web.HttpContext.Current.get returned null. What gives?

Like I said, it works when there is a physical user clicking the button, but ultimately I want to automate the task of opening the Access db, as well as refreshing the web form (page?) with the new data.

This is the button click code. The commented lines are what I've already tried:

    protected void btnRefresh_Click(object sender, EventArgs e)
    {
        Process.Start(@"C:\Users\mperea\Desktop\DC8SortMonitor.accdb");           

        HttpContext.Current.Response.Redirect("~/Default.aspx");

        //Response.Redirect("~/Default.aspx");
        //Page.Response.Redirect(Page.Request.Url.ToString(), true);
        //Server.TransferRequest(Request.Url.AbsolutePath, false);

        //Trying open access with this didn't work:
        //Access.Application oAccess = new Access.ApplicationClass();//oAccess.OpenCurrentDatabase(@"C:\Users\mperea\Desktop\DC8SortMonitor.accdb");
    } 

This is the timer object meant to programmatically execute the method that calls the click event:

    protected void Page_Load(object sender, EventArgs e)
    {

        Timer aTimer;

        //Create a timer and set a two second interval.
        aTimer = new System.Timers.Timer();
        aTimer.Interval = 15000;

        //ElapsedEventHandler btnRefresh_Click = null;
        //Hook up the Elapsed event for the timer. 
        aTimer.Elapsed += timer;

        //Have the timer fire repeated events(true is the default)
        aTimer.AutoReset = true;

        //Start the timer
        aTimer.Enabled = true;
     }

and the method which the timer calls:

    public void timer(object sender, EventArgs e)
    {
        btnRefresh_Click(null, null);
    }

Answer 1

First thing that jumps out at me is that the button_click method is expecting a object sender and EventArgs e , which you are passing in as null, because obviously you don't have the information to give it.

You should not call your button event handlers in this way. Instead you should move the shared code out from your click method into a different method which doesn't require those parameters that you can call from anywhere.

protected void btnRefresh_Click(object sender, EventArgs e)
{
    Refresh();
} 

Now your Refresh Method

public void Refresh()
{
    Process.Start(@"C:\Users\mperea\Desktop\DC8SortMonitor.accdb");

    Response.Redirect("~/Default.aspx");

    //HttpContext.Current.Response.Redirect("~/Default.aspx");
    //Page.Response.Redirect(Page.Request.Url.ToString(), true);
    //Server.TransferRequest(Request.Url.AbsolutePath, false);

    //Access.Application oAccess = new Access.ApplicationClass();//oAccess.OpenCurrentDatabase(@"C:\Users\mperea\Desktop\DC8SortMonitor.accdb");
}

And timer

 public void timer(object sender, EventArgs e)
{
    Refresh();
}

The click event handler should only really handle the event specifically, and any functionality relating to what the button does should be in separate methods. Usually the work of a button is so small that it isn't necessary, but situations like this illustrate why it is generally a good idea.

Answer 2

You can't use a timer and code that runs server side to click/run another button/code sever side and expect that the controls values can be used, read, grabbed, or modified. This will not work.

Remember, when you actually click on the button, YOU START running browser side code, and the VERY first thing that occurs is a full web page post-back to the server. It is then (and only then), that your actual event code written in c# starts to run.

If you just call other code bits, or even a button event, then the code can run, but will run without a full web page post back. It is the transfer of the whole web page to the server that has to occur BEFORE any of you code can look at/grab/modify the values of controls.

The only way to actually make this work is to setup some JavaScript that runs browser side as a timer, and then when it fires (clicks) the actual button you want to click on the web page (not server side code),then the button click event will:

Do a full web page post back to the server (and transfer the contents of all controls/text boxes etc.).

Then after the web page transfer has occurred, then the button code for the given click event will now run, and when it does run as noted it will have use of the data in the web page.

So, you can't use a timer in c# that will be running server side code, and expect the button code to do work, since you have to first do a full page post back on the client side to transfer the current value(s) in controls.

In your case, the button code via the timer will run, but the web page not been posted (sent back) to the server side. So, your code will run, but without any use of data in the web page.

So, say while a user is typing things into the web page, your timer code can run the button event, but the user is still typing away on the web page, and the data on the web page will NOT have yet been transferred to the server. So you need a full post back before that button event written in c# can examine any values on the page.

As noted, you could write browser side JavaScript and such code is able to view and use and look at any control before a full post back. And such JavaScript code of course can also fire/click that button. So values in/from the web browser have to be transferred to the server side for c# to be able to look at or use current values in the control(s) on that page.