How can I correctly pass an input parameter to an event handler method in C#?

Question

I am pretty new in C# and .NET and I have the following problem.

In my code I have something like this:

Table table = new Table();

lstLabelType.SelectedIndexChanged += new System.EventHandler(SelectDocumentTypeChanged);

that is attaching an event handler to a dropdown element into a view.

So when the user change the chosen value into this dropdown the SelectDocumentTypeChanged() method is performed, this one:

private void SelectDocumentTypeChanged(object sender, EventArgs e)
{
    Debug.WriteLine("SelectDocumentTypeChanged() STARTED");

    SPWeb contextWeb = SPContext.Current.Web;

    DropDownList listaTipiDocumenti = (DropDownList)sender;
    tipoDocumentoSelezionato = listaTipiDocumenti.SelectedValue;
    this.renderizzaEtichetteFacoltative(tipoDocumentoSelezionato, table);

    string url = contextWeb.Url;
    string link = url + "/ARXEIA WEBPART/Stampa Etichetta.aspx?IsDlg=1&postazione=" + macchina + "&tipoDoc=" + tipoDocumentoSelezionato;
    SPUtility.Redirect(link, SPRedirectFlags.Default, Context);


}

Now I have a problem. I have to pass to this event handler method the Table table object created in my code because into this method I have to use it.

How can I correctly implement this behavior? How can I pass this Table table object to the SelectDocumentTypeChanged() method when it is automatically called when the user change the value in my dropdown?

Is SelectDocumentTypeChanged and table are part of the same class? — Chetan, Feb 13 '19 at 09:50

Enigmativity · Accepted Answer · 2019-02-13T09:54:31.727

Try writing your code like this:

Table table = new Table();

lstLabelType.SelectedIndexChanged += (sender, e) =>
{
    Debug.WriteLine("SelectDocumentTypeChanged() STARTED");

    SPWeb contextWeb = SPContext.Current.Web;

    DropDownList listaTipiDocumenti = (DropDownList)sender;
    tipoDocumentoSelezionato = listaTipiDocumenti.SelectedValue;
    this.renderizzaEtichetteFacoltative(tipoDocumentoSelezionato, table);

    string url = contextWeb.Url;
    string link = url + "/ARXEIA WEBPART/Stampa Etichetta.aspx?IsDlg=1&postazione=" + macchina + "&tipoDoc=" + tipoDocumentoSelezionato;
    SPUtility.Redirect(link, SPRedirectFlags.Default, Context);
};

Now you can simply use table directly in the event handler.

You can even ignore the sender now and do this:

lstLabelType.SelectedIndexChanged += (s, e) =>
{
    Debug.WriteLine("SelectDocumentTypeChanged() STARTED");

    SPWeb contextWeb = SPContext.Current.Web;

    tipoDocumentoSelezionato = lstLabelType.SelectedValue;
    this.renderizzaEtichetteFacoltative(tipoDocumentoSelezionato, table);

    string url = contextWeb.Url;
    string link = url + "/ARXEIA WEBPART/Stampa Etichetta.aspx?IsDlg=1&postazione=" + macchina + "&tipoDoc=" + tipoDocumentoSelezionato;
    SPUtility.Redirect(link, SPRedirectFlags.Default, Context);
};

You can do this if you want to keep a separate method:

Table table = new Table();

lstLabelType.SelectedIndexChanged += (o, e) => SelectDocumentTypeChanged(lstLabelType, table);


private void SelectDocumentTypeChanged(DropDownList lstLabelType, Table table)
{
    Debug.WriteLine("SelectDocumentTypeChanged() STARTED");

    SPWeb contextWeb = SPContext.Current.Web;

    tipoDocumentoSelezionato = lstLabelType.SelectedValue;
    this.renderizzaEtichetteFacoltative(tipoDocumentoSelezionato, table);

    string url = contextWeb.Url;
    string link = url + "/ARXEIA WEBPART/Stampa Etichetta.aspx?IsDlg=1&postazione=" + macchina + "&tipoDoc=" + tipoDocumentoSelezionato;
    SPUtility.Redirect(link, SPRedirectFlags.Default, Context);
}

@AndreaNobili - Why do you prefer to do that? It's poor encapsulation. — Enigmativity, Feb 13 '19 at 09:52
To keep this logic separate and have less code into my "main" class — AndreaNobili, Feb 13 '19 at 09:54
@AndreaNobili - It really depends on your point-of-view. My method keeps the logic together with your event code making refactoring easier and it enables proper encapsulation. — Enigmativity, Feb 13 '19 at 09:56
Nice Solution. But be aware of a [problem](https://stackoverflow.com/a/2465054/9364460) that comes with it (even though i cant come up with a better solution) — Hyarus, Feb 13 '19 at 09:57
@Hyarus - It's a super easy problem to solved. Just keep a reference to the handler to detach later or use `System.Reactive.Disposables.Disposable.Create` (NuGet `System.Reactive`) to create an `IDisposable` that you can use to clean up the handler - you don't need to even keep a reference to the original handler then. — Enigmativity, Feb 13 '19 at 10:00

How can I correctly pass an input parameter to an event handler method in C#?

1 Answers1