Asp.net Frequently Asked Questions by azamsharp


Introduction:

I post a lot on Asp.net forums, most of the times answering questions and sometimes asking questions.

Q1: How do I open a new window from the server side code?

You can open a new window from the server side code by attaching the attributes to the server controls. Let’s see that how you can open a new window on the button click event. Just type the line below in the InitializeComponent() method which is automatically generated by Asp.net.

this.Button1.Attributes.Add("onclick","window.open('http://www.yahoo.com')");

┬áThis will simply open the webpage for www.yahoo.com. Sometimes you need more flexibility like supplying more attributes to the opened window. Let’s see how we can attach more attributes to the new window that opens. Write the following code in the InitializeComponent() of the page.

string str = @"window.open('http://www.yahoo.com',height=300,width=100,scrollbars=0,toolbar=1)"; this.Button1.Attributes.Add("onclick",str);

As you can see that I have placed all the attributes of the window.open into a string and simply supplied the string to the Button1.Attributes.Add method which make our code little bit cleaner.

Q2: How do I iterate through the listbox and find which items have been selected?

Sometimes we have a listbox controls with several values inside it and we want to find that which of those values are selected. Let’s see how this can be done.

foreach(ListItem l in ListBox1.Items){if(l.Selected)
{
Response.Write(l.Value);
}
}

As you can see that how simple it is. We are just iterating through the listbox items and when we find an item selected we just print out the value of that particular item.

Q3: How do I pass the values from one page to another?

Passing values from one page to another can be done is many ways hence I will show you two of the most commonly used ways. One will be using QueryString and other will be using Session variables. In both the examples we will be passing values from Page1.aspx to Page2.aspx.

Page1.aspx (QueryString):

privatevoid Button1_Click(object sender, System.EventArgs e){string myValue = "azamsharp"; // The value being passed
Response.Redirect("Page2.aspx?Name="+myValue);
}

Page2.aspx (QueryString):

if(!Page.IsPostBack){if(Request.QueryString["Name"] != null)
{
string str = Request.QueryString["Name"].ToString();
Response.Write(str);
}
}

In Page2.aspx we are first checking that is QueryString is not null and if not than printing out the value in the querystring onto the form.

Now let’s see that how we can pass the values from Page1.aspx to Page2.aspx using Session variables.

Page1.aspx (Session)

privatevoid Button2_Click(object sender, System.EventArgs e){Session["Name"] = "azam";
Response.Redirect("Page2.aspx");
}

Page2.aspx (Session):

// Checking for Session string myString = Session["Name"].ToString();Response.Write(myString);

Q4: Is that possible to not to fire validation when a button is clicked?

This is one of the most commonly asked questions on www.asp.net forums.

Q5: How can I redirect to another page after the exception has thrown?

Have you ever tried redirecting your page in the catch block. It won’t work if you don’t supply the correct parameters. Let’s say you try something like this:

try{// Do some work here and suppose the exception is thrown
}
catch(Exception ex)
{
Response.Redirect("ErrorPage.aspx");
}

You should be expecting that you will be redirected to the “ErrorPage.aspx” page but unfortunately this will throw “System.Threading” exception. If you want to redirect to the “ErrorPage.aspx” than you will have to do something like this:

try{// Do some work here and suppose the exception is thrown
}
catch(Exception ex){
Response.Redirect("ErrorPage.aspx",true);}

As you can see that I have provided the second parameter to “true” for the Response.Redirect method which terminates the execution of the current page and redirects to the new page successfully.

Q6: How can I populate one dropdownlist based on the selected of another dropdownlist?

This is one of the most frequently asked questions. Suppose you have a dropdownlist and you want that when you select any item from that dropdownlist then another dropdownlist should be populated. I have created two dropdownlist “ddlParent” and “ddlChild”. Let’s see how this be done.

First let’s populate the “ddlParent”:

if(!Page.IsPostBack) {SqlDataAdapter ad = new SqlDataAdapter("SELECT PersonID FROM Person",myConnection);
DataSet ds = new DataSet();
ad.Fill(ds,"Person");
ddlParent.DataSource = ds;
ddlParent.DataTextField = "PersonID";
ddlParent.DataValueField = "PersonID";
ddlParent.DataBind();
}

As you can see that populated the dropdownlist was not that hard. Now in the selectedIndexChanged event of the “ddlParent” you will implement the code that will fill the “ddlChild”. Let’s see how this is accomplished:

privatevoid ddlParent_SelectedIndexChanged(object sender, System.EventArgs e){int personID = Convert.ToInt32(ddlParent.SelectedItem.Value);
string query = "SELECT PhoneNumber,PhoneID FROM Phone WHERE PersonID = "+personID;
SqlDataAdapter ad = new SqlDataAdapter(query,myConnection);
DataSet ds = new DataSet();
ad.Fill(ds,"Phone");
ddlChild.DataSource = ds;
ddlChild.DataTextField = "PhoneNumber";
ddlChild.DataValueField = "PhoneID";

ddlChild.DataBind();

}

If you want to instantly fill the child dropdownlist when the item is selected from the parent than make the “IsPostBack” property of the “ddlParent” to true.
I hope you like the article happy coding.