ASP.NET

Re: Instansiating a List(of Double) to nothing or no value

2-Oct-07 3:55

Hi,
I have a problem with List(of Double). below is just an example what happens in List(of Double).

Example:

''Say i have a arraylist filled with data

Dim arrlst as new Arraylist
arrlst.add(10.50)
arrlst.add(Nothing)
arrlst.add(0.0)

''Now I am creating a generic type list with data in above arraylist by looping

Dim lst as new List(of Double)

For i as integer = 0 to arrlst.count-1
lst.add(arrlst(i))
Next

Finally when I checked the data in the List
lst(0) = 10.50
lst(1) = 0.0 ''Here there should be Nothing or novalue, Instead it was 0.0
lst(2) = 0.0

The second value lst(1) should be nothing, Intead it becomes 0.0 while Instantiating List object. Then whats the point in using this object for generic types.

Please help me out, I am in a situation to use only List or collection(Generic type) because of our companies new standard.

I would appreciate if you give me a example.

Thanks in advance
Abdul

kubben2-Oct-07 5:24

Re: Instansiating a List(of Double) to nothing or no value

2-Oct-07 5:24

Your issue is with the double type. It can not be null. If you switch this to a nullable type then you will get more of the results you are expecting.

Hope that helps.
Ben

Md Arif2-Oct-07 5:39

Re: Instansiating a List(of Double) to nothing or no value

2-Oct-07 5:39

Please let me know. how to switch this to a nullable type.

It would appreciable, if you give a small example.

Thanks
Abdul

kubben2-Oct-07 6:26

Re: Instansiating a List(of Double) to nothing or no value

2-Oct-07 6:26

I believe the correct syntax is:
List(Nullable(Of Double))

You should probably do some reading on nullable types before you go into this. There are plenty of articles out there.

Ben

Md Arif2-Oct-07 8:08

Re: Instansiating a List(of Double) to nothing or no value

2-Oct-07 8:08

As per your reply, I have used List(of nullable(of double)replacing arraylist. The reason I am asking about collection is a different case, because Chart FX datasource allow either arraylist or collection. if you give me declaration (example) for "collection" object of generic type that would be very halp full.

Thanks
Abdul

Guffa2-Oct-07 7:00

Guffa

2-Oct-07 7:00

You should use Option Strict On, then the compiler would tell you what the problem is. Actually, that should be part of the company standard, rather than restricting the classes to use.

Compiling the line

lst.add(arrlst(i))

with Option Strict On will give you the error message that implicit conversion from Object to Double is not allowed. With Option Strict Off, the compiler will turn the line into:

lst.add(Convert.ToDouble(arrlst(i)))

which explains why it turns your null value (Nothing) into a zero. Calling Convert.ToDouble with a null reference does actually return zero.

---
single minded; short sighted; long gone;

Re: Instansiating a List(of Double) to nothing or no value

Md Arif2-Oct-07 7:30

2-Oct-07 7:30

Thanks for your timely explanation. In another case I am binding arraylist as chart fx datasource, As i have to remove all arraylist from my application(as per my company new standards). the only choice I have is binding a collection of generic type like double, string.

So please give a small example or declaration part for building a collection of generic type.

Thanks in advance.
Abdul

Help with UpdatePanel...

Dio222-Oct-07 2:38

Dio22

2-Oct-07 2:38

I'm trying to update a drop down list (DropDownList2) after selecting an item from another drop down list (DropDownList1). Both are set to autopostback. I've set the update panel to both conditional and always with the same result. Nothing happens. However, it works without the update panel. So I suppose that is the source of my problem..
Please help me. Smile | :)

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
    //open database connection..
    SqlConnection QADatabase = new SqlConnection();
    QADatabase.ConnectionString = SqlDataSource1.ConnectionString;
    QADatabase.Open();

    //find the records I need to put in the second DropDownList.
    SqlCommand command = new SqlCommand("SELECT Machines FROM Machines WHERE DeptID=" + DropDownList1.SelectedIndex ,QADatabase);

    SqlDataReader reader = command.ExecuteReader();
    while (reader.Read())
    {
        //Add the items to DropDownList..
        DropDownList2.Items.Add((string)reader["Machines"]);
        //Refresh panel..
        UpdatePanel1.Update();
    }
    QADatabase.Close();
}

kubben2-Oct-07 3:25

2-Oct-07 3:25

Clearly there are a couple of things wrong with your code. First you don't clear out the dropdownlist2 before the while statement. If you don't do this you will continue to add new items to your dropdown everytime. Second are you sure you select command is returning any rows. I am guessing it is not since you are setting the deptID = to the selectedIndex. I think you would want to set it to the SelectedValue, assuming you set up this dropdown with the selected value containing the deptid.

Hope that helps.
Ben

Dio222-Oct-07 3:37

Dio22

2-Oct-07 3:37

The SQL statement returns all Machines in the database with deptID = to the selected index. This is what I want, and I've tested by just putting the data into a string while debugging and it returns the correct values. It returns the correct values when not using the UpdatePanel. It just reloads the entire page. I just haven't implemented the UpdatePanel right.. (P.s I added code to clear the list, thanks for noticing that.)

kubben2-Oct-07 3:45

2-Oct-07 3:45

Reading up on updatepanels, it appears that you don't have to call the update method. Check out this example:
http://www.asp.net/ajax/documentation/live/tutorials/IntroductionUpdatePanel.aspx[^]

Hope that helps.
Ben

Dio222-Oct-07 4:44

Dio22

2-Oct-07 4:44

Thanks Ben. I wasn't adding all of my controls and my data source to the update panel. Just the output control. I added the rest of the related controls and my data source and it updated correctly. Smile | :)

kubben2-Oct-07 5:22