how to handle nulls in a RadioButtonList

My webform RadioButtonList is bound to a column from a Sql Server dataSource. The radioButtonList is in a formView which starts up in edit mode. My question is how to handle null values because the only way I could get the page to come up when the columnn value is null is to have an item in the radio list which is setup for nulls such as, "Unknown" in the Text property and -1 in the value property. I understand that the RadioButtonList control is supposed to default to nulls which means that nothing in the radio list is selected - this is the behavior that I want, but I get the error below complaining that null (-1) is not valid because it does not exist as a valid value in the radio list: I have tried -1, '', etc. with no success.

'RadioButtonList2' has a SelectedValue which is invalid because it does not exist in the list of items.
Parameter name: value

How should I handle nulls at page startup?

Thanks.

While setting SelectedValue = null will normally allow no default selection for the RadioButtonList control, trying to set it to null in binding (using either Eval or Bind) doesn't work and throws an exception. Going through a method, however, seems to work as follows:

[ASPX]

<ItemTemplate>

<asp:RadioButtonListID="RadioButtonList1"runat="server"SelectedValue='<%# myFunc(Eval("NumValue").ToString()) %>'>

<asp:ListItemValue="1"Text="Item 001"></asp:ListItem>

<asp:ListItemValue="2"Text="Item 002"></asp:ListItem>

<asp:ListItemValue="3"Text="Item 003"></asp:ListItem>

<asp:ListItemValue="4"Text="Item 004"></asp:ListItem>

<asp:ListItemValue="5"Text="Item 005"></asp:ListItem>

</asp:RadioButtonList>

</ItemTemplate>

[code-behind: C#]

publicstring myFunc(string val)

{

if (val =="")

returnnull;

else

return val;

}

Thank you, the myFunc worked great for handling nulls, But it caused another problem with the update statement. My update statement no longer works because it has lost the identity of the bound variable somehow.

The default SelectedValue works fine:

aspx:

SelectedValue='<%# Bind("myField") %>'

UpdateCommand="update myTable setmyField=@.myField

But with your custom SelectedValue code I get the error at the bottom:

SelectedValue='<%# myFunc(Eval("r1").ToString()) %>'

UpdateCommand="update myTable setmyField=@.myField

here is the error:

Must declare the variable'@.myField'.

[SqlException (0x80131904): Must declare the variable'@.myField'.]

The customization to SelectedValue caused this, any idea why?

Thanks.

I can see why this would be an issue when trying to update since it deviates from the binding expected by the framework. Interesting situation you have here. I'll have to look into this before giving you an answer. I wouldn't be surprised if someone already faced such situation and resolved it.
Thanks. I am appreciative of your help.
I tried to see if there's a workaround for this issue, but I'm afraid there isn't as far as I can see. I thought that there might be a way to bind a NULL value into SelectedValue property (which can be done if a direct null value is assigned), but when binding to a table column, it rejects it for some reason. This problem/behavior may be related to the fact that RadioButtonList isn't meant to provide an option for "no selection" (i.e. once a selection is made, you cannot go back to a no-selection state), although the starting point allows no selection state. Perhaps the DropDownList control serves your purpose better since you can have a default value that represents no selection. Another option might be to add another item for your RadioButtonList that represents no selection and default what would have been null value to that choice. Sorry I don't have a better news for you.

Here's another way to handle this. Create a user control with your RadioButtonList in it, and create a property in it called SelectedValue such as following:

public string SelectedValue
{
get
{
return RadioButtonList1.SelectedValue;
}
set
{
if (value != "")
RadioButtonList1.SelectedVAlue = value;
}
}

This way you conditionally avoid setting the RadioButtonList value when it's null. Then you can happily use the SelectedValue='<%# Bind("FieldName") %>' syntax in your user control tag declaration.

How to Handle Null Value from <%# Bind(...

I am trying to retrieve the values for a selected row in a FormView to allow editing. One of the fields ("photo") can be null and when the following statement gets executed it throws an error that it cannot convert a null to an integer.

<gs:PhotopickerID="Photopicker1"runat="server"ImageId='<%# Bind("photo") %>'/>

I recall that in 1.1 I could replace the olde EVAL with a function that could check for a null and return a zero instead? Can that be done here, and if so, what is the syntax?

Hi Wayne,

Bind is a two-way data operation -- is that what you are trying to use with this?

Otherwise, you could just do an <%# Eval( CallSomeFunction("photo")) %> and define the "CallSomeFunction" (or another name) on your page or control to convert it to an appropriate value.

Alternatively, you could define ImageId on the control to accept null and handle the conversion there.

Hope this helps,

Scott

I am actually trying to modify the Club Starter Kit so most of the original code is from that kit. I did figure out that Bind was a 2 way process and I believe that the way that is written, the 2 way binding is necessary. I guess I may have to re-code that whole section?

Could you expand on defining Imageid on the control? I am not familiar with that approach.

Assuming you are talking about the control defined in photos_formpicker.ascx:

The easiest fix is to add a new property to the control for the ImageID, and have it take object rather than string. You could change the type on the existing property, but you'd then also have to change the code throughout the control. This way you just need to change the property name used to bind to the database from ImageID to PhotoImageID:

constint DEFAULTIMAGEID = 0;

privateint ImageId

{

get

{

EnsureChildControls();

if (viewswitch.ActiveViewIndex == 1)

{

returnConvert.ToInt32(selectedimage.Value);

}

else

{

object id = ViewState["ImageID"];

if (id ==null)

{

ViewState["ImageID"] = DEFAULTIMAGEID;

return DEFAULTIMAGEID;

}

else

{

returnConvert.ToInt32(id);

}

}

}

set { ViewState["ImageID"] =value; }

}

publicobject PhotoImageID

{

get {return ImageId; }

set { ViewState["ImageID"] = (value ==DBNull.Value) ? DEFAULTIMAGEID :Convert.ToInt32(value); }

}

Thank you very much. That is all a little beyond my current understanding so I'll have to do a bit of reading to make sure I impliment it correctly.

I appreciate the very complete response.

How to handle events for controls within templates?

I have a FormView control, containing an EditItemTemplate with a dropdown listbox. I want to handle the SelectedIndexChanged event so I can hide or show part of the form...

If I double-click the dropdown to create the event handler, I get the function dropdown1_SelectedIndexChanged()... but without " handles Dropdown1.SelectedIndexChanged"

Obviously the dropdown1 control is inside the EditItemTemplate - but it still generates events? how does one get these?

This is getting frustrating: every time I try to design stuff logically in a componentised way, I get kicked in the teeth with a "gotcha". It was easier using classic ASP and just throwing the html out using Response.Write!!

Hello.

I think that you're using vb.net, right? in c# I don't see any trouble with this. Doesn't something like this work in vb.net:

<asp:DropDownListrunat="server"id="T"OnSelectedIndexChanged="p"AutoPostBack="true">

<asp:ListItemText="1"/>

<asp:ListItemText="2"/>

</asp:DropDownList>

then just define the p method without using the handles (but with the normal parameter list).

Ah, many many hanks: I didn't realise thats how it worked (instead of event-based handling). It so frustrated me last night i turned off the PC and polished off a bottle of wine.

That part works at last!

Now all I need to do is figure out how to control my datasource and formview to tell them to open in edit or insert mode by default (and not select).

How to handle cancel command on a formview inside of a datalist?

I have no problem using the findcontrol method to work with the formview, but I want to hide the formview when the cancel button is pressed. I am new and at a loss... been looking everywhereI can't tell if the Cancel button is on your FormView or on your DataList, but either way, you'd do the same thing. When you handle the Cancel button (you have to do it manually in DataList, in FormView you can handle the OnModeChanging event and see if CancelingEdit in the event args is true), use FindControl to find the FormView and set FormView.Visible = false.
Hope this helps...