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C# What does setting the SelectedIndex of a combobox to -1 do?

When I was working on one of my projects I was trying to write to a datasourced combobox and then write a value into the combobox like below:
//Create list for combobox
List<string> companyList= new List<string>() { "", "Company1", "Company2" };
//Datsource list to combobox
cbCompanyName.DataSource = companyList;
//If form is set to import data and the billing address is not null
if (importAddress && StaticValues.billAddress != null)
{
//Fill all fields with data from Static Values class
cbCompanyName.Text = StaticValues.billAddress.CompanyName;
cbCountry.Text = StaticValues.billAddress.Country;
}
else
{
//Set country to US
cbCountry.SelectedIndex = 0;
}
however the line cbCompanyName.Text = StaticValues.billAddress.CompanyName; ran without writing any text to the combobox, until I set the selected index of the combobox to -1. What does setting the combobox selected index to -1 do that would change this as apposed to setting the selected index to 0?

Setting the SelectedIndex on a ComboBox to -1 deselects (SelectedItem is NULL). Setting to 0 selects the first item in Items

Combobox needs to know what is my value and display member,
giving the datasource is not enough for it.
I think you can use like this or
// comboBox.DisplayMember = "Text";
// comboBox.ValueMember = "Value";
int =0;
companyList.forEach(x=> comboBox.Items.Add(new { Text = x.toString(), Value =i++ }));
comboBox1.SelectedIndex = 0;
you can look this article
similar question and answers

How to set combobox first element in winform application in c#

I am new in Winform application, here i am using Entity framework, when i binding values to a combobox from sql table, i need to set first combobox item as "Please select", how can i set this..?
var qry = context.Tbl_EmployeeDetails.Where(x => x.IsDeleted == false).ToList();
if(qry!=null)
{
drpname.ValueMember = "RecordId";
drpname.DisplayMember = "Name";
drpname.DataSource = qry;
}
how can i set this first item as "please select" and value '0'
another suggestion in winform that..
how can i set value in a datagridview linkbutton column when i set 'Edit', 'Delete' columns are as linkbutton

You can insert that item in the beginning of the list.
Code:
var items = db.Tbl_EmployeeDetails.Where(x => x.IsDeleted == false).ToList();
items.Insert(0,new Tbl_EmployeeDetail() { RecordId= 0, Name = "[Please Select an Item]" });
drpname.DropDownStyle = ComboBoxStyle.DropDownList; //optional
drpname.ValueMember = "RecordId";
drpname.DisplayMember = "Name";
drpname.DataSource = items;
drpname.SelectedIndex = 0;
Screenshot:
Note
To have a hint in ComboBox without adding an item, take a look at following post:
Set hint or watermark or default text for ComboBox without adding it as item:

After you load your var qry which will be a List<string>
you want to do the following
var qry = context.Tbl_EmployeeDetails.Where(x => x.IsDeleted == false).ToList();
if(qry!=null)
{
drpname.ValueMember = "RecordId";
drpname.DisplayMember = "Name";
drpname.DataSource = qry;
drpname.Items.Insert(0, "--Please Select--");
drpname.SelectedIndex = 0;
}
Or you could have easily added it to qry since you are returning the data ToList()
for example
var qry = context.Tbl_EmployeeDetails.Where(x => x.IsDeleted == false).ToList();
qry.Insert(0, "--Please Select--");
if(qry!=null)
{
drpname.ValueMember = "RecordId";
drpname.DisplayMember = "Name";
drpname.DataSource = qry;
drpname.SelectedIndex = 0;
}

Your combobox cannot contain additional items if you are binding it to a data source. You have a few options here:
Don't bind it to a data source, i.e. populate it manually from your data set by looping through the items in your query and adding them individually to the combobox; this will allow you the opportunity to first add your "Please Select" item. This can be complicated because you will actually have to add the string names as the items in the combobox, and then you can set the item's Tag property to the actual query row to reference it later.
Include a fake item returned from your query that has a display member of "please select". Note, I don't recommend this approach because you're allowing your presentation logic to leak into your data/business layer.
Use a third-party control instead of the win forms combobox. I've used Infragistics and they have the ability to display a "suggestion" that's not actually in the list when nothing is selected.

change selecteditem of dropdownlist after binding

i have a droplist in page that i bind this with code
Category catObj = new Category();
dropCat.DataSource = catObj.GetAllCategory();
dropCat.DataTextField = "Title";
dropCat.DataValueField = "CategoryID";
dropCat.DataBind();
i want change selected item of droplist with ths code
dropCat.SelectedIndex = Convert.ToInt32(catObj.ParentId);
but this code cant change the selected item
please help me
thank you all

I'll assume that you're trying to select by value and not by index since catObj.ParentId is likely to contain a CategoryId. In case my assuption is correct, you'll need to do this
dropCat.SelectedValue = catObj.ParentId.ToString();
DropDownList.SelectedIndex property refers to a position inside Items collection and not about the value of the item.

Setting dropdownlist selecteditem programmatically

I want to set the selecteditem attribute for an ASP.Net dropdownlist control programmatically.
So I want to pass a value to the dropdownlist control to set the selected item where the item is equal to the passed value.

Assuming the list is already data bound you can simply set the SelectedValue property on your dropdown list.
list.DataSource = GetListItems(); // <-- Get your data from somewhere.
list.DataValueField = "ValueProperty";
list.DataTextField = "TextProperty";
list.DataBind();
list.SelectedValue = myValue.ToString();
The value of the myValue variable would need to exist in the property specified within the DataValueField in your controls databinding.
UPDATE:
If the value of myValue doesn't exist as a value with the dropdown list options it will default to select the first option in the dropdown list.

ddlData.SelectedIndex will contain the int value To select the specific value into DropDown :
ddlData.SelectedIndex=ddlData.Items.IndexOf(ddlData.Items.FindByText("value"));
return type of ddlData.Items.IndexOf(ddlData.Items.FindByText("value")); is int.

Here is the code I was looking for :
DDL.SelectedIndex = DDL.Items.IndexOf(DDL.Items.FindByText("PassedValue"));
Or
DDL.SelectedIndex = DDL.Items.IndexOf(DDL.Items.FindByValue("PassedValue"));

Well if I understood correctly your question. The Solution for setting the value for a given dropdownlist will be:
dropdownlist1.Text="Your Value";
This will work only if the value is existing in the data-source of the dropdownlist.

If you need to select your list item based on an expression:
foreach (ListItem listItem in list.Items)
{
listItem.Selected = listItem.Value.Contains("some value");
}

Just Use this oneliner:
divisions.Items.FindByText("Some Text").Selected = true;
divisions.Items.FindByValue("some value").Selected = true;
where divisions is a dropdownlist control.
Hope it helps someone.

var index = ctx.Items.FirstOrDefault(item => Equals(item.Value, Settings.Default.Format_Encoding));
ctx.SelectedIndex = ctx.Items.IndexOf(index);
OR
foreach (var listItem in ctx.Items)
listItem.Selected = Equals(listItem.Value as Encoding, Settings.Default.Format_Encoding);
Should work.. especially when using extended RAD controls in which FindByText/Value doesn't even exist!

ddList.Items.FindByText("oldValue").Selected = false;
ddList.Items.FindByText("newValue").Selected = true;

On load of My Windows Form the comboBox will display the ClassName column of my DataTable as it's the DisplayMember also has its ValueMember (not visible to user) with it.
private void Form1_Load(object sender, EventArgs e)
{
this.comboBoxSubjectCName.DataSource = this.Student.TableClass;
this.comboBoxSubjectCName.DisplayMember = TableColumn.ClassName;//Column name that will be the DisplayMember
this.comboBoxSubjectCName.ValueMember = TableColumn.ClassID;//Column name that will be the ValueMember
}

Safety check to only select if an item is matched.
//try to find item in list.
ListItem oItem = DDL.Items.FindByValue("PassedValue"));
//if exists, select it.
if (oItem != null) oItem.Selected = true;

ddlemployee.DataSource = ds.Tables[0];
ddlemployee.DataTextField = "Employee Name";
ddlemployee.DataValueField = "RecId";
ddlemployee.DataBind();
ddlemployee.Items.Insert(0, "All");

Adding custom options in bound dropdown in asp.net

I have a bound dropdown list populated with a table of names through a select, and databinding. it shoots selectedindexchanged that (through a postback) updates a certain gridview.
What happens is, since it runs from changing the index, the one that always comes selected (alexander) can only me chosen if you choose another one, then choose alexander. poor alexander.
What I want is to put a blanc option at the beginning (default) and (if possible) a option as second.
I can't add this option manually, since the binding wipes whatever was in the dropdown list and puts the content of the datasource.

Set the AppendDataBoundItems property to True. Add your blank, then data bind.
ddl.AppendDataBoundItems = true;
ddl.Items.Add("Choose an item");
ddl.DataSource = foo;
ddl.DataBind();
The AppendDataBoundItems property
allows you to add items to the
ListControl object before data
binding occurs. After data binding,
the items collection contains both the
items from the data source and the
previously added items.

protected void SetAddrList()
{
TestDataClassDataContext dc = new TestDataClassDataContext();
dc.ObjectTrackingEnabled = false;
var addList = from addr in dc.Addresses
from eaddr in dc.EmployeeAddresses
where eaddr.EmployeeID == _curEmpID && addr.AddressID == eaddr.AddressID && addr.StateProvince.CountryRegionCode == "US"
select new
{
AddValue = addr.AddressID,
AddText = addr.AddressID,
};
if (addList != null)
{
ddlAddList.DataSource = addList;
ddlAddList.DataValueField = "AddValue";
ddlAddList.DataTextField = "AddText";
ddlAddList.DataBind();
}
ddlAddList.Items.Add(new ListItem("<Add Address>", "-1"));
}
I created this code example using adventure works to do a little practice with Linq and it is very similar to the previous answer. Using linq still shouldn't matter for the answer the last dddlAddList.Items.Add is what you need. "Add Address" = first selected option and -1 = the value.