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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

Two different datagridview rows getting selected when one should be selected

I am working with winform project. i have two datagridview which has same number of columns and same structure. frist column is checkbox column.
here is code to bind two datagrid with same kind of data.
List<AllGroupNames> grpDtl = GetAllGroups(Nodes);
List<AllGroupNames> grpDtl1 = GetAllGroups(Nodes); //grpDtl.GetRange(0, grpDtl.Count);
//bind two grid with all groups name
if (_grpDtl != null && _grpDtl.Count > 0)
{
dgSingleGroups.AutoGenerateColumns = false;
dgSingleGroups.DataSource = grpDtl;
dgSingleGroups.Columns[0].DataPropertyName = "Select";
dgSingleGroups.Columns[1].DataPropertyName = "GroupName";
dgSingleGroups.Columns[1].ReadOnly = true;
dgSingleGroups.Columns[0].Width = 47;
dgSingleGroups.Columns[1].Width = 346;
dgAllGroups.AutoGenerateColumns = false;
dgAllGroups.DataSource = grpDtl1;
dgAllGroups.Columns[0].DataPropertyName = "Select";
dgAllGroups.Columns[1].DataPropertyName = "GroupName";
dgAllGroups.Columns[1].ReadOnly = true;
dgAllGroups.Columns[0].Width = 47;
dgAllGroups.Columns[1].Width = 346;
}
grpDtl1 = null;
grpDtl = null;
_grpDtl = null;
GetAllGroups() iterate in treeview node collection and accumulate node name.
private List<AllGroupNames> GetAllGroups(TreeNodeCollection tnCollection)
{
//accumulate group name in recursive fashion
foreach (TreeNode tn in tnCollection)
{
if (tn.Tag != null)
{
if (((object)tn.Tag).GetType().ToString().Contains("TunerDetails"))
{
_grpDtl.Add(new AllGroupNames { Select = false, GroupName = tn.Text });
}
GetAllGroups(tn.Nodes);
}
}
return _grpDtl;
}
Now problem is when i check second grid checkbox then my first grid checkbox is getting checked invisibly means when i am reading first grid's first column value in loop then i am getting checkbox value true. where as i have not checked my first grid's any checkbox.
when i select any row of second grid that same row is getting automatically selected in first grid.
I just select one row from right side grid and same row of left side grid automatically gets selected....which is problem for me. Why two grid syncing automatically. screen shot attached.
why it is happening i am not able to capture the reason. please help me what to change in code to get rid of this problem.

Apparently every object of class AllGroupName has a Boolean property Select.
You created Column[0] such, that the checkbox is checked whenever this boolean is true, and vice versa: whenever the operator checks this checkbox, the corresponding AllGroupName.Select will become true.
You decided to use the same DataSource as object in two DataGridViews.
Operator checks the checkbox of row[4] in Dgv1
This means that DataSource[4].Select = true;
This means that all objects that have this DataSource will be notified that [4] has changed
Row[4] in Dgv1 and Dgv2 will be updated: checkbox is checked.
If you don't want that changes in Dgv1 are refelected in Dgv2, then you should make a copy of the datasource.
List<...> data1 = ...; // shouldn't this be an BindingList<...> ?
Dgv1.DataSource = data1;
List<...> data2 = new List<...>(data1); // clone data1
Dgv2.DataSource = data2;

How to set combobox default value?

In windows form, It has a ComboBox, Which have data binded by the DataSource.
When going to set the text property for a ComboBox.
Selected ComboBox -> Property -> Text : "--Select--".
Design page shows the given text. But when run the application the given text disappeard and the initial index value of a comboBox item appeared, Which is from the DataSource.
So i gave the ComboBox text in the Form load. I mean in the Constructor
public myform()
{
InitializeComponent();
ComboBox.Text="--Select--";
}
link revised and more. But ..
Setting default item in combo box
https://msdn.microsoft.com/en-us/library/system.windows.forms.combobox.text(v=vs.110).aspx
Searched lot of question in SO depends to ComboBox. But those never solve my case
Edited
In that combobox, Click the right top corner , From that i choosed data for my combobox by using Datasouce. I didn't write any code for add items into combobox.

You can do something like this:
public myform()
{
InitializeComponent(); // this will be called in ComboBox ComboBox = new System.Windows.Forms.ComboBox();
}
private void Form1_Load(object sender, EventArgs e)
{
// TODO: This line of code loads data into the 'myDataSet.someTable' table. You can move, or remove it, as needed.
this.myTableAdapter.Fill(this.myDataSet.someTable);
comboBox1.SelectedItem = null;
comboBox1.SelectedText = "--select--";
}

Suppose you bound your combobox to a List<Person>
List<Person> pp = new List<Person>();
pp.Add(new Person() {id = 1, name="Steve"});
pp.Add(new Person() {id = 2, name="Mark"});
pp.Add(new Person() {id = 3, name="Charles"});
cbo1.DisplayMember = "name";
cbo1.ValueMember = "id";
cbo1.DataSource = pp;
At this point you cannot set the Text property as you like, but instead you need to add an item to your list before setting the datasource
pp.Insert(0, new Person() {id=-1, name="--SELECT--"});
cbo1.DisplayMember = "name";
cbo1.ValueMember = "id";
cbo1.DataSource = pp;
cbo1.SelectedIndex = 0;
Of course this means that you need to add a checking code when you try to use the info from the combobox
if(cbo1.SelectedValue != null && Convert.ToInt32(cbo1.SelectedValue) == -1)
MessageBox.Show("Please select a person name");
else
......
The code is the same if you use a DataTable instead of a list. You need to add a fake row at the first position of the Rows collection of the datatable and set the initial index of the combobox to make things clear. The only thing you need to look at are the name of the datatable columns and which columns should contain a non null value before adding the row to the collection
In a table with three columns like ID, FirstName, LastName with ID,FirstName and LastName required you need to
DataRow row = datatable.NewRow();
row["ID"] = -1;
row["FirstName"] = "--Select--";
row["LastName"] = "FakeAddress";
dataTable.Rows.InsertAt(row, 0);

how to set the default value to the drop down list control?

I have a drop down list control on my web page. I have bind the datatable to the dropdownlist control as follows -
lstDepartment.DataTextField = "DepartmentName";
lstDepartment.DataValueField = "DepartmentID";
lstDepartment.DataSource = dtDept;
lstDepartment.DataBind();
in the page load event i want to set the default value to the drop down list control from my other table field.
how to do this?

After your DataBind():
lstDepartment.SelectedIndex = 0; //first item
or
lstDepartment.SelectedValue = "Yourvalue"
or
//add error checking, just an example, FindByValue may return null
lstDepartment.Items.FindByValue("Yourvalue").Selected = true;
or
//add error checking, just an example, FindByText may return null
lstDepartment.Items.FindByText("Yourvalue").Selected = true;

if you know the index of the item of default value,just
lstDepartment.SelectedIndex = 1;//the second item
or if you know the value you want to set, just
lstDepartment.SelectedValue = "the value you want to set";

Assuming that the DropDownList control in the other table also contains DepartmentName and DepartmentID:
lstDepartment.ClearSelection();
foreach (var item in lstDepartment.Items)
{
if (item.Value == otherDropDownList.SelectedValue)
{
item.Selected = true;
}
}

lstDepartment.DataTextField = "DepartmentName";
lstDepartment.DataValueField = "DepartmentID";
lstDepartment.DataSource = dtDept;
lstDepartment.DataBind();
'Set the initial value:
lstDepartment.SelectedValue = depID;
lstDepartment.Attributes.Remove("InitialValue");
lstDepartment.Attributes.Add("InitialValue", depID);
And in your cancel method:
lstDepartment.SelectedValue = lstDepartment.Attributes("InitialValue");
And in your update method:
lstDepartment.Attributes("InitialValue") = lstDepartment.SelectedValue;

ASP .NET - Retrieve values from selected Listview control row?

My Listview control contains 4 columns and 30 rows.
I can retrieve the row number by using:
//get row of listview item
ListViewDataItem item1 = e.Item as ListViewDataItem;
int findMe = item1.DisplayIndex;
How do I then get values from one or all 4 columns?
I was trying:
this.lblReponseRoute.Text = item1.FindControl("routenameLabel").ID.ToString();
UPDATE1:
The final solution is:
//get row of listview item
ListViewDataItem item1 = e.Item as ListViewDataItem;
int findMe = item1.DisplayIndex;
//find label value
var routeLabel = (Label)ListView1.Items[findMe].FindControl("routenameLabel");
this.lblReponseRoute.Text = routeLabel.Text;

If routenameLabel is a server control, I believe you're going to have to cast it as such prior to accessing the properties:
var routeLabel = (Label)item1.FindControl("routenameLabel");
lblResponseRoute.Text = routeLabel.ID.ToString();
Do you get an error on the code you've posted?
Edit: Note that in your real code you'd want to test for null before casting to the Label.

var routeLabel = (Label)item1.FindControl("routenameLabel");
lblResponseRoute.Text = routeLabel.ID.ToString();
It should be:
var routeLabel = (Label)item1.FindControl("routenameLabel");
lblResponseRoute.Text = routeLabel.Text.ToString();
.Text not .ID, we already know the name of the label.