[c#] 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.

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);