虽然DataTextField已检索，但DataValueField未检索值

Question

while I'm trying to use the DataValueField of my dropdownlist as an input int parameter in a method by using parse method int.parse(DataValueField) as its value should come from the database it gives me this runtime error

Input string was not in a correct format

so I debugged the website to see where is the problem I found that it didn't retrieved its value int its SelectedIndexChanged method ,although the dropdownlist was filled in the Page_Load method
here is the page_load method :

protected void Page_Load(object sender, EventArgs e)
{
writerddl.DataSource = DS.show_all_writers();
writerddl.DataValueField = "writerid";
writerddl.DataTextField = "writersname:;
writerddl.DataBind(); }  

and this is the SelectedIndexChanged method:

protected void writerddl_SelectedIndexChanged(object sender, EventArgs e)
{
writer _writer = DS.select_writer_by_id(int.Parse(writerddl.DataValueField)).Single();
usernametxt.Text = _writer.username;
passwordtxt.Text = _writer.password;
nametxt.Text = _writer.writersname;
}

Note that I'm using LinQtoSql ,so DS."method"() is a stored procedure

Answer 1

Replace this:

writer _writer = DS.select_writer_by_id(int.Parse(writerddl.DataValueField)).Single();

with:

int i = int.Parse(writerddl.SelectedItem.Value);
writer _writer = DS.select_writer_by_id(i).Single();

Answer 2

In msdn DataValueField

Use this property to specify the field that contains the value of each item in a list control.

So its simply property name, not value. For getting selected value you need use

1. SelectedValue
2. SelectedItem

Answer 3

To select the selected value from dropdown list, you should use writerddl.SelectedValue instead off writerddl.DataValueField

Thanks Parvati