值不能为null参数名称：在ASP.NET中进行数据绑定时的键

Question

I am triing to bind the data to a listbox from sql server then got the error "Value cannot be null.Parameter name: key"

ddlCountry = new Obout.ComboBox.ComboBox();
ddlCountry.Width = 200;
ddlCountry.Height = 200;
ddlCountry.DataTextField = "Country";
ddlCountry.DataValueField = "Country";

sqlCommand = "SELECT [Country] FROM [tbl_LookupCountry] where [Country] IS NOT NULL";
SqlConnection sqlConCountry = new SqlConnection(connectString);
SqlCommand sqlCommCountry = new SqlCommand();
sqlCommCountry.Connection = sqlConCountry;
sqlCommCountry.CommandType = System.Data.CommandType.Text;
sqlCommCountry.CommandText = sqlCommand;
sqlCommCountry.CommandTimeout = 300;
sqlConCountry.Open();
reader = sqlCommCountry.ExecuteReader();
ddlCountry.DataSource = reader;
ddlCountry.DataBind();
sqlConCountry.Close();

Does anyone meet this problem before?

Answer 1

Ideally you should return the columns similar to "Id" and the "Name" of the country. The Id would be the value for the dropdown and the Name would be the text which the user would see in the dropdown.

Make sure you have the DataTextField and DataValueField of the dropdownlist assigned the "Id" and "Name" columns respectively for teh above example.

EDIT :- Pretty odd though, If this still does not work, could you try setting the SelectedValue of the dropdown to null just before the databind?

The other option to try to move the databinding code out of the If(!IsPostBack). This is odd again.