Won't add a default custom row on combobox - vb.net

Question

been having an issue displaying an empty item as default selected item on a combobox. This combobox is a filter for my gridview and currently my datagridview displays all data based on the first item of the combobox but what i wanted is to have an empty first combobox item so that datagridview will pull all data from database. When i run the app it says
System.ArgumentException: 'Input string was not in a correct
format.Couldn't store <> in ailment_id Column. Expected type is
Int32.'
My code to populate combobox from database:
Private Sub populateComboAilment()
data_adapter = New MySqlDataAdapter("SELECT * FROM ailment", myconnection.open())
Dim data_table As New DataTable
data_adapter.Fill(data_table)
'assign default value
Dim row As datarow = data_table.NewRow()
row(0) = ""
data_table.Rows.InsertAt(row, 0)
comboAilment.DataSource = data_table
With comboAilment
.DisplayMember = "name"
.ValueMember = "ailment_id"
End With
myconnection.close()
End Sub
Update: All is working now when I changed combobox ValueMember to a
name since it's an id before. but when I select another item it
displays nothing on the gridview because my datagridview query doesn't
know the name, only id since it's a foreign key.
Any help would be greatly appreciated!

Answer 1

Ah, I just looked more carefully at the code and you're not actually assigning a String containing "<>" but rather an empty String. It doesn't really matter though. A String is a String and you can't put one where a number is required. You might be able to use a String that contained digits because that could be implicitly converted to a number but a empty String doesn't qualify.
If you want an empty field in a DataRow then you need to do what ADO.NET does to represent a database null, i.e. use DBNull.Value. That means an object of type DBNull, which is required because ADO.NET predates nullable value types. You can't just use Nothing because would work for reference types but not value types, e.g. if you assign Nothing to an Integer variable you are effectively setting it to zero.
In short, you need to change this:
Dim row As datarow = data_table.NewRow()
row(0) = DBNull.Value
row(1) = DBNull.Value
data_table.Rows.InsertAt(row, 0)
In the case of the name column, its data type is String so you could use an empty String there (preferably String.Empty rather than "" but either works) but null is more appropriate because it is actually no value, rather than a value containing no characters.