Convertemptystringtonull not working in gridview - asp.net

Question

I'm trying to convert an empty string to null when editing a table in my gridview. I've tried several iterations and methods with no success. I'm suspecting that the issues is in my ObjectDataSource but don't know the correct syntax I need.
<asp:GridView
ID="grdMyDistributors"
DataSourceID = "srcGetMyDistributors"
CssClass="GridViewStyle"
DataKeyNames = "ID_Distributor"
AutoGenerateColumns = "false" AutoGenerateEditButton = "true"
GridLines="None"
runat="server" >
<Columns>
<asp:BoundField
DataField="DistributorName"
HeaderText="Distributor"
convertemptystringtonull="true"
ReadOnly = "True" />
<asp:BoundField
DataField="Distributor_Email_1"
HeaderText="Distributor Email 1"
convertemptystringtonull="true" >
<ItemStyle BackColor="#f6f17c"/>
</asp:BoundField>
<asp:BoundField
DataField="Distributor_Email_2"
HeaderText="Distributor Email 2"
convertemptystringtonull="true" >
<ItemStyle BackColor="#f6f17c"/>
</asp:BoundField>
<asp:BoundField
DataField="Minimum_Purchase_Amount"
HeaderText="Minimum Purchase Amount"
dataformatstring="{0:c}"
convertemptystringtonull="true" >
<ItemStyle BackColor="#f6f17c" />
</asp:BoundField>
</Columns>
</asp:GridView>
Here is my ObjectDataSource:
<asp:ObjectDataSource
ID="srcGetMyDistributors"
TypeName = "AAA.Distributors"
SelectMethod = "GetDistributorsPagingFiltered"
UpdateMethod = "UpdateDistributors"
EnableCaching="false"
ConvertNulltoDBNull = "true"
runat="server" >
<SelectParameters>
<asp:ProfileParameter Name="ID_Account" PropertyName="ID_Account" />
</SelectParameters>
<UpdateParameters>
<asp:ControlParameter ControlID="grdMyDistributors" Name="ID_Distributor" PropertyName="SelectedValue" Type="Int32" />
</UpdateParameters>
</asp:ObjectDataSource>
Here is my update method
Public Shared Sub UpdateDistributors(ByVal ID_Distributor As Integer, ByVal Distributor_Email_1 As String, ByVal Distributor_Email_2 As String, ByVal Minimum_Purchase_Amount As Decimal)
' Initialize command
Dim con As New SqlConnection(_connectionString)
Dim cmd As New SqlCommand("UpdateDistributors", con)
cmd.CommandType = CommandType.StoredProcedure
cmd.Connection = con
' Initialize parameters
cmd.Parameters.AddWithValue("#ID_Distributor", A2AID_Restaurant_To_Distributor)
cmd.Parameters.AddWithValue("#Distributor_Email_1", Distributor_Email_1)
cmd.Parameters.AddWithValue("#Distributor_Email_2", Distributor_Email_2)
cmd.Parameters.AddWithValue("#Minimum_Purchase_Amount", Minimum_Purchase_Amount)
' Execute command
Using (con)
con.Open()
cmd.ExecuteNonQuery()
End Using
End Sub
When I run the code as is, I get an error saying "Object of type 'System.DBNull' cannot be converted to type 'System.String'." It's most likely erroring when it's trying to send to the Update method and expecting a string. If I take out "ConvertNulltoDBNull = "true" from the ObjectDataSource it complains that the SQL statement is expecting a value (meaning it's not sending the Null). I've tried so many variations of this and can't find any info in the class library. Any help would be greatly appreciated!
Thanks in advance!

Answer 1

use eval() for your gridview Columns
<%# Eval("DistributorName") == DBNull.Value ? "" : value.ToString()%>
(its in C#)
<asp:GridView
ID="grdMyDistributors"
DataSourceID = "srcGetMyDistributors"
CssClass="GridViewStyle"
DataKeyNames = "ID_Distributor"
AutoGenerateColumns = "false"
AutoGenerateEditButton = "true"
GridLines="None"
runat="server" >
<Columns>
<asp:TemplateField HeaderText="Seniority">
<ItemTemplate>
<%# Eval("DistributorName") == DBNull.Value ? "" :Eval("DistributorName").ToString()%>
</ItemTemplate>
</asp:TemplateField>
</Columns>