gridview SelectedDataKey - do I use this right?

I try to get the Social security number (SSN) of the selected element from a gridview when the user selects a row.

                <asp:GridView ID="PeopleGrid" runat="server" AutoGenerateColumns="False" CellPadding="4"
                ForeColor="#333333" GridLines="None" 
                onselectedindexchanged="GetDataForPerson" DataKeyNames="SSN">
                <AlternatingRowStyle BackColor="White" />

               <Columns>
                <asp:CommandField ShowSelectButton="True" ButtonType="Button" SelectText="Select" />
                <asp:BoundField HeaderText="NameHeader" DataField="NameProp" />
                <asp:BoundField HeaderText="AddressHeader" DataField="PersAddressProp" />
               </Columns>

protected void GetDataForPerson(object sender, EventArgs e)
{
    va开发者_JS百科r x= PeopleGrid.SelectedDataKey.Value.ToString()  

   }

x should be the SSN of the selected person, but PeopleGrid.SelectedDataKey is null. What is wrong here?

This is how I bind the data:

PeopleGrid.DataSource = PeopleCollection;
            PeopleGrid.DataBind();

PeopleCollection is a collection of People. Class people contains SSN, NameProp, PersAddressProp

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if you want to find selected row "SSN" value then you need to use Gridview's DataKeys property, like GridView1.DataKeys["your_SelectedDataKey_index"]["your_selected_row_index"]. This is a read only property.