execute code when select is clicked in GridView - asp.net

Question

I have a GridView and a select link in the GrdiView, when the item is selected I want it to read the contents to a textbox below the GridView. The only way I can think is by accessing the behind code for the onClick function of the select link. The problem is I am unsure how to do this.
UPDATED jams method:
C# code
protected void GridView1_SelectedIndexChanging(object sender, GridViewSelectEventArgs e)
{
GridView1.SelectedIndex = e.NewSelectedIndex;
TextBox1.Text = GridView1.Rows[e.NewSelectedIndex].Cells[0].Text;
}
front code
<asp:GridView ID="GridView1" OnSelectedIndexChanging="GridView1_SelectedIndexChanging" runat="server" AutoGenerateColumns="False" CellPadding="4" ForeColor="#333333"
GridLines="None">
UPDATE my method:
protected void GridView1_SelectedIndexChanging(object sender, GridViewSelectEventArgs e)
{
int i = GridView1.SelectedIndex;
ds.Tables["Comments"].Rows[i]["Comment"] = TextBox1.Text;
}
also tried this but I get an error -> System.IndexOutOfRangeException: There is no row at position -1.
Thanks.

Answer 1

You need to attach OnSelectedIndexChanged event with your gridview
<asp:GridView OnSelectedIndexChanged="GridView1_SelectedIndexChanged" />
Then in code behind you could do:
void GridView1_SelectedIndexChanged(Object sender, EventArgs e)
{
GridViewRow row = CustomersGridView.SelectedRow;// you will get the selected row
someLabel.Text = row.Cells[0].Text;
}

Answer 2

Once you attach to the selectedindexchanged event you need to also set up some datakeys so you can retrieve values from your grid. See sample here.
void CustomersGridView_SelectedIndexChanged(Object sender, EventArgs e)
{
// Determine the index of the selected row.
int index = CustomersGridView.SelectedIndex;
// Display the primary key value of the selected row.
Message.Text = "The primary key value of the selected row is " +
CustomersGridView.DataKeys[index].Value.ToString() + ".";
}

Answer 3

it is better to use oORowCommand event of the grid view and pass the command argument and commandName to the same event as a attribute value. Then i think it will work.