ViewState for private string with button - asp.net

Question

I have gridview with a Rowcommand event that let's me get the column ActivityID cell value and puts it in a private string called ActivityIDString.
private string ActivityIDString; // To be used in btUpdate
protected void gwActivity_RowCommand(object sender, GridViewCommandEventArgs e)
{
if (e.CommandName == "Page") return;
GridViewRow row = ((e.CommandSource as Control).NamingContainer as GridViewRow);
string ActivityID = row.Cells[1].Text;
ActivityIDString = ActivityID;
}
This all works when the row is selected however when i press my "btUpdate" button the ActivityIDString becomes NULL.
protected void btUpdate_Click(object sender, EventArgs e)
{
string CS = ConfigurationManager.ConnectionStrings["DBCS"].ConnectionString;
using (SqlConnection con = new SqlConnection(CS))
{
SqlCommand cmd = new SqlCommand("sp_tblActivityUpdate", con);
cmd.CommandType = System.Data.CommandType.StoredProcedure;
cmd.Parameters.AddWithValue("#ActivityID", ActivityIDString);
con.Open();
cmd.ExecuteNonQuery();
BindGridviewActivity();
}
}
I understand the issue and why it becomes NULL but I don't know exactly how to solve this issue and where to use the ViewState.
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
ViewState["ActivityIDStringText"] = "ActivityIDString"; // <-- SHOULD I CREATE THIS?????
}
}

Answer 1

this method should be something like this
protected void gwActivity_RowCommand(object sender, GridViewCommandEventArgs e)
{
if (e.CommandName == "Page") return;
GridViewRow row = ((e.CommandSource as Control).NamingContainer as GridViewRow);
string ActivityID = row.Cells[1].Text;
ViewState["ActivityIDString"] = ActivityID;
}
then retrieve it in button method
protected void btUpdate_Click(object sender, EventArgs e)
{
string CS = ConfigurationManager.ConnectionStrings["DBCS"].ConnectionString;
string ActivityIDString= Convert.ToString(ViewState["ActivityIDString"]);
using (SqlConnection con = new SqlConnection(CS))
{
SqlCommand cmd = new SqlCommand("sp_tblActivityUpdate", con);
cmd.CommandType = System.Data.CommandType.StoredProcedure;
cmd.Parameters.AddWithValue("#ActivityID", ActivityIDString);
con.Open();
cmd.ExecuteNonQuery();
BindGridviewActivity();
}
}

Answer 2

You can use Session here. Whenever a row is selected add the ID in the session and retrieve the value of the session object in the btUpdate click event and then null the session .
Inside the gwActivity_RowCommand()
Session["ActivityId"] += ";ActivityId";
and inside btUpdate_Click()
if(Session["ActivityId"]!=null){
var ActivityIds = Session["ActivityId"].Split(';'); // For Mulitple Ids if Required
Session["ActivityId"] = null;
}