Hi ASPbun ,

You could disable paging temporarily and will re-bind the grid so that now we have access to all records . Code below is for your reference :

  <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" AllowPaging="true"
     OnPageIndexChanging="grdData_PageIndexChanging" PageSize="2">
    <Columns>
        <asp:BoundField ItemStyle-Width="150px" DataField="CustomerID" HeaderText="Customer ID" />
        <asp:BoundField ItemStyle-Width="150px" DataField="ContactName" HeaderText="Contact Name" />
      
    </Columns>
</asp:GridView>
        <asp:Button ID="Button1" runat="server" Text="Button"  OnClick="Button1_Click"/>
        <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>

Code behind :

        protected void Page_Load(object sender, EventArgs e)
        {
            if (!this.IsPostBack)
            {
                this.BindGrid();
            }
        }

        public void BindGrid()
        {
            DataTable dt = new DataTable();
            dt.Columns.Add(new DataColumn("CustomerID", typeof(string)));
            dt.Columns.Add(new DataColumn("ContactName", typeof(string)));
            dt.Rows.Add("1", "1");
            dt.Rows.Add("1", "1");
            dt.Rows.Add("3", "1");
            dt.Rows.Add("2", "1");
            GridView1.DataSource = dt;
            GridView1.DataBind();
        }


        protected void grdData_PageIndexChanging(object sender, GridViewPageEventArgs e)
        {
            GridView1.PageIndex = e.NewPageIndex;
            BindGrid();
        }

        protected void Button1_Click(object sender, EventArgs e)
        {
            var a = GridView1.PageIndex;
            GridView1.AllowPaging = false;        
            BindGrid();
            GridView1.DataBind();
             var distinctRows = (from GridViewRow row in GridView1.Rows
                                select row.Cells[0].Text
                  ).Distinct().Count();
            GridView1.AllowPaging = true;
            GridView1.PageIndex = a;
            BindGrid();
            Label1.Text = distinctRows.ToString();

        }

According to CustomerID , label shows "3" .

Best Regards,

Nan Yu