How to handle a selected row in GridView on the client-side

1. Create hidden field onto your page like following:

2. Then add script block at your aspx page:

function select(rowIndex) {
        document.getElementById('<%=inp_row_index.ClientID %>').value = rowIndex;
        <%=Page.GetPostBackEventReference(inp_row_index) %>
    }

3. In RowCreated event handler add following code snipet:

if (e.Row.RowType == DataControlRowType.DataRow)
            { 
                e.Row.Attributes.Add("onclick", String.Format("select({0});",e.Row.RowIndex));
            }

4. At the end add to code-behind:

protected void Page_Load(object sender, EventArgs e)
        {
            inp_row_index.ServerChange += new EventHandler(inp_row_index_ServerChange);
        }

        void inp_row_index_ServerChange(object sender, EventArgs e)
        {
            int index = int.Parse(inp_row_index.Value);
            gvTemplates.SelectedIndex = index;
            Response.Write("Selected Index:"+index.ToString());
        }