//default.aspx

//First step:Take a grid view and checkbox

           <asp:GridView ID="GridView1" runat="server" Height="131px" Width="500px" DataKeyNames="Id"

            AutoGenerateColumns="False" Style="margin-top: 2px; margin-left: 0px; margin-bottom: 1px;">

            <Columns>

                <asp:TemplateField HeaderText="Action">

                    <ItemTemplate>

                        <asp:CheckBox ID="chkRow" runat="server" />

                    </ItemTemplate>

                </asp:TemplateField>

            </columns>

//default.cs

//second step:take a button for select all checkbox

             //double click on button and write code

protected void Button2_Click(object sender, EventArgs e)

    {

        CheckState(b);//this is function 

    }

//make global variable

  bool b=true;

//make a function

 private void CheckState(bool p)

    {

        foreach (GridViewRow row in GridView1.Rows)

        {

            CheckBox chkcheck = (CheckBox)row.FindControl("chkRow");

            chkcheck.Checked = p;

        }

       

    }