Here is the our aspx page that contains a JavaScript method and a hidden filed and button.
<script type="text/javascript">
  function SetValue() {
    var sValue = "Hello World";
    document.getElementById("Hidden1").value = str;
  }
</script>
<html>
  <head>
    <title>How to set c# variable value in javascript</title>
  </head>
  <body>
    <div>
      <asp:hiddenfield id="hdnVlaue" runat="server"></asp:hiddenfield>
      <asp:button id="btnClick" runat="server" onclientclick="SetValue()" onclick="&rdquo;" text="Click Me">         </asp:button>
    </div>
   </body>
</html>

Now in the click event of the btnClick, we will set value received by JavaScript function

protected void btnClick_Click(object sender, EventArgs e)
{
  string sValue = hdnVlaue.Value;
  Response.Write("The Received Value is " + sValue);


}
using System;
using System.Net.Mail;
using System.Net.Security;
public partial class SendEmailPage : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        MailMessage oMailMessage = new MailMessage("Your Email Address ", "Recepent’s Email Address ");
        oMailMessage.Subject = "Write Your Mail Here";
        oMailMessage.Body = "Write Your Email Message";
        oMailMessage.IsBodyHtml = false;
        SmtpClient oSmtpClient = new SmtpClient("smtp.gmail.com", 587);
        oSmtpClient.Credentials = new System.Net.NetworkCredential("Email Username", "Email Password");
        oSmtpClient.EnableSsl = true;
        oSmtpClient.Send(oMailMessage);
    }
}


using System;
using System.Web;
using System.Web.UI;

public partial class GetDeviceType : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
    }
    protected override void OnInit(EventArgs e)
    {
        string sUserAgent = Request.UserAgent.ToLower();
        if (sUserAgent.Contains("tablet"))
        {
            this.MasterPageFile = "/Tablet.master";
        }
        else
        {
            this.MasterPageFile = "/Mobile.master";
        }
    }

}
using System;
using System.Web;
using System.Web.UI;
using System.Web.UI.HtmlControls;

public partial class AddMetaTageProgrammatically : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        // Adding Title
        Page.Title = "How to add meta tags programmatically.";

        //Adding Meta Descriptions
        HtmlMeta oHtmlMetaDescription = new HtmlMeta();
        oHtmlMetaDescription.Name = "description";
        oHtmlMetaDescription.Content = "How to add meta tags programmatically in asp.net with c#.";
        Page.Header.Controls.Add(oHtmlMetaDescription);
        oHtmlMetaDescription = null;

        //Adding Meta Keywords
        HtmlMeta oHtmlMetaKeywords = new HtmlMeta();
        oHtmlMetaKeywords.Name = "keywords";
        oHtmlMetaKeywords.Content = "meta tags, adding meta tags programmatically, meta tags in asp.net";
        Page.Header.Controls.Add(oHtmlMetaKeywords);
        oHtmlMetaKeywords = null;
    }
}


SqlCommand cmd;
        SqlConnection con;
        SqlDataAdapter da;
        DataSet ds;

        private void button1_Click(object sender, EventArgs e)
        {
            con = new SqlConnection("your_database_connection_sting");
            cmd =new SqlCommand ("select * from emp", con);
            da = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            da.Fill(ds);

            for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
            {
                comboBox1.Items.Add(ds.Tables[0].Rows[i][0] + " " + ds.Tables[0].Rows[i][1] + " " + ds.Tables[0].Rows[i][2]);  
            }
        }