How to use local variable of Page_OnLoad method in OnClick event in asp.net C#

I am designing a website in which I need to update a table Company in my database through CompanyDetails page with respect to the auto increment field CompanyID which is being passed through Query string from previous page named Company and only one button is there for insert and update. So my problem is I am unable to get the value of Companyid of Page_OnLoad event in SaveButtonClick event.

Note: I have already tried Session and View state, IsPostBack but in Onclick event even their value are not being maintained and are updated to 0 or null.

Here is my code......(Please ignore my coding mistakes)

using System;
using System.Web.UI;
using System.Data;
using System.Data.SqlClient;

public partial class CompanyDetails : System.Web.UI.Page
{
    int Companyid = 0;
    string cmdName = null;
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            Companyid = Convert.ToInt32(Request.QueryString["CompanyID"]);
            cmdName = Request.QueryString["CommandType"];
            Session["something"] = Companyid;
        }
            if (cmdName == "Details")
            {
                BindTextBoxvalues();
            }
    }

    protected void SaveButton_Click(object sender, EventArgs e)
    {
        string x = Session["something"].ToString();
        try
        {
            if (SaveButton.Text == "Save")
            {
                SqlCommand cmd = new SqlCommand();
                String mycon = "Data Source=.; Initial Catalog=something; Integrated Security=True";
                SqlConnection con = new SqlConnection(mycon);
                cmd = new SqlCommand("spInsertCompany", con);
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.Parameters.Add("@CompanyName", SqlDbType.VarChar).Value = Name.Text;
                cmd.Parameters.Add("@CompanyCode", SqlDbType.VarChar).Value = CompanyCode.Text;
                cmd.Parameters.Add("@LegalName", SqlDbType.VarChar).Value = LegalName.Text;
                cmd.Parameters.Add("@TaxID", SqlDbType.Int).Value = TaxID.Text;
                cmd.Parameters.Add("@BusinessPhone", SqlDbType.VarChar).Value = BusinessPhone.Text;
                cmd.Parameters.Add("@Extension", SqlDbType.VarChar).Value = Extension.Text;
                cmd.Parameters.Add("@FaxNumber", SqlDbType.VarChar).Value = FaxNumber.Text;
                cmd.Parameters.Add("@Description", SqlDbType.VarChar).Value = Description.Value;
                bool isstatus = IsActiveCheckBox.Checked;
                cmd.Parameters.Add("@Status", SqlDbType.Int).Value = Convert.ToInt32(isstatus);
                con.Open();
                cmd.Connection = con;
                cmd.ExecuteNonQuery();
                Response.Write("<script language='javascript'>window.alert('Saved Successfully.');window.location='Company.aspx';</script>");
            }
            else if (SaveButton.Text == "Update")
            {
                SqlCommand cmd = new SqlCommand();
                String mycon = "Data Source=.; Initial Catalog=something; Integrated Security=True";
                SqlConnection con = new SqlConnection(mycon);
                con.Open();
                cmd = new SqlCommand("spUpdateCompany", con);
                cmd.CommandType = CommandType.StoredProcedure;
                int a = Convert.ToInt32(Companyid);
              // I need the value here but it is being updated to zero here.
                cmd.Parameters.Add("@CompanyID", SqlDbType.Int).Value = Companyid;
                cmd.Parameters.Add("@CompanyName", SqlDbType.VarChar).Value = Name.Text;
                cmd.Parameters.Add("@CompanyCode", SqlDbType.VarChar).Value = CompanyCode.Text;
                cmd.Parameters.Add("@BusinessPhone", SqlDbType.VarChar).Value = BusinessPhone.Text;
                cmd.Parameters.Add("@Extension", SqlDbType.VarChar).Value = Extension.Text;
                cmd.Parameters.Add("@FaxNumber", SqlDbType.VarChar).Value = FaxNumber.Text;
                cmd.Parameters.Add("@TaxID", SqlDbType.Int).Value = TaxID.Text;
                cmd.Parameters.Add("@LegalName", SqlDbType.VarChar).Value = LegalName.Text;
                cmd.ExecuteNonQuery();
                cmd.Dispose();
                con.Close();
                Response.Write("<script language='javascript'>window.alert('Updated Successfully.');window.location='Company.aspx';</script>");
            }
        }
        catch (SqlException ex)
        {
            ScriptManager.RegisterStartupScript(this, this.GetType(), "Message",
                "alert('Oops!! following error occured : " + ex.Message.ToString() + "');", true);
        }

    }
    protected void CancelButton_Click(object sender, EventArgs e)
    {
        Response.Redirect("Company.aspx");
    }
    private void BindTextBoxvalues()
    {
        SaveButton.Text = "Update";
        string constr = "Data Source=.; Initial Catalog=something; Integrated Security=True";
        SqlConnection con = new SqlConnection(constr);
        SqlCommand cmd = new SqlCommand("select * from Company where CompanyID=" + Companyid, con);
        DataTable dt = new DataTable();
        SqlDataAdapter da = new SqlDataAdapter(cmd);
        da.Fill(dt);
        Name.Text = dt.Rows[0][1].ToString();
        CompanyCode.Text = dt.Rows[0][2].ToString();
        LegalName.Text = dt.Rows[0][15].ToString();
        TaxID.Text = dt.Rows[0][14].ToString();
        BusinessPhone.Text = dt.Rows[0][3].ToString();
        Extension.Text = dt.Rows[0][13].ToString();
        FaxNumber.Text = dt.Rows[0][12].ToString();
        Description.Value = dt.Rows[0][4].ToString();
        IsActiveCheckBox.Checked = Convert.ToBoolean(dt.Rows[0][11]);
    }
}

Any values stored in local variables need to be read from Request on every postback.

So do following

int Companyid = 0;
string cmdName = null;
protected void Page_Load(object sender, EventArgs e)
{

        Companyid = Convert.ToInt32(Request.QueryString["CompanyID"]);
        cmdName = Request.QueryString["CommandType"];


        if (!IsPostBack)
          {
            if (cmdName == "Details")// be sure about string case
            {
                BindTextBoxvalues();
            }
        }
}

Or make viewstate properties