Amount Calculation logic

Question

Package Name:

ddlPackName

No. of Extra Connection:

txtNoConn

Charges per Connection(If any):

txtChargeConn

Installation Charges:

txtInstlChrge

Discount (if any):

txtDisc

Amount to Pay:

txtAmntToPay

Paying Amount:

txtPayAmnt

Balance:

txtBlnc

How to get final result1 (txtAmntToPay.text)? and final result2 (txtBlnc)


1).if I select 'ddlPackName' then its price display in 'txtAmntToPay.text'
2).if I enter 'txtNoConn' and 'txtChargeConn' then multiple this both and add with 'ddlpackname' then display in 'txtamnttoPay'

if I not select 'ddlpackname' then display multiplied values.

3).like this all textboxes values add with 'txtPayAmnt' and subtract 'txtDisc'

I written like below code but when i enter value second time in other textboxes assing/subtracting again

public void regCustCalculation()

{

double amountToPay = double.Parse(txtAmntToPay.Text);

double noOfConn = double.Parse(txtNoOfConn.Text);

double chargePerConn = double.Parse(txtChargePerConn.Text);

double installCharge = double.Parse(txtInstallCharge.Text);

double discount = double.Parse(txtDiscount.Text);

double packPrice = double.Parse(txtAmntToPay.Text);

if (ddlPackName.SelectedIndex != 0)

{

txtAmntToPay.Text = "0";

packPrice = amountToPay;

packPrice = amountToPay + (noOfConn * chargePerConn);

packPrice = packPrice + installCharge;

packPrice = packPrice - discount;

txtAmntToPay.Text = packPrice.ToString();

}

if (ddlPackName.SelectedIndex==0)

{

txtAmntToPay.Text = "0";

packPrice = noOfConn * chargePerConn;

packPrice = packPrice + installCharge;

packPrice = packPrice - discount;

txtAmntToPay.Text = packPrice.ToString();

}

}

protected void txtNoOfConn_TextChanged(object sender, EventArgs e)

{

//regCustCalculation();

}

protected void txtChargePerConn_TextChanged(object sender, EventArgs e)

{

regCustCalculation();

}

protected void txtInstallCharge_TextChanged(object sender, EventArgs e)

{

regCustCalculation();

}

protected void txtDiscount_TextChanged(object sender, EventArgs e)

{

regCustCalculation();

}

public void ddlPackPriceBind()

{

try

{

//open the db connection if it is closed...

if (connection.State == ConnectionState.Closed)

connection.Open();

string packNameEdit = ddlPackName.SelectedItem.ToString();

command = new SqlCommand();

command.CommandText = "sp_Get_PackPrice";

command.CommandType = CommandType.StoredProcedure;

command.Parameters.Add("@packageName", packNameEdit);

da = new SqlDataAdapter(command);

command.Connection = connection;

ds = new DataSet();

da.Fill(ds);

txtAmntToPay.Text = "";

txtAmntToPay.Text = ds.Tables[0].Rows[0]["PackagePrice"].ToString();

}

protected void ddlPackName_SelectedIndexChanged(object sender, EventArgs e)

{

ddlPackPriceBind();

regCustCalculation();

}