How do I pass a value from one form to a variable in another that actually changes the default value of the variable?

Question

I am trying to pass the value in one form (Sales Tax) to a variable in another (Invoice Total). I want the new value to override the default value. When I step through the code the value of the SalesTaxPct variable changes, but then when I select calculate, the default value (7.75) is still used in the calculation. Any help would be much appreciated.

Code for Sales Tax Form:

    public frmSalesTax()
    {
        InitializeComponent();
    }

    private void btnOK_Click(object sender, EventArgs e)
    {
        if (IsValidData())
        {
            this.SaveData();
        }  
    }

    private void SaveData()
    {
        string salesTaxPct = Convert.ToString(txtSalesTaxPct.Text);
        this.Tag = salesTaxPct;
        this.DialogResult = DialogResult.OK;
    }

Code for Invoice Total Form:

    public frmInvoiceTotal()
    {
        InitializeComponent();
    }
    //removed the constant
    decimal SalesTaxPct = 7.75m;

    private void btnChangePercent_Click(object sender, EventArgs e)
    {
        Form salesTaxForm = new frmSalesTax();
        DialogResult selectedButton = salesTaxForm.ShowDialog();
        if (selectedButton == DialogResult.OK)
        {
            decimal SalesTaxPct = Convert.ToDecimal(salesTaxForm.Tag);
            lblTax.Text = "Tax(" + SalesTaxPct + "%)";
        }
    }

    private void btnCalculate_Click(object sender, EventArgs e)
    {
        if (IsValidData())
        {
            decimal productTotal = Convert.ToDecimal(txtProductTotal.Text);
            decimal discountPercent = .0m;

            if (productTotal < 100)
                discountPercent = .0m;
            else if (productTotal >= 100 && productTotal < 250)
                discountPercent = .1m;
            else if (productTotal >= 250)
                discountPercent = .25m;

            decimal discountAmount = productTotal * discountPercent;
            decimal subtotal = productTotal - discountAmount;
            decimal tax = subtotal * SalesTaxPct / 100;
            decimal total = subtotal + tax;

            txtDiscountAmount.Text = discountAmount.ToString("c");
            txtSubtotal.Text = subtotal.ToString("c");
            txtTax.Text = tax.ToString("c");
            txtTotal.Text = total.ToString("c");

            txtProductTotal.Focus();
        }
    }

Answer 1

If you will notice in btnChangePercent_Click you create a new, local variable by declaring it's type ( decimal SalesTaxPct ) which is correctly set by the return from the SalesTax form:

if (selectedButton == DialogResult.OK)
{
    // In the net line you're declaring a new, local
    // variable instead of using the class level variable
    decimal SalesTaxPct = Convert.ToDecimal(salesTaxForm.Tag);
    lblTax.Text = "Tax(" + SalesTaxPct + "%)";
}

However, the class-level variable, SalesTaxPct , has NOT been set. If you remove the decimal declaration, it will work as you expect:

if (selectedButton == DialogResult.OK)
{
    SalesTaxPct = Convert.ToDecimal(salesTaxForm.Tag);
    lblTax.Text = "Tax(" + SalesTaxPct + "%)";
}