How do I stop a single button from hiding my textbox during a specific event?

Question

I have two questions:

1) Is this the correct way to make my default value = 0 for variables, then pass a value given by the user into that variable?

protected void btnCheck_Click(object sender, EventArgs e)
    {
        lblYesNo.Text = "";
        //default int values are set to 0
        int remainder = 0;
        int guess = 0;

        remainder = int.Parse(txtRemainder.Text);

        guess = int.Parse(txtAnswer.Text);
        answer = (int)Session["answer"];
        if (guess == answer)
        {
            lblYesNo.Text = lblYesNo.Text + "Correct!";
        }
        else
        {
            lblYesNo.Text = lblYesNo.Text + "Try Again..";
        }
    }//END Check Answer

2) how can I stop the Check_Click(submit button) from hiding the txtRemainder(textbox)? The reason it 'auto-hides' now is because I set the default value to 'txtRemainder.Visible = false;' in the Page_Load, which will make it hide unless the math problem is division. When I click on the btnDiv_Click(divide button) it resets it to 'txtRemainder.Visible = true;', because this provides a division question to be solved.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class BasicMath : System.Web.UI.Page
{
    int number1;
    int number2;
    int answer;

    protected void Page_Load(object sender, EventArgs e)
    {
        txtRemainder.Visible = false;
    }

    protected void GetRandom()
    {
        Random rand = new Random();
        number1 = rand.Next(0, 10);
        number2 = rand.Next(0, 10);
        txtAnswer.Text = "";
        txtRemainder.Text = "";
        lblYesNo.Text = "";
    }//END Get Random Number

    protected void btnAdd_Click(object sender, EventArgs e)
    {
        GetRandom();
        lblEquation.Text = number1.ToString() + " + " + number2.ToString();
        answer = number1 + number2;
        Session["answer"] = answer;
    }//END Addition Button

    protected void btnSub_Click(object sender, EventArgs e)
    {
        GetRandom();
        if (number2 > number1)
        {
            answer = number2 - number1;
            lblEquation.Text = number2.ToString() + " - " + number1.ToString();
        }
        else
        {
            answer = number1 - number2;
            lblEquation.Text = number1.ToString() + " - " + number2.ToString();
        }
        Session["answer"] = answer;

    }//END Subtraction Button

    protected void btnMult_Click(object sender, EventArgs e)
    {
        GetRandom();
        lblEquation.Text = number1.ToString() + " x " + number2.ToString();
        answer = number1 * number2;
        Session["answer"] = answer;
    }//END Multiplication Button

    protected void btnDiv_Click(object sender, EventArgs e)
    {

        Random rand = new Random();
        number1 = rand.Next(1, 10);
        number2 = rand.Next(1, 10);
        /*will only display the txtRemainder(textbox) while using the Divide button,
           txtRemainder will auto-hide when using another button because
           it's default setting of '.Visible = false' is placed in the Page_Load*/
        txtRemainder.Visible = true;
        lblEquation.Text = number1.ToString() + " / " + number2.ToString(); 
        answer = number1 / number2;
        Session["answer"] = answer;

    }//END Division Button

    protected void btnCheck_Click(object sender, EventArgs e)
    {
        lblYesNo.Text = "";
        //default int values are set to 0
        int remainder = 0;
        int guess = 0;

        remainder = int.Parse(txtRemainder.Text);

        guess = int.Parse(txtAnswer.Text);
        answer = (int)Session["answer"];
        if (guess == answer)
        {
            lblYesNo.Text = lblYesNo.Text + "Correct!";
        }
        else
        {
            lblYesNo.Text = lblYesNo.Text + "Try Again..";
        }
    }//END Check Answer
}

If this question doesn't make sense please ask for clarification.

Answer 1

Question 1:

Yes, that is the correct way to initialize your variables to default values of zero, although int variables are initialized to zero by default, so even if you omitted the assignment, they would still default to zero.

The method you're using to accept the user input is correct, but could throw an exception if the values entered by the user are not Int32 values (think 3.2). As it stands, you have no try..catch blocks to deal with these exceptions. You could either add these try..catch blocks, or you could use the TryParse() method to check that the values are valid. For example:

protected void btnCheck_Click(object sender, EventArgs e)
{
    lblYesNo.Text = "";
    //default int values are set to 0
    int remainder = 0;
    int guess = 0;

    if (!Int32.TryParse(txtRemainder.Text, out remainder))
    {
        // do something here to inform the user that remainder is invalid
        return;
    }

    if (!Int32.TryParse(txtAnswer.Text, out remainder))
    {
        // do something here to inform the user that answer is invalid
        return;
    }

    answer = (int)Session["answer"];
    if (guess > answer)
    {
        lblYesNo.Text = lblYesNo.Text + "Try Again..";
    }
    else if (guess < answer)
    {
        lblYesNo.Text = lblYesNo.Text + "Try Again..";
    }
    else
    {
        lblYesNo.Text = lblYesNo.Text + "Correct!";
    }
}//END Check Answer

TryParse() will convert the value into the output variable if it's a valid value & return true to indicate success, otherwise it will return false if it was not able to perform the conversion.

Question 2:

In order to hide txtRemainder when the page first loads & then to keep it hidden if any button was clicked other than btnDiv, here is a proposed solution. First, the code:

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack) {
        lblTest.Visible = false;
    } else {
        foreach (string ctrl in Request.Form) {
            Control c = FindControl(ctrl);
            if (c is Button) {
                txtRemainder.Visible = c.ID == "btnDiv";
                return;
            }
        }
    }
}

Now for some explaining. When the page initially loads, the label is hidden (by the !IsPostBack check). If the request is indeed a postback, we go through the collection of returned form controls (contained in the Request.Form collection) & check to see if a button is present that could have caused the postback. Buttons are rendered in the HTML as <input type="submit" .. /> elements, and upon postback, only the button that was clicked is sent back in the Form collection, even if there are multiple submits on the page. A more detailed explanation can be found here

Now, if a button did indeed cause the postback, then we check to see if this button was btnDiv. If it was, the comparison returns true & txtRemainder is displayed. If not, it is hidden.

This way, there is no need to show or hide txtRemainder in each event handlers. You wouldn't even need to set it to visible in the event handler for btnDiv.

EDIT

Based on the requirement listed in the comments, I've altered my answer to question 2 so as to provide an alternative method to having to show or hide the label in each button event.

Answer 2

Question1: If you are using .Net 4.0 and up, consider using the TryParse method, and check the result for success, this way you have better control over parsing values and trapping exceptions e.g.

int number = 0;
bool result = false;

result = int.TryParse("123", out number);

if (!result)
{
    throw new InvalidCastException("Error converting number");
}

Question2: It looks like you're using Asp.Net, so try testing for Page.IsPostBack in your Page_Load event, this way you can determine whether the page loads because of an initial load, or because you sent data to the server i.e. clicked a button.

Hope this helps!