C# Temp Converter Program.

I don't know what's wrong with my application. I'm suppose to satisfy these 4 conditions but it's not working. Please help me fix it.

1: User enters data inside both Celsius and fahrenheit text boxes: You should enter data in only one textbox. Press Clear to start over.

2: User leaves text boxes empty and tries to convert: You must enter the temperature that you want to convert. Press Clear to start over.

3: User enters any character that is not a number: "Characters are invalid. Press clear to start over."

4: User enters data below absolute zero (If the Celsius temperature is less than -273.15, Fahrenheit less than -459.67): "You entered an invalid data. Press clear to start over."

Below is the code I have done so far.

public partial class Form1 : Form { double celsius; double fahrenheit; int flag;

public Form1() { InitializeComponent(); } public void ToCelsius(double fah) { celsius = (fah - 32) * 5 / 9; celsiusTextBox.Text = celsius.ToString("N2"); } public void ToFahrenheit(double cel) { fahrenheit = (cel * 9 / 5) + 32; fahrenheitTextBox.Text = fahrenheit.ToString("N2"); }

private void convertButton_Click(object sender, EventArgs e) {

if (celsiusTextBox.Text != "")

{

if (fahrenheitTextBox.Text != "")

{

messageLabel.Text = "You should enter data in only one textbox. Press Clear to start over.";

}

}

if (string.IsNullOrEmpty(celsiusTextBox.Text) && string.IsNullOrEmpty(fahrenheitTextBox.Text))

{

messageLabel.Text = "You must enter the temperature that you want to convert. Press Clear to start over.";

}

if (celsiusTextBox.Text != "")

{ long number = 0; bool canConvertNum = long.TryParse(celsiusTextBox.Text, out number); if (canConvertNum == false) {

messageLabel.Text = "Characters are invalid. Press clear to start over.";

}

if (fahrenheitTextBox.Text != "")

{ long number = 0; bool canConvertNum = long.TryParse(fahrenheitTextBox.Text, out number); if (canConvertNum == false) {

messageLabel.Text = "Characters are invalid. Press clear to start over."; }

}

if { long valueCelsius = 0; long valueFahrenheit = 0;

bool canConvertNumCel = long.TryParse(celsiusTextBox.Text, out valueCelsius);

bool canConvertNumFah = long.TryParse(fahrenheitTextBox.Text, out valueFahrenheit);

if (canConvertNumCel == true && valueCelsius < -273.15) {

messageLabel.Text = "You entered an invalid data. Press clear to start over."

}

if (canConvertNumFah == true && valueFahrenheit < -459.67) {

messageLabel.Text = "You entered an invalid data. Press clear to start over."

} }

if (flag == 1) { celsius = Convert.ToDouble(celsiusTextBox.Text); ToFahrenheit(celsius); } else if (flag == 0) { fahrenheit = Convert.ToDouble(fahrenheitTextBox.Text); ToCelsius(fahrenheit); } }

private void clearButton_Click(object sender, EventArgs e) { celsiusTextBox.Text = ""; fahrenheitTextBox.Text = ""; }

private void exitButton_Click(object sender, EventArgs e) { this.Close(); }

private void celsiusTextBox_TextChanged(object sender, EventArgs e) { flag = 1; }

private void fahrenheitTextBox_TextChanged(object sender, EventArgs e) { flag = 0; } } }