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Input string was not in a correct format in a Formula

I was try to create a formula that will calculate the total amount and show it in a textbox , but there's an error that says
" Input string was not in a correct format."
private void txtQUANTITYPOS_TextChanged(object sender, EventArgs e)
{
txtTOTALPOS.Text = (Convert.ToDouble(txtPricePOS.Text) * Convert.ToDouble(txtQUANTITYPOS.Text)).ToString();
}
Can you help me please , thank you again in advance :)

One possible scenario - You may have enter a number in your txtQUANTITYPOS textbox but txtPricePOS textbox is still empty. Therefore, you probably getting this error because your txtPricePOS textbox doesn't contains a number yet.
You could use Double.TryParse instead to ensure your are not multiplying other character rather than numbers:
Following an example of how you could implement it:
private void txtQUANTITYPOS_TextChanged(object sender, EventArgs e)
{
double pricePos;
double qtyPos;
if (!double.TryParse(txtPricePOS.Text, out pricePos))
return; // Will return in case txtPricePOS.Text is not a number
if (!double.TryParse(txtQUANTITYPOS.Text, out qtyPos))
return; // Will return in case txtPricePOS.Text is not a number
txtTOTALPOS.Text = (pricePos * qtyPos).ToString();
}

Format error clearing TextBox parsed as integer

I am using Windows Form Application and am having an issue clearing a user textbox. The textbox is getting parsed as an int. I have a button near the bottom of the app that allows a user to clear different parts of the form, labels textboxes etc.
The labels clear fine but any textbox that has been parsed to int I get error on the TextChanged method.
xxx = textbox name
yyy = new int var used in other parts of code
when xxx.Clear(); is called I get exception below:
An unhandled exception of type
System.FormatException' occurred in mscorlib.dll
Additional information: Input string was not in a correct format.
Here is example of code
private void xxx_TextChanged(object sender, EventArgs e){
// parse user text and convert to int
yyy = int.Parse(xxx.Text);
}
private void btnExit_Click(object sender, EventArgs e){
// close program
this.Close();
}
private void btnClear_Click(object sender, EventArgs e){
xxx.Clear();
}

put the textBoxName.Text in a variable and check if its value is null or not.
int xxx = textbox.Text ;
if(xxx == ""){
//do something
}

well when you use int32.parse you should be carefull that your string be not null and all contain numbers like
"123456789"
if you think you string might be null ot contain not number characters you should use:
int32.TryParse(string String, out int iString);
and use it like this:
int i = 0; // default
string s = "somthing123somthing456blah";
int32.TryParse(s, out i);
result should be: 123456 as integer
EDIT:
if you dont what TextChanged fire:
mytextbox.TextChanged -= myTextBox_TextChanged;
make you change and:
mytextbox.TextChanged += myTextBox_TextChanged;

I suspect something to do with code re-entry. When xxxTxtBox.Clear() is called, the TextChanged method is called, meaning int.parse is called on an empty textbox (null or string.empty can't be parsed to an integer), throwing an exception.
A preferred way to fix that may be setting a flag upon re-entry, something like this.
The easy way is put your TextChanged() logic in a try/catch, meaning that upon re-entry, the int.parse() fails but your user doesn't see a problem.

You need evaluate if the textbox is empty:
enter private void xxx_TextChanged(object sender, EventArgs e)
{
// parse user text and convert to int if value is't Empty
yyy = String.IsNullOrEmpty(xxx.Text) ? 0 : int.Parse(xxx.Text);
}
And try this:
private void btnClear_Click(object sender, EventArgs e){
xxx.Text = String.Empty;
}

To clarify, as some others have pointed out, the problem you are experiencing is due to the fact that xxx_TextChanged() is being called when xxx.Clear() is called and the textbox content changes to an empty string. This causes int.Parse() to execute against an empty string, which produces said error.
To circumvent this problem, you should change the xxx_TextChanged() method to something similar to as follows:
private void xxx_TextChanged(object sender, EventArgs e)
{
// Parse user text and convert to integer, when able:
if ((string.IsNullOrEmpty(xxx.Text)) ||
(!int.TryParse(xxx.Text, out yyy)))
{
yyy = 0;
}
}
Notice the key difference is the "null or empty" check against the text box contents. There is no need to parse a null value or an empty string. Also, instead of using int.Parse(), we can use the int.TryParse() method so that invalid numeric input (e.g., "abc") does not produce an error, but instead is assigned some default value (in this case, 0).
The rest of the code can remain as-is.

How to set TextBox to only accept numbers?

I have already checked other questions here but the answers are not related to my issue. the following code allows textbox1 to only accept numbers if the physical keyboard (laptop) is pressed:
private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
char ch = e.KeyChar;
if ( !char.IsDigit(ch))
{
e.Handled = true;
}
}
but this is not what I wanted (I dont use physical laptop keyboard).
As shown in screenshot, I have windows form with buttons and a textbox. I designed this keyboard and it works well but I want textbox1 to only accept numbers and the ".".
There are only two lines of code inside each button (and only code in the project) which is:
private void buttonName_Click(object sender, EventArgs e)
{
// each button only has this code.
textBox1.Focus();
SendKeys.Send(buttonName.Text);
}
I know how to set txtbox to accept numbers if the physical (laptop ) keys are pressed but here in this case I have control buttons in windwos form and I want to set textBox1 to only accept numbers and the ".". Please help in how to achieve this. Thank you

Declare a string variable at form level, use it to store the last valid text and to restore it when an invalid text is entered on the TextChanged event of your textbox.
string previousText;
public Form1()
{
InitializeComponent();
previousText = String.Empty;
}
private void textBox1_TextChanged(object sender, EventArgs e)
{
int dummy, changeLenght, position;
if (!String.IsNullOrWhiteSpace(textBox1.Text) && !int.TryParse(textBox1.Text, out dummy))
{
position = textBox1.SelectionStart;
changeLenght = textBox1.TextLength - previousText.Length;
textBox1.Text = previousText;
textBox1.SelectionStart = position - changeLenght;
}
else
{
previousText = textBox1.Text;
}
}
position and changeLenght are used to keep the cursor where it was before restoring the text.
In case you want to accept numbers with decimals or something bigger than 2147483647, just change dummy to double and use double.TryParse instead of int.TryParse.
private void textBox1_TextChanged(object sender, EventArgs e)
{
int changeLenght, position;
double dummy;
if (!String.IsNullOrWhiteSpace(textBox1.Text) && !double.TryParse(textBox1.Text, out dummy))
{
...
}
}

Suppose button1 is your button control, you could do this:
private void allButtons_Click(object sender, EventArgs e)
{
Button btn = sender as Button;
char c = btn.Text[0]; //assuming all buttons have exactly 1 character
if(Char.IsDigit(c) || c == '.')
{
//process
textBox1.Focus();
SendKeys.Send(btn.Text);
}
//otherwise don't
}
I'm assuming you put this in a common handler, to which you already wired all your buttons (i.e. allButtons_Click).
Problem with this approach, it allows you to type values like 0.0.1, which are most likely invalid in your context. Another way to handle this is to process TextChanged event, store previous value, and if new value is invalid, restore the old one. Unfortunately, TextBox class does not have TextChanging event, which could be a cleaner option.
The benefit of you determining the invalid value is modularity. For example, if you later decide your user can enter any value, but only numbers can pass validation, you could move your check from TextChanged to Validate button click or similar.
Why users may want that - suppose one of the options for input is copy/paste - they want to paste invalid data and edit it to become valid, for example abc123.5. If you limit them at the entry, this value will not be there at all, so they now need to manually paste into Notepad, cut out in the invalid characters, and paste again, which goes against productivity.
Generally, before implementing any user interface limitation, read "I won't allow my user to...", think well, whether it's justified enough. More often than not, you don't need to limit the user, even for the good purpose of keeping your DB valid etc. If possible, never put a concrete wall in front of them, you just need to guide them correctly through your workflow. You want users on your side, not against you.

Preventing user from entering zero but allowing other numbers that contain zero

I have a TextBox control. I have set my KeyPress event of the TextBox so that the user is only allowed to enter digits:
private void txtbxRecurEveryXWeeks_KeyPress(object sender, KeyPressEventArgs e)
{
if (!char.IsControl(e.KeyChar) && !char.IsDigit(e.KeyChar))
{
e.Handled = true;
}
}
But in addition to that I do not want the user to set the value of the TexBot to zero "0". But they still should be allowed to enter "10" or "3400" as the value. How can I achieve that?

It's probably best to ignore the keypress event and handle the lost focus event and check and ensure the value isn't 0 when the user is done typing.
EDIT: It was pointed out that the user could type "000" and lose focus which would cause this to fail. You can check and confirm the textbox has a value and parse it as an int to confirm it isn't 0.
private void txtbxRecurEveryXWeeks_LostFocus(object sender, EventArgs e)
{
int value;
if(!string.IsNullOrWhitespace(txtbxRecurEveryXWeeks.Text)
&& int.TryParse(txtbxRecurEveryXWeeks.Text, out value)
&& value > 0)
{
//Do something like clear the textbox value
}
}

In the assumption that you use winforms, consider using the NumericUpDown control to enter numbers. It has the Minimum property, which you could set to 1 to avoid zero input.

Use NumericUpDown with minimum value = 1 and hidden up and down buttons.
numericUpDown1.Minimum = 1;
numericUpDown1.Controls[0].Hide();

New C# Programmer - Adding two numericUpDown values together?

I'm fairly new to C# programming.
I am making a program for fun that adds two numbers together, than displays the sum in a message box. I have two numericUpDowns and a button on my form. When the button is pushed I want it to display a message box with the answer.
The problem is, I am unsure how to add the twp values from the numericUpDowns together.
So far, I have this in my button event handler:
private void button1_Click(object sender, EventArgs e)
{
MessageBox.Show(this.numericUpDown1.Value + this.numericUpDown2.Value);
}
But obviously, it does not work. It gives me 2 compiler errors:
1. The best overloaded method match for 'System.Windows.Forms.MessageBox.Show(string) has some invalid arguments
2. Argument '1': cannot convert decimal to 'string'
Thanks!

this.numericUpDown1.Value + this.numericUpDown2.Value is actually evaluating properly to a number, so you're actually very close. The problem is that the MessageBox.Show() function, needs a string as an argument, and you're giving it a number.
To convert the result to a string, add .ToString() to it. Like:
private void button1_Click(object sender, EventArgs e)
{
MessageBox.Show((this.numericUpDown1.Value + this.numericUpDown2.Value).ToString());
}
For reference, if you want to do more advanced formatting, you'd want to use String.Format() instead of ToString(). See this page for more info on how to use String.Format().

This works.
decimal total = this.numericUpDown1.Value + this.numericUpDown2.Value;
MessageBox.Show(total.ToString());
MessageBox.Show expects a string as a parameter (that's the first error message).

Try this:
private void button1_Click(object sender, EventArgs e)
{
MessageBox.Show((this.numericUpDown1.Value + this.numericUpDown2.Value).ToString());
}
It takes the values from the numericUpDown components and adds them to get an object of the type Decimal. This is then converted to a String, which MessageBox takes.

Now that's easy. NumericUpDown.Value has a type of Decimal. Messagebox.Show() expects a String. All you need to do is
MessageBox.Show((this.numericUpDown1.Value + this.numericUpDown2.Value).ToString());
to convert the result of the addition to a string.