Validate number input on text boxes

Question

I want to validate the input on a textbox on a Windows Forms application i.e. show a message box with an error if the number entered is lower than 1 or greater than 24 or if any other character besides an integer is entered. How would I go about doing this?

Answer 1

I would consider something like:

    //make sure that we have a valid number in the text box with no other characters       
    //loop through the string, examining each character
    for (int i = 0; i < txtHour.Text.Length; i++)
    {
        //if this character isn't a number, then don't close the form or continue           
        if (!char.IsNumber(txtHour.Text[i]))
        {
            MessageBox.Show("Value for 'txtHour' must be a number from 1 to 24");
            return;
        }
    }
    //now that we know we have a valid number, convert the string to int and make sure it's not less than 1 or greater than 24
    int testInt = Convert.ToInt32(txtHour.Text);
    if (testInt < 1 || testInt > 24)
    {
        MessageBox.Show("Value for 'txtHour' must be a number from 1 to 24");
        return;
    }

For the method example that you asked for in your comment you could do something more like:

    //////////////////////////////////////////////////////////////////////
    //in your main code:
    if (!isValidHour(textBox1.Text))
        MessageBox.Show("Value for field must be a number from 1 to 24");

    if (!isValidHour(textBox2.Text))
        MessageBox.Show("Value for field must be a number from 1 to 24");

    //////////////////////////////////////////////////////////////////////

    ///method to validate if text field is an INT from 1 to 24  
    bool isValidHour (string stringToValidate)
    {
        //make sure that we have a valid number in the text box with no other characters       
        //loop through the string, examining each character
        for (int i = 0; i < stringToValidate.Length; i++)
        {
            //if this character isn't a number, then don't close the form or continue           
            if (!char.IsNumber(stringToValidate[i]))
            {
                //MessageBox.Show("Value for 'txtHour' must be a number from 1 to 24");
                return false;
            }
        }
        //now that we know we have a valid number, convert the string to int and make sure it's not less than 1 or greater than 24
        int testInt = Convert.ToInt32(stringToValidate);
        if (testInt < 1 || testInt > 24)
        {
            //MessageBox.Show("Value for 'txtHour' must be a number from 1 to 24");
            return false;
        }

        return true;
    }

Answer 2

try{
  if((int)item.value >= 1 && (int)item.value <=25){
    //match.
  }else{
    //error.
  }
}catch (Exception e){
  //type error
}

//or---

var itemValue = default(int);
if(int.TryParse(item.value, out itemValue)){
  if(itemValue >= 1 && itemValue <= 25){
    //match.
  }else{
    //error.
  }
}else{
  //item.value is not numeric.
}

Answer 3

You can add a private method and call it wherever necessary by passing textbox control which needs to be validated.

 private void ValidateText(TextBox textbox)
        {
            int value;

            bool isConverted = Int32.TryParse(textbox.Text.Trim(), out value);
            if (!isConverted)
            {
                MessageBox.Show("Only numbers allowed");
                return;
            }

            if (value < 1 || value > 24)
            {
                MessageBox.Show("Please enter a value between 1-24");
            }
        }

Validating txtHour by invoking above method

ValidateText(txtHour);

Answer 4

Look into doing your check with the OnKeyPress/OnKeyDown/OnKeyUp events:

http://msdn.microsoft.com/en-us/library/system.windows.forms.control.keypress.aspx

Answer 5

This is a very simplistic implementation but, this will achieve the validation that you are asking for:

int hoursEntered;
bool isInteger;
isInteger = int.TryParse(txtHour.Text, out hoursEntered);

if (hoursEntered < 1 || hoursEntered > 24 && isInteger == true)
   MessageBox.Show("Your number is either less than 1, greater than 24 or you didn't enter a number", "Warning!", MessageBoxButtons.OK);  

There are a few things to consider here though. I wrote this code and had it tied to the event handler for a button click. If that is not your implementation, then I would strongly consider using the OnKeyPress/OnKeyUp/OneKeyDown events to do your validation. Otherwise, you can just copy this code into whichever button click event handler you already have in-place.