Validating a textbox to allow only numeric values

Question

Possible Duplicate:
How do I make a textbox that only accepts numbers?

I have a phone number that I wish to store as a string.

I read this in using

txtHomePhone.Text

What I think I need is some kind of is numeric but can't get it working

if (txtHomePhone.Text == //something.IsNumeric)
{
    //Display error
}
else
{
    //Carry on with the rest of program ie. Add Phone number to program.
}

What is the best way to allow only numeric values to be entered?

Answer 1

Since txtHomePhone represents a TextBox, you may use the KeyPress event to accept the characters you would like to allow and reject what you would not like to allow in txtHomePhone

Example

public Form1()
{
    InitializeComponent();
    txtHomePhone.KeyPress += new KeyPressEventHandler(txtHomePhone_KeyPress);
}
private void txtHomePhone_KeyPress(object sender, KeyPressEventArgs e)
{
    if (e.KeyChar >= '0' && e.KeyChar <= '9' || e.KeyChar == '') //The  character represents a backspace
    {
        e.Handled = false; //Do not reject the input
    }
    else
    {
        e.Handled = true; //Reject the input
    }
}

Notice: The following character (which is not visible) represents a backspace.
Notice: You may always allow or disallow a particular character using e.Handled.
Notice: You may create a conditional statement if you would like to use -, , ( or ) only once. I would recommend you to use Regular Expressions if you would like to allow these characters to be entered in a specific position.

Example

if (e.KeyChar >= '0' && e.KeyChar <= '9' || e.KeyChar == '') //The  character represents a backspace
{
    e.Handled = false; //Do not reject the input
}
else
{
    if (e.KeyChar == ')' && !txtHomePhone.Text.Contains(")"))
    {
        e.Handled = false; //Do not reject the input
    }
    else if (e.KeyChar == '(' && !txtHomePhone.Text.Contains("("))
    {
        e.Handled = false; //Do not reject the input
    }
    else if (e.KeyChar == '-' && !textBox1.Text.Contains("-"))
    {
        e.Handled = false; //Do not reject the input
    }
    else if (e.KeyChar == ' ' && !txtHomePhone.Text.Contains(" "))
    {
        e.Handled = false; //Do not reject the input
    }
    else
    {
        e.Handled = true;
    }
}

Thanks,
I hope you find this helpful :)

Answer 2

I'm assuming that you are using Windows Forms here, have a look at the MaskedTextBox. It allows you to specify an input mask of characters.

txtHomePhone.Mask = "##### ### ###";

Since this allows you to restrict the input values, you can safely parse the value to an integer.

Note: If you are using WPF, I don't think there is a MaskedTextBox in the base libraries, however there are extensions available on NuGet which may provide similar functionality.

Answer 3

To check to see if a numeric value has been entered, you can use Integer.TryParse

int num;
bool isNum = Integer.TryParse(txtHomePhone.Text.Trim(), out num);

if (!isNum)
    //Display error
else
    //Carry on with the rest of program ie. Add Phone number to program.

BUT remember that telephone numbers aren't necessarily only numeric. Refer to Trevor Pilley answer for a masked textbox.

Answer 4

Try this

if (!txtHomePhone.Text.All(c=> Char.IsNumber(c)))
{
    //Display error
}
else
{
    //Carry on with the rest of program ie. Add Phone number to program.
}

Answer 5

Usually I would use Integer.TryParse as davenewza recommended, but another alternative is to use the VisualBasic IsNumeric function from C#.

Add a reference to the Microsoft.VisualBasic.dll file and then use the following code.

if (Microsoft.VisualBasic.Information.IsNumeric(txtHomePhone.Text))
{
    //Display error
}
else
{
    //Carry on with the rest of program ie. Add Phone number to program.
}

Answer 6

The best way I've found to do this is to only allow the user to enter the numeric keys into the text box, thanks for your help.