ASP.NET: produce an error message when a non-numeric value is entered in a textbox?

If a user enters a non-numeric value into a TextBox and presses a Button, I want to show an error m开发者_JAVA百科essage on a Label.

How can I achieve this?

If you're working with ASP.NET webforms, perhaps you have some markup like this:

<asp:TextBox runat="server" ID="TextBox1" Text="Default Text!" />
<asp:Button ID="Button1" runat="server" Text="Click Me" OnClick="ChangeIt" />
<br />
<br />
<asp:Label ID="Label1" runat="server" Text="default!" />

Then your code-behind will need a method to handle the button's click event. This will cause a post-back.

protected void ChangeIt(Object sender,EventArgs e)
{
   // ensure that the value in the textbox is numbers only.
   // there are always questions here whether you care about 
   // decimals, negative numbers, etc. Implement  it as you see fit.
   string userEnteredText = TextBox1.Text.Trim();
   long resultantNumber;

   if (!long.TryParse(userEnteredText, out resultantNumber))
   {
      Label1.Text = string.Format( "It looks like this isn't a number: '{0}'",
                    userEnteredText);
   }
}

Are you saying you want to allow numbers only?

<asp:TextBox runat="server" ID="TextBox1" />
<asp:RegularExpressionValidator runat="server" ID="RegularExpressionValidator1" ControlToValidate="TextBox1" ErrorMessage="Digits only, please" ValidationExpression="^\d+$" />

If this will allow numbers only, but will also allow you to skip the box entirely. If you want to make it required, add this:

<asp:RequiredFieldValidator runat="server" ID="RequiredFieldValidator1" ControlToValidate="TextBox1" ErrorMessage="Required" />

Update: If you would like to accept decimal values like "3.5" in addition to just "3", modify the ValidationExpression in the RegularExpressionValidator I supplied above to read "^\d+(\.\d+)?$"

Assuming you want to check whether the input is a number or a string (and display an error if the input is a string), you could do something like this using the int.TryParse function:

protected void Button1_Click(object sender, EventArgs e)
{
    int readValue;
    if (!int.TryParse(TextBox1.Text, out readValue))
        Label1.Text = "Error";
}