Validation For Digits only.

you can easily use Regular expression validator for this:
In the validation expression property keep ^\d+$.

<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="TextBox1" ErrorMessage="Please Enter Only Numbers" ValidationExpression="^\d+$" ValidationGroup="check"></asp:RegularExpressionValidator>

You also might want to consider a MaskedTextBox[^] which would only allow numerics...

The regex pattern you need to use would be "^\d+(\.\d+)?$" (without the quotes) as the previously suggested ones wouldn't account for the decimal point. You could use it in the following ways:

Using a RegularExpressionValidator Control:

<pre lang="xml"><%@ language="C#" %>
<form id="form1" runat="server">
    <asp:TextBox ID="textBox1" runat="server"/>
    <asp:Button ID="btnSubmit" runat="server" Text="Submit" />
    <asp:RegularExpressionValidator ID="regexpName" runat="server"
                                    ErrorMessage="This is not a valid number."
                                    ControlToValidate="textBox1"
                                    ValidationExpression=@"^\d+(\.\d+)?$"/>
</form>

Or using the Regex class:

if(System.Text.RegularExpressions.Regex.IsMatch(textBox1.Text, @"^\d+(\.\d+)?$"))
{
    doSomething();
}

use regular expression validation and put this ^[0-9]*\.?[0-9]+$ in expression