How do I check to see the value (if its empty or not) in TextBox1 from my class file (ASP.Net, C#)

I would like to make a static class that check to see if  a checkbox has something in it or not. And if  it does, I want to execute my code.  I just don’t know how to check the textbox 

public class costCL
{
    
    public double rs;
   
    public void Add(double c, double s, double st)
	{
        if(don't know what goes here)
            double ftax = 5.00; 
            
            rs = Math.Round(c + s + st + ftax, 2);
    }

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bmanmike39Asked:
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Jerry MillerCommented:
You can use the IsNullOrEmpty method to check the value.

If Not String.IsNullOrEmpty(TextBox.Text) then
execute your code


http://msdn.microsoft.com/en-us/library/system.string.isnullorempty.aspx
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bmanmike39Author Commented:
I don't know how to access the text box from my class file
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Vikram Singh SainiSoftware Engineer cum AD DeveloperCommented:
Hi,

Please check link: http://www.vss-tech.com/access-page-control-from-class-file-in-app_code/

to see how to access text box from .aspx page in class file(.cs) from App_Code.

Let us know if you face any problem.
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Manoj PatilSr. Software EngineerCommented:
Hey, if you want to restrict the user to enter the value in textbox then you can use regular expression n your .aspx page
Try it your self here
http://www.w3schools.com/aspnet/showasp.asp?filename=demo_reqfieldvalidator

Or you can check it by using java-script


function msgBox(objtxt)

{
var objtxt=document.getElementById('<%=TextBox2.ClientId%>');
if (objtxt.value.length == 0)

{
alert('TextBox shouldnot be empty');

objtxt.focus();

}

}



<asp:TextBox ID="TextBox2" onkeyup ="msgBox(this);" runat="server"></asp:TextBox><br />
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