How do you define a method for text box validation?

Dear Friend,

Thank you very much for your reply. But I need a small clarification about this coding. That is I jz want to know whether Javascript coding can be used in C # coding. Here, I'm developing a windows application, so... Can you apply these coding to validate text boxes in widows form?

Thanks in advance !

You should call a javascript function in onclick or onkeypress event.
I will give you javascript function for the validation.

//To check whether a adress is proper email address or not.

function isEmail(s){  
    if (s.search(/^[a-zA-Z][\w\.-]*[a-zA-Z0-9]@[a-zA-Z0-9][\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z]$/) == -1){
     return false;
    }
    else{
     return true;
    }
}

//To check input is dizit or Character. This function will only allow user to enter number

function ValidatePhoneNo(Obj){
  if(((window.event.keyCode >= 48) && (window.event.keyCode <= 57))){
      var Value = Obj.value;
      var len = Value.length;
       if (len == 3 || len == 7 ){
             Obj.value = Value + '-';
              return true;
        }
    }
   else{
        alert("Phone No Cannot be a Character");
        return false;
    }
}

//the following function will disable a textbox call it in onload.My textbox id is txtVendorName

Function disable()
{
  document.getElementById("txtVendorName").disable = true;
}

Yes. At client side.