Individual Field Validation in ASP .NET

When you have text box on your form (or drop down lists that include invalid values) it can make sense to validate the field immediately after the field has been updated rather than waiting until the entire form has been filled out. By doing this the person filling out the form can know immediately that the value that they have entered is invalid allowing them to correct their entry before filling out the rest of the form. In some cases telling them that the first field is invalid straight away may let them know that they are filling out the wrong form before they waste time filling out all of the fields.

We can validate text boxs straight away using the onblur event handler. When we add this event handler to our text boxs and text areas the appropriate validation routine will be called as soon as the person moves on to another field.

Of course we will still need to validate the same field immediately prior to submitting the form just in case someone bypasses accessing the field at all. We can easily add the same validation to both places by placing all of the code for the validation into a function and calling that function from both places.

Here is what the code for a text boxlooks like with the validation processing added:

<asp:TextBox ID="txtEmail" runat="server" onblur="ValidateControl(this)" ></asp:TextBox>

<asp:RequiredFieldValidator ID="reqTxtEmail" runat="server" SetFocusOnError="true"
ControlToValidate="txtEmail" ErrorMessage=" required" EnableClientScript="true" ></asp:RequiredFieldValidator>

<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" SetFocusOnError="true" ControlToValidate="txtEmail" ErrorMessage=" invalid"
ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*" ></asp:RegularExpressionValidator>

In this example ValidateControl(this) is called OnBlur of the TextBox. This method finds all validator controls for this text box in Page_Validators array, then call ValidatorValidate function of Asp.Net to validate. Javascript to validate is,

function ValidateControl(controltovalidate) {

if (typeof (Page_Validators) != "undefined") {

for (i = 0; i < Page_Validators.length; i++) {

val = Page_Validators[i];

if (val.controltovalidate == controltovalidate.id) {

ValidatorValidate(val);

if (val.isvalid==false) {

return;

}

}

}

}

}