how to validate SharePoint:formfield

Question

I have a data view web part and it does insert,delete ,in edit mode it save and cancel button.The data is based on a custom list.In the list i have set the max to 4 but in DWP it doesn't seem to work. When in Edit mode or in Insert mode i want to validate this field

<SharePoint:formfield runat="server" id="ff9{$Pos}" ControlMode="Edit" FieldName="Quantity" ItemId="{@ID}" __designer:bind="{ddwrt:DataBind('u',concat('ff9',$Pos),'Value','ValueChanged','ID',ddwrt:EscapeDelims(string(@ID)),'@Quantity')}" />

I want make sure the number being added is not greater that 4. Any ideas? Thanks

Answer 1

I think you should create a customfield

and overrides the validate method

create custom field : http://msdn2.microsoft.com/en-us/library/aa981226.aspx

    public override void Validate()
{
   //make your validation here

}

Answer 2

Fixed. I used the function PreSaveAction() to fix this