Example of how to access a field on a WFFM form outside the validator's field context.

gistfile1.cs

protected override bool OnServerValidate(string value)
{
  // get the form
  var form = WebUtil.GetParent<SimpleForm>(this);
  
  // find another field within our form
  var field = (SingleLineText)WebUtil.FindFirstOrDefault(form, c => c is SingleLineText && (c as SingleLineText).Result.FieldName == "City");

  // compare values between this field (State) and the value of a different field (City)
  bool isBoston = field != null && field.Result.Value.ToString() == "Boston";
  bool isMassachusetts = !string.IsNullOrEmpty(value) && value == "MA";
  return isBoston && isMassachusetts;
}