How it works...

On startup, the application is as shown in the following screenshot:

When the Register button is clicked, the validation mechanism is triggered through the following steps:

1. Retrieving a TCustomer instance based on the form data.
2. Using the Validate class method of the TValidator class validates the instance just retrieved.
3. If the validation is successful, a success message is shown; otherwise, errors are shown:

procedure TMainForm.btnRegisterClick(Sender: TObject);
var
  LCustomer: TCustomer;
  LErrors: TArray<string>;
begin
  LCustomer := GetCustomer;
  try
    LErrors := [];
    if TValidator.Validate(LCustomer, LErrors) then
      ShowMessage('Well done! You''re now registered...')
    else
      ShowErrors(LErrors);
  finally
    LCustomer.Free;
  end;
end;

The concepts of querying, consideration, and application are contained in this code snippet:

// iterate over object properties
  for p in rt.GetProperties do
    // iterate over attributes of this property
    for a in p.GetAttributes do
    begin
      // ensure that is a ValidationAttribute
      if not(a is ValidationAttribute) then
        continue;
      // reference to Validate method
      m := FContext.GetType(a.ClassType).GetMethod('Validate');
      if m = nil then
        continue;
      // invoke method
      LResult := m.Invoke(a, [p.GetValue(AObject).AsString])
        .AsType<TValidationResult>;
      // eventually add error to errors array
      if not LResult.Validation then
        AErrors := AErrors + [LResult.ErrorMessage];
    end;

Specifically, in the validated method of TValidator, the properties of the BOs are iterated to understand whether the validation attributes have been applied. If yes, the Validate method of the specific attribute is applied to the current value of the object property. If the property value does not pass validation, the error message specified in the attribute declaration phase is added to the errors array.

In the following screenshot, you can see the mechanism in place: