Fluent Validation not Working in Bootstrap 4 Modal - asp.net-core

Question

I was using data annotation validation on my Razor Pages which worked fine. For more complex valitaion I am using Fluent Validation, which works fine for everything unless it is in a Modal.
public class MyModel
{
[RegularExpression(#"^Test|Prod", ErrorMessage = "Please Choose A Type")]
public string Type { get; set; }
}
public class MyValidator : MyValidator<MyModel>
{
public MyValidator()
{
RuleFor(x => x.Username)
.Empty()
.When(support => x.Found == false)
.WithMessage("Not Found");
RuleFor(x => x.IsComplete)
.Must(x => x.Equals(true))
.WithMessage("Please confirm");
}
}
Below is example modal code - I have simiplified it so sorry for any mistakes. The code in the main body validates fine, but nothing I put in a modal with Fluent Validation works - it is just submitted without checks. I have several modals all doing the same thing. Data annotation validation works fine.
<div id="" class="modal" tabindex="-1" role="dialog">
<div class="modal-dialog-700" role="document">
<div class="modal-content modal-size">
<div class="modal-header">
<h6 class="modal-title">Test</h6>
<button type="button" class="close" data-dismiss="modal" aria-hidden="true">×</button>
</div>
<div class="modal-body">
<div class="container">
<div class="row justify-content-md-center">
<form asp-page-handler="update" method="post">
<div class="form-group">
<label asp-for="MyModel.Username" class="control-label">Test Code</label>
<textarea asp-for="MyModel.Username" class="form-control" rows="2"></textarea>
<span asp-validation-for="MyModel.Username" class="text-danger"></span>
</div>
<div class="form-group">
<button class="btn">
<span class="text">Update</span>
</button>
</div>
</form>
</div>
</div>
</div>
</div>
</div>
#section Scripts {
#{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}
I have also tried:
RuleFor(support => support.Found.ToString())
.Matches(#"^True$")

Answer 1

The real reason is that FluentValidation does not support all rules for client side validation.
Quoted from FluentValidation docs:
Note that not all rules defined in FluentValidation will work with ASP.NET’s client-side validation. For example, any rules defined using a condition (with When/Unless), custom validators, or calls to Must will not run on the client side. Nor will any rules in a RuleSet (although this can be changed - see the section below on “RuleSet for client-side messages”). The following validators are supported on the client:
NotNull/NotEmpty
Matches (regex)
InclusiveBetween (range)
CreditCard
Email
EqualTo (cross-property equality comparison)
MaxLength
MinLength
Length
So this is a behavior by design.

Answer 2

FluentValidation is a server-side framework, and does not provide any client-side validation directly. The following validators are supported on the client:
NotNull/NotEmpty
Matches (regex)
InclusiveBetween (range)
CreditCard
Email
EqualTo (cross-property equality comparison)
MaxLength
MinLength
Length
If you want to use validation rules on client-side, you can try FormHelper. Form Helper transforms server-side validations to client-side.
Nuget Package: https://www.nuget.org/packages/FormHelper
GitHub: https://github.com/sinanbozkus/formhelper