What is the best way to get a configured value to a class library for validation? - validation

Question

I currently have a Blazor app that references a class library. One of the pages in the web app is used for updating an instance of a class model in the class library. To validate I'm using Validation attributes on the class model. One of the fields for input is email which, for our software, is validated via a configurable regular expression (because each of our sites can be different).
I think the best way to do this is using a custom ValidationAttribute but I don't know the best way to get a value from the web app's app settings to the custom Validation class.
The following code is an example of what I'm trying to accomplish:
Blazor page:
<EditForm Model="#name" OnValidSubmit="HandleValidSubmit">
<DataAnnotationsValidator />
<ValidationSummary />
<div>
<label for="FirstName">First:</label>
</div>
<div>
<input id="FirstName" class="form-control profile-form-control" #bind-value="name.FirstName" #bind-value:event="oninput" type="text" maxlength="30" autocomplete="off" />
</div>
<div>
<label for="LastName">Last:</label>
</div>
<div>
<input id="LastName" class="form-control profile-form-control" #bind-value="name.LastName" #bind-value:event="oninput" type="text" maxlength="100" autocomplete="off" />
</div>
<div>
<label for="Email">Email:</label>
</div>
<div>
<input id="Email" class="form-control profile-form-control" #bind-value="name.Email" #bind-value:event="oninput" type="text" maxlength="100" autocomplete="off" />
</div>
<div>
<button type="submit" class="btn btn-primary">
Save
</button>
</div>
</EditForm>
Model (in separate class library):
public class Person
{
[Required(ErrorMessage = "First name required")]
public string FirstName { get; set; }
[Required(ErrorMessage = "Last name required")]
public string LastName { get; set; }
[Required(ErrorMessage = "Email required")]
[EmailFromRegexValidator(ErrorMessage = "Email not valid")]
public string Email { get; set; }
}
Custom Validation:
public class EmailFromRegexValidator : ValidationAttribute
{
private const string defaultEmailValidationRegex = "^[\\w-]+(\\.[\\w-]+)*#([a-zA-Z0-9-]+(\\.[a-zA-Z0-9-]+)*?\\.[a-zA-Z]{2,6}|(\\d{1,3}\\.){3}\\d{1,3})(:\\d{4})?$";
protected override ValidationResult IsValid(object value, ValidationContext context)
{
string emailRegexString = null;
var configurationBuilder = new ConfigurationBuilder();
var path = Path.Combine(Directory.GetCurrentDirectory(), "appsettings.json");
if (File.Exists(path))
{
configurationBuilder.AddJsonFile(path, false);
var root = configurationBuilder.Build();
emailRegexString = root.GetSection("AppConfiguration").GetSection("EmailRegex").Value;
}
emailRegexString = emailRegexString ?? defaultEmailValidationRegex;
Regex emailRegex = new Regex(emailRegexString);
if (value is string && emailRegex.IsMatch(value as string))
{
return ValidationResult.Success;
}
else
{
return new ValidationResult(FormatErrorMessage(context.DisplayName));
}
}
}
The above code works but building configuration from a file path within a class library does not feel optimal. So I was curious if anyone had any better ideas for how to get a configurable value to the EmailFromRegexValidator?
Thanks!

Answer 1

Welcome! You can use ValidationContext to access a service that provides your configuration value.
protected override ValidationResult IsValid(object value, ValidationContext context)
{
var appConfig = (AppConfiguration) validationContext
.GetService(typeof(AppConfiguration));
string emailRegexString = appConfig.EmailRegex;
...
}
public class AppConfiguration
{
public string EmailRegex { get; set; }
}
You can bind AppConfiguration from the config file using to make it available in your DI container:
services.Configure<AppConfiguration>(configuration.GetSection("AppConfiguration"))