C# Custom ValidationAttribute using lookup table from DB

Question

Hello I am facing a small design problem with my application (.NET Core 2.2 API).

I want to create a custom ValidationAttribute that would be able to check if a certain property matches a Regex pattern.

What differs this attribute from built-in RegularExpressionAttribute is that patterns will be stored in the DB lookup table in format [column_name|regex_patter].

For simplicity, let's say that a class that is used to load this lookup table is called IDataRepository and a method that returns that dictionary is called LoadRegexPatterns()

I would like to be able to use the attribute like:

[RegexPattern("human_name")]
public string FirstName {get;set;}

[RegexPattern("human_name")]
public string LastName {get;set;}

I am not sure which place would be the best to load the lookoup dictionary and how to handle passing it, in the way custom ValidationAttribute could use it. I would appreciate any tips.

I can't use IDataRepository inside an attribute class, because I would have to inject it there.
Maybe a "singleton lookup dictionary" with values from the BD? But still - how to handle it inside the Startup class so it could be used by ValidationAttribute

public sealed class RegexPatternAttribute : ValidationAttribute { private readonly string _columnName;

public RegexPatternAttribute(string columnName)
{
    _columnName = columnName;
}

protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
    // var regexPattern = LOAD REGEX PATTERN SHOULD BE LOADED HERE FROM SOME KIND OF LOOKUP TABLE
    string inputValue = value.ToString();

    if (Regex.IsMatch(inputValue,regexPattern))
    {
        return ValidationResult.Success;
    }
    else
    {
        return new ValidationResult(ErrorMessage);
    }
}

}

Create an object that implement the required IDataRepository interface and pass it to the custom validation attribute. Check the following post: Passing parameter to a custom validation attribute in asp.net mvc — Victor
– Victor, Commented Sep 1, 2021 at 12:41

pfx · Accepted Answer · 2021-09-01 17:14:06Z

0

Since ValidationContext implements IServiceProvider, you can use its GetService method to retrieve any registered dependency from within an attributes IsValid method.

Note that you can retrieve any service, e.g. one (in StartUp.cs) preconfigured with the data from the database.
The ValidationContext as a IServiceProvider offers a means to retrieve data that can't be configured via the properties of an attribute.

protected override ValidationResult IsValid(object value, ValidationContext validationContext)
{
    var dataRepository = validationContext.GetService(typeof(IDataRepository ));    
    
    // ...  
}

Make sure you have that IDataRepository registered in your container.

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddScoped<IDataRepository, DataRepository>();
        // or any other lifetime scope.

        // ...

    }
}

edited Sep 1, 2021 at 17:14

answered Sep 1, 2021 at 12:51

pfx

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5 Comments

Kamil Turowski Over a year ago

I will check that in a second, but I already see the problem with this approach. It would call the Database on each Attribute check. There is no need for that in my case, as this lookup data will not change frequently.

pfx Over a year ago

You van keep a field in the attribute with the data you need, only when it is null you call that repository.

GH DevOps Over a year ago

Don't use service locator pattern in validation attributes. I wouldn't call the db either. You're just setting yourself up for trouble. What if there's an exception? Then what do you do? Intercept your route config and redirect and blahh...The best approach is to validate inside the controller, add any errors to the model state, and then check if the model state is valid.

pfx Over a year ago

@GHDevOps How does that differ when an exception occurs? Both setups will end up in an error, or at best in an exception handler middleware if one is present.

mxmissile Over a year ago

I've used service location in my validation attributes for years, no issues.

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