How To validate Appsetting.json Configuration Values In .NET

Introduction

In this article, we are going to learn how to validate appsetting configuration values.

Generally, we store the application configuration in setting and reading those values through IConfiguration, and Option pattern, but sometimes there is are possibility of having this configuration error prone like.

Forgot to add settings values
Incorrect section name while reading the value
Data type mismatch
Typo error in property name

If anything above happens, the application won't throw an error at the application start, and the application can behave differently at runtime or throw an exception at runtime.

How to resolve this issue?

We have 3 ways to validate this.

We can use the Data Annotation to validate the configuration class.
Options Validation using Delegates.
We can integrate FluentValidation using the IValidationOptions interface.

First, we will see how to Add the Data Annotation to validate and read the setting configuration.

"Smtp": {
  "Server": "mail.whatever.com",
  "Port1": "25",
  "FromAddress": "[email protected]"
}

ConfigurationModel

public class SmtpOptions
{
    [Required(AllowEmptyStrings =false)]
    public string Server { get; set; }

    [Required(AllowEmptyStrings = false)]
    public string Port { get; set; }

    [Required(AllowEmptyStrings = false)]
    public string FromAddress { get; set; }
}

Add the code below to the Program.cs.

services.AddOptions<SmtpOptions>()
    .BindConfiguration("Smtp") // Bind the smtp section in config
    .ValidateDataAnnotations() //Enable the validation
    .ValidateOnStart(); //Validate on app start

To Read the value, inject the IOption<SmtpOption> in the Controller as shown below.

 [Route("api/[controller]")]
 [ApiController]
 public class EmployeeController : ControllerBase
 {
     private readonly SmtpOptions _smtp;

     public EmployeeController(IOptions<SmtpOptions> smtp)
     {

         _smtp = smtp.Value;
     }

     [HttpGet]
    public IActionResult GetEmployee()
     {
         return Ok();
     }

 }

Now try to run the application, and when this Employee Controller hits, we can see we are able to read and validate the app settings; at this time, our configuration is valid, so it won't throw any exception.

1app application

Now let's change the configuration intentionally, from Port to Port1, to see whether validation is happening or not.

2 Appsettings validation

When the application starts at that time, it will throw an above exception as the Port key is not available in the configuration as it is marked required in the configuration class.

Now, we will see the second approach with Options Validation using Delegates.

3 Appsetting Validate Delegates

The third approach is with IValidationOptions, which we can use for any complex validation, and the code can be placed outside of the Program. cs. For this, we need to create a Class SmtpConfigurationValidation which implements IValidateOptions<SmtpOptions> and this interface has a method called Validate; we can write our custom validation logic and return ValidationResult as shown in the code snippet below.

public class SmtpConfigurationValidation : IValidateOptions<SmtpOptions>
{
    public ValidateOptionsResult Validate(string? name, SmtpOptions options)
    {
        if (string.IsNullOrEmpty(options.Port))
        {
            return ValidateOptionsResult.Fail("Port is required field");
        }
        if(string.IsNullOrEmpty(options.Server))
        {
            return ValidateOptionsResult.Fail("Server is required field");
        }
        if (string.IsNullOrEmpty(options.FromAddress))
        {
            return ValidateOptionsResult.Fail("FromAddress is required field");
        }
        return ValidateOptionsResult.Success;
    }
}

Now we need to add this class to the DI container, as shown below. Bind the SMTP section and ValidateOnStart code.

services.TryAddSingleton<IValidateOptions<SmtpOptions>, SmtpConfigurationValidation>();

services.AddOptions<SmtpOptions>()
    .BindConfiguration("Smtp") // Bind the smtp section in config    
    .ValidateOnStart(); //Validate on app start

Now try to run the code, and here we can see it's throwing an error as we have changed key Port to Port1.

Appsettings Required validation

Now we will change Port1 to Port so that the configuration is valid, and it should read the configuration value as expected.

1 Appsettings read

git commit id. Configuration Validation Code

Summary

In this article, we learn about app settings configuration validation using Data Annotation, using Delegate, and IValidationOption interface.

I hope you enjoyed this article. Thanks for reading.