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Showing content from http://valitdocs.readthedocs.io/en/latest/validation-errors/index.html below:

Validation errors — Valit 0.1.0 documentation

Having a validation result (so succeeded or failed) is fine but in most cases we want to provide more details to the end-user or other parts of the system. Valit provides two different mechanisms for such a thing:

• error messages
• error codes

In order to add an error message to the specific validation rule use WithMessage() extension. All messages are stored inside ErrorMessages property in the IValitResult object. The example below present the usage of error messages:

public class RegisterModel
{
    public string Email { get; set; }
    public int Age { get; set; }
}

void ValidateModel(RegisterModel model)
{
    IValitResult result = ValitRules<RegisterModel>
        .Create()
        .Ensure(m => m.Email, _=>_
            .Required()
                .WithMessage("Email is required")
            .Email()
                .WithMessage("Email is incorrect"))
        .For(model)
        .Validate();

    if(!result.Succeeded)
    {
        foreach(var message in result.ErrorMessages)
        {
            Console.WriteLine(message);
        }
    }
}

WithMessage() has also an overload which accepts Func<string> as parameter:

public class RegisterModel
{
    public string Email { get; set; }
    public int Age { get; set; }
}

void ValidateModel(RegisterModel model)
{
    Func<string> getRequiredMessage = () => "Email is required";
    Func<string> getIncorrectMessage = () => "Email is incorrect";

    IValitResult result = ValitRules<RegisterModel>
        .Create()
        .Ensure(m => m.Email, _=>_
            .Required()
                .WithMessage(getRequiredMessage)
            .Email()
                .WithMessage(getIncorrectMessage)
        .For(model)
        .Validate();

    if(!result.Succeeded)
    {
        foreach(var message in result.ErrorMessages)
        {
            Console.WriteLine(message);
        }
    }
}

Note

If no error message is specified for particular rule, Valit uses its own, default message template. The full list of default error messages can be found here.

The above example is cool but it has one disadvantage - all messages are hardcoded. Of course it’s nothing really bad for a small projects but in a typical scenario you typically store that kind of data somewhere else like database. That’s why besides error messages Valit supports also message providers. The usage is fairly simple:

1. Create a class which implements generic interface IValitMessageProvider<TKey>. This interface provides one method called GetByKey(TKey key).
2. Pass the instance of your provider to ValitRules using WithMessageProvider() extension.
3. Use WithMessageKey() extension on each rule you want to extend with custom message.

Here’s an example:

public class RegisterModel
{
    public string Email { get; set; }
    public int Age { get; set; }
}

public class MyMessageProvider : IValitMessageProvider<string>
{
    private readonly IReadOnlyDictionary<string, string> _messages = new Dictionary<string, string>
    {
        { "Key1", "Email is required" },
        { "Key2", "Email is incorrect" },
    };

    public string GetByKey(string key)
        => _messages[key];
}

void ValidateModel(RegisterModel model)
{
    IValitResult result = ValitRules<RegisterModel>
        .Create()
        .WithMessageProvider(new MyMessageProvider())
        .Ensure(m => m.Email, _=>_
            .Required()
                .WithMessageKey("Key1")
            .Email()
                .WithMessageKey("Key2"))
        .For(model)
        .Validate();

    if(!result.Succeeded)
    {
        foreach(var message in result.ErrorMessages)
        {
            Console.WriteLine(message);
        }
    }
}

Notice that MyMessageProvider uses string as a key type, but you can choose any other type you want.

Valit also supports error codes as an alternative approach to messages. You can add one using WithErrorCode() extension on each validation rule. All error codes are stored inside ErrorCodes property in the IValitResult object. The example below present the usage of error codes:

public class RegisterModel
{
    public string Email { get; set; }
    public int Age { get; set; }
}

void ValidateModel(RegisterModel model)
{
    IValitResult result = ValitRules<RegisterModel>
        .Create()
        .WithMessageProvider(new MyMessageProvider())
        .Ensure(m => m.Email, _=>_
            .Required()
                .WithErrorCode(1103)
            .Email()
                .WithErrorCode(1107))
        .For(model)
        .Validate();

    if(!result.Succeeded)
    {
        foreach(var code in result.ErrorCodes)
        {
            Console.WriteLine(code);
        }
    }
}

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