How to test some conditions by code then validate or invalidate and show error message?

Question

I want to test if customer name is unique in database. if customer name added before then send validation error message.

In the past we test this add new error to ModelState like this:

ModelState.AddModelError("Name", "Some message");

How to do like this in BLAZOR???

Answer 1

I assume since you reference ModelState you want to know how forms and validation works in Blazor. Have you looked at the documentation?

https://learn.microsoft.com/en-us/aspnet/core/blazor/forms-validation?view=aspnetcore-3.0

This explains how to use a validation process to show errors in a form. As well as built-in validations ( [Required] etc) you can also create custom validations, e.g. How to create Custom Data Annotation Validators

Alternatively you can use a more powerful library such as Fluent Validation - see these articles for more help integrating this with Blazor:

https://blog.stevensanderson.com/2019/09/04/blazor-fluentvalidation/

https://chrissainty.com/using-fluentvalidation-for-forms-validation-in-razor-components/

Answer 2

I used FluentValidation with Steve Sanderson's implementation https://gist.github.com/SteveSandersonMS/090145d7511c5190f62a409752c60d00#file-fluentvalidator-cs

I then made a few modifications to it.

First I added a new interface based on IValidator.

public interface IMyValidator : IValidator
{

    /// <summary>
    /// This should be the objects primary key.
    /// </summary>
    object ObjectId { get; set; }
}

Then I changed the FluentValidator to implement IMyValidator and added a new parameter.

public class FluentValidator<TValidator> : ComponentBase where TValidator: IMyValidator,new()
{
    /// <summary>
    /// This should be the objects primary key.
    /// </summary>
   [Parameter] public object ObjectId { get; set; }

   ... continue with the rest of Steve Sanderson's code 
}

For my FluentValidation AbstractValidator I did the following.

public class InvestigatorValidator:AbstractValidator<IAccidentInvestigator>,IMyValidator
{

    public object ObjectId { get; set; }

    public InvestigatorValidator()
    {
        RuleFor(user=>user.LogonName).NotEmpty().NotNull().MaximumLength(100);
        RuleFor(user=>user.Email).NotEmpty().NotNull().MaximumLength(256);
        RuleFor(user=>user.FullName).NotEmpty().NotNull().MaximumLength(100);
        RuleFor(user=>user.RadioId).MaximumLength(25);
        RuleFor(user=>user.LogonName).MustAsync(async (userName, cancellation)=>
                                               {
                                                   var exists = await GetUserNameExists(userName);
                                                   return !exists;
                                               }).WithMessage("UserName must be unique.");
        RuleFor(user=>user.Email).MustAsync(async (email, cancellation)=>
                                               {
                                                   var exists = await GetEmailExists(email);
                                                   return !exists;
                                               }).WithMessage("Email must be unique.");
    }

    private async Task<bool> GetUserNameExists(string userName)
    {
        if(ObjectId is int badge)
        {
            await using var db = MyDbContext;
            var retVal = await db.AccidentInvestigators.AnyAsync(a=>a.Badge != badge && string.Equals(a.LogonName.ToLower(), userName.ToLower()));
            return retVal;
        }

        return false;
    }
    private async Task<bool> GetEmailExists(string email)
    {
        if(ObjectId is int badge)
        {
            await using var db = DbContext;
            var retVal = await db.AccidentInvestigators.AnyAsync(a=>a.Badge != badge && string.Equals(a.Email.ToLower(), email.ToLower()));
            return retVal;
        }
        return false;
    }
}

Then in my Razor Component Form I changed the FluentValidator to set the ObjectId.

<EditForm Model="_investigator" OnValidSubmit="Save">
    <FluentValidator TValidator="InvestigatorValidator" ObjectId="@_investigator.Badge"/>

     ... put the rest of your layout here
</EditForm>