I have implemented a custom validation attribute to check column uniqueness. I want to check if the provided value already exists in the database or not. Here is my code:

[AttributeUsage(AttributeTargets.Property, AllowMultiple = true, Inherited = true)]
public class UniqueAttribute : ValidationAttribute
{
    public UniqueAttribute()
    {
    }

    public override bool RequiresValidationContext => true;

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        return ValidationResult.Success;
    }
    
}

the validationContext in IsValid method always returns null. How it can be fixed?

CodePudding user response：

Below is an example to check if the provided value already exists in the database or not, you can refer to it. UniqueAttribute:

 public class UniqueAttribute : ValidationAttribute
    {
        protected override ValidationResult IsValid(object value,ValidationContext validationContext)
        {
            var context = (MvcMovieContext)validationContext.GetService(typeof(MvcMovieContext));//change the MvcMovieContext to your DbContext
            if (!context.Movie.Any(a => a.Company == value.ToString()))
            {
                return ValidationResult.Success;
            }
            return new ValidationResult("Company exists");
        }
}

Movie:

public class Movie
    {           
        [Unique]
        public string Company { get; set; }          
    }

Create view:

@model MvcMovie.Models.Movie

@{
    ViewData["Title"] = "Create";
}

<h1>Create</h1>

<h4>Movie</h4>
<hr />
<div >
    <div >
        <form asp-action="Create">
            <div asp-validation-summary="ModelOnly" ></div>                
            <div >
                <label asp-for="Company" ></label>
                <input asp-for="Company"  />
                <span asp-validation-for="Company" ></span>
            </div>               
            <div >
                <input type="submit" value="Create"  />
            </div>
        </form>
    </div>
</div>

@section Scripts {
    @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
}

result:

CodePudding user response：

@Quin Guo Here is my Startup.cs

public void ConfigureServices(IServiceCollection services)
        {
            services.AddApiVersioning(o =>
            {
                o.ReportApiVersions = true;
                o.AssumeDefaultVersionWhenUnspecified = true;
                o.DefaultApiVersion = new ApiVersion(1, 0);
            });

            services.AddDbContext<ApplicationDbContext>(options =>
                options.UseSqlServer(
                    Configuration.GetConnectionString("DefaultConnection")));

            services.AddDatabaseDeveloperPageExceptionFilter();

            services.AddDefaultIdentity<ApplicationUser>(options => options.SignIn.RequireConfirmedAccount = false)
                .AddRoles<IdentityRole>()
                .AddEntityFrameworkStores<ApplicationDbContext>();

            services.AddIdentityServer()
                .AddApiAuthorization<ApplicationUser, ApplicationDbContext>();

            services.AddAuthentication()
                .AddIdentityServerJwt();

            services.AddScoped<IClaimsTransformation, ClaimsTransformation>();

            RegisterRepository(services);
            RegisterServices(services);
            RegisterAutoMapper(services);

            services.AddControllersWithViews()
            .AddJsonOptions(opts => opts.JsonSerializerOptions.PropertyNamingPolicy = null);

            services.AddRazorPages();
            // In production, the Angular files will be served from this directory
            services.AddSpaStaticFiles(configuration =>
            {
                configuration.RootPath = "ClientApp/dist";
            });
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
                app.UseMigrationsEndPoint();
            }
            else
            {
                app.UseExceptionHandler("/Error");
                // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
                app.UseHsts();
            }

            app.UseHttpsRedirection();
            app.UseStaticFiles();
            if (!env.IsDevelopment())
            {
                app.UseSpaStaticFiles();
            }

            app.UseRouting();

            app.UseAuthentication();
            app.UseIdentityServer();
            app.UseAuthorization();
            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllerRoute(
                    name: "default",
                    pattern: "{controller}/{action=Index}/{id?}");
                endpoints.MapRazorPages();
            });

            app.UseSpa(spa =>
            {
                // To learn more about options for serving an Angular SPA from ASP.NET Core,
                // see https://go.microsoft.com/fwlink/?linkid=864501

                spa.Options.SourcePath = "ClientApp";

                if (env.IsDevelopment())
                {
                    spa.UseAngularCliServer(npmScript: "start");
                }
            });
        }

Model class

public class Category
    {
        [Unique]
        public string Name { get; set; }
        public string Description { get; set; }
    }