ASP.NET Core - Custom model validation

Question

In MVC when we post a model to an action we do the following in order to validate the model against the data annotation of that model:

if (ModelState.IsValid) 

If we mark a property as [Required], the ModelState.IsValid will validate that property if contains a value or not.

My question: How can I manually build and run custom validator?

P.S. I am talking about backend validator only.

Answer 1

In .NET Core, you can simply create a class that inherits from ValidationAttribute. You can see the full details in the ASP.NET Core MVC Docs.

Here's the example taken straight from the docs:

public class ClassicMovieAttribute : ValidationAttribute {     private int _year;      public ClassicMovieAttribute(int Year)     {         _year = Year;     }      protected override ValidationResult IsValid(object value, ValidationContext validationContext)     {         Movie movie = (Movie)validationContext.ObjectInstance;          if (movie.Genre == Genre.Classic && movie.ReleaseDate.Year > _year)         {             return new ValidationResult(GetErrorMessage());         }          return ValidationResult.Success;     } } 

I've adapted the example to exclude client-side validation, as requested in your question.

In order to use this new attribute (again, taken from the docs), you need to add it to the relevant field:

[ClassicMovie(1960)] [DataType(DataType.Date)] public DateTime ReleaseDate { get; set; } 

Here's another, simpler example for ensuring that a value is true:

public class EnforceTrueAttribute : ValidationAttribute {     public EnforceTrueAttribute()         : base("The {0} field must be true.") { }      public override bool IsValid(object value) =>         value is bool valueAsBool && valueAsBool; } 

This is applied in the same way:

[EnforceTrue] public bool ThisShouldBeTrue { get; set; } 

Edit: Front-End Code as requested:

<div asp-validation-summary="All" class="text-danger"></div> 

The options are All, ModelOnly or None.