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)
         


        

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:

The options are All, ModelOnly or None.