Custom error message on model validation ASP.NET Core MVC

Question

I'm working with ASP.NET Core MVC project, in which we want to set custom message to required field with filed name instead of generic message given by framework.
For that I I have created a custom class as below:

public class GenericRequired : ValidationAttribute
{
    public GenericRequired() : base(() => "{0} is required")
    {

    }

    public override bool IsValid(object value)
    {
        if (value == null)
        {
            return false;
        }
        string str = value as string;
        if (str != null)
        {
            return (str.Trim().Length != 0);
        }
        return true;
    }
}

And using that class in a model.

[GenericRequired]
[DisplayName("Title")]        
public string Name { get; set; }

On view page:

<span asp-validation-for="Name" class="text-danger"></span>

But message not displaying or validation doesn't work. Is there any other way to make it work?

Solution

Your GenericRequired implementation works only for server-side validation. When creating a subclass of ValidationAttribute, you will only get server-side validation out of the box. In order to make this work with client-side validation, you would need to both implement IClientModelValidator and add a jQuery validator (instructions further down the linked page).

As I suggested in the comments, you can instead just subclass RequiredAttribute to get what you want, e.g.:

public class GenericRequired : RequiredAttribute
{
    public GenericRequired()
    {
        ErrorMessage = "{0} is required";
    }
}

All this does is change the ErrorMessage, leaving both the server-side and client-side validation in tact, which is far simpler for your use case.

Comments

[Kirk Larkin]

What's wrong with just using [Required(ErrorMessage = "{0} is required")]?

[Divyang Desai]

Thanks! @KirkLarkin I don't want to write is required every time, it will take time while changing the text.

[Kirk Larkin]

Yes, I thought you might come back with that. Your code works for me, so it must be something else you're missing. But anyway, I'd recommend you create a subclass of the RequiredAttribute and don't reinvent the IsValid functionality.

[Divyang Desai]

@KirkLarkin: Did you check with Core? it works? okay, I will give a try with RequiredAttribute

[Divyang Desai]

@KirkLarkin: Do I need to register GenericRequired class anywhere? I have checked with another solution, not working :(

[Divyang Desai]

Here to go:RazorPagesDemo customer model

[Divyang Desai]

I'm following this validation sample project for client side validation, however it also work server side :(

[Kirk Larkin]

Yes, of course. It implements both server-side and client-side validation, whereas your GenericRequired implements server-side only.