Array must contain 1 element

Question

I have the following class:

public class CreateJob
{
    [Required]
    public int JobTypeId { get; set; }
    public string RequestedBy { get; set; }
    public JobTask[] TaskDescriptions { get; set; }
}

I'd like to have a data annotation above TaskDescriptions so that the array must contain at least one element? Much like [Required]. Is this possible?

Solution 1

I've seen a custom validation attribute used for this before, like this:

(I've given sample with a list but could be adapted for array or you could use list)

public class MustHaveOneElementAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        var list = value as IList;
        if (list != null)
        {
            return list.Count > 0;
        }
        return false;
    }
}

[MustHaveOneElementAttribute (ErrorMessage = "At least a task is required")]
public List<Person> TaskDescriptions { get; private set; }

// as of C# 8/9 this could be more elegantly done with     
public class MustHaveOneElementAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        return value is IList {Count: > 0};
    }
}

Credit to Antonio Falcão Jr. for elegance

Solution 2

It can be done using standard Required and MinLength validation attributes, but works ONLY for arrays:

public class CreateJob
{
    [Required]
    public int JobTypeId { get; set; }
    public string RequestedBy { get; set; }
    [Required, MinLength(1)]
    public JobTask[] TaskDescriptions { get; set; }
}

Solution 3

Please allow me a side note on using MinLengthAttribute with .NET Core.

Microsoft recommends using Razor Pages starting with .NET Core 2.0.

Currently, The validation with MinLengthAttribute on a property within the PageModel does not work:

[BindProperty]
[Required]
[MinLength(1)]
public IEnumerable<int> SelectedStores { get; set; }

ModelState.IsValid returns true when SelectedStores.Count() == 0.

Tested with .NET Core 2.1 Preview 2.

Solution 4

Here is a bit improved version of @dove solution which handles different types of collections such as HashSet, List etc...

public class MustHaveOneElementAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        var collection = value as IEnumerable;
        if (collection != null && collection.GetEnumerator().MoveNext())
        {
            return true;
        }
        return false;
    }
}

Solution 5

You have to use 2 standard annotation attribute

public class CreateJob
{
    [MaxLength(1), MinLength(1)]
    public JobTask[] TaskDescriptions { get; set; }
}

View more solutions

Comments

[John MacIntyre]

Apparently it's only available in .net 4.5+ as well. :-(

[Pluc]

A little more info about this attribute: You should use [Required] in conjunction with [MinLength(1)] because MinLength will not trigger if the array is null (not empty, null). Also important to note, this is not supported (correct me if I'm wrong) with default client side validators. It will only trigger the ModelState.IsValid.

[Cat_Clan]

That attribute also works for objects that implements the ICollection interface and strings.

[Mike Devenney]

You should add @Pluc's comment to your answer. Having the [Required] attribute makes your answer better than the accepted answer above.

[Lostaunaum]

Thank you this is exactly what I was looking for.

[whyleee]

[MinLength(1)] works with IEnumerable<T> in ASP.NET Core

[monty]

MinLength seems not to work correctly in .NET Core 2.2 (nor with arrays, IEnumerables, ICollections, IList....)

[Antonio Falcão Jr.]

As of C # 8/9, all of this code can be replaced by this: return value is IList {Count: > 0};

[tiennen07]

yeah, it appears to be broken on .NET 5, as well.

[Daniël J.M. Hoffman]

[MinLength(1)] attribute is working for my requirement to have a least 1 element in my List<T>, with .NET 6 Blazor.