Array must contain 1 element

问题

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?

回答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; }

回答2:

It can be done using standard MinLengthAttribute validation attribute, but works ONLY for arrays:

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

回答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.

回答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;
    }
}

回答5:

MinLength attribute considers the value as valid if it's null. Therefore just initialize your property in the model as an empty array and it'll work.

MinLength(1, ErrorMessageResourceName = nameof(ValidationErrors.AtLeastOneSelected), ErrorMessageResourceType = typeof(ValidationErrors))]
int[] SelectedLanguages { get; set; } = new int[0];

来源：https://stackoverflow.com/questions/13361500/array-must-contain-1-element