CollectionMinimumLengthAttribute.cs

public sealed class CollectionMinimumLengthAttribute : ValidationAttribute
{
    private const string ErrorMessageWithMax = "{0} must contain between {1} and {2} item(s).";
    private readonly int _minLength;

    public CollectionMinimumLengthAttribute(int min)
    {
        _minLength = min;
        MaxLength = null;

        ErrorMessage = "{0} must contain at least {1} item(s).";
    }

    public CollectionMinimumLengthAttribute(int min, int max)
        : this(min)
    {
        _minLength = min;
        MaxLength = max;
    }

    public int MinLength
    {
        get { return _minLength; }
    }

    public int? MaxLength { get; private set; }
    //Override default FormatErrorMessage Method  
    public override string FormatErrorMessage(string name)
    {
        if (MaxLength != null)
        {
            return string.Format(ErrorMessageWithMax, name, _minLength, MaxLength.Value);
        }
        return string.Format(ErrorMessage, name, _minLength);
    }

    public override bool IsValid(object value)
    {
        var list = value as IEnumerable<object>;

        if (list != null && list.Count() >= _minLength && (MaxLength == null || list.Count() <= MaxLength))
        {
            return true;
        }
        return false;
    }
}