// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

namespace System.ComponentModel.DataAnnotations
{
    /// <summary>
    ///     Validation attribute to indicate that a property, field or parameter is required.
    /// </summary>
    [AttributeUsage(AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter,
        AllowMultiple = false)]
    public class RequiredAttribute : ValidationAttribute
    {
        /// <summary>
        ///     Default constructor.
        /// </summary>
        /// <remarks>
        ///     This constructor selects a reasonable default error message for
        ///     <see cref="ValidationAttribute.FormatErrorMessage" />
        /// </remarks>
        public RequiredAttribute()
            : base(() => SR.RequiredAttribute_ValidationError)
        {
        }

        /// <summary>
        ///     Gets or sets a flag indicating whether the attribute should allow empty strings.
        /// </summary>
        public bool AllowEmptyStrings { get; set; }

        /// <summary>
        ///     Override of <see cref="ValidationAttribute.IsValid(object)" />
        /// </summary>
        /// <param name="value">The value to test</param>
        /// <returns>
        ///     Returns <see langword="false" /> if the <paramref name="value" /> is null or an empty string.
        ///     If <see cref="AllowEmptyStrings" /> then <see langword="true" /> is returned for empty strings.
        /// </returns>
        public override bool IsValid(object? value)
        {
            if (value is null)
            {
                return false;
            }

            // only check string length if empty strings are not allowed
            return AllowEmptyStrings || value is not string stringValue || !string.IsNullOrWhiteSpace(stringValue);
        }
    }
}