FileExtensionsAttribute.cs

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using System.Collections.Generic;
using System.Globalization;
using System.IO;
using System.Linq;
 
namespace System.ComponentModel.DataAnnotations;
 
[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter, AllowMultiple = false)]
public sealed class FileExtensionsAttribute : DataTypeAttribute
{
    private string _extensions;
 
    public string Extensions
    {
        get
        {
            if (!string.IsNullOrWhiteSpace(_extensions))
            {
                return _extensions;
            }
            return "png,jpg,jpeg,gif";
        }
        set
        {
            _extensions = value;
        }
    }
 
    private string ExtensionsFormatted => ExtensionsParsed.Aggregate((string left, string right) => left + ", " + right);
 
    private string ExtensionsNormalized => Extensions.Replace(" ", string.Empty).Replace(".", string.Empty).ToLowerInvariant();
 
    private IEnumerable<string> ExtensionsParsed => from e in ExtensionsNormalized.Split(',')
        select "." + e;
 
    public FileExtensionsAttribute()
        : base(DataType.Upload)
    {
        base.DefaultErrorMessage = System.SR.FileExtensionsAttribute_Invalid;
    }
 
    public override string FormatErrorMessage(string name)
    {
        return string.Format(CultureInfo.CurrentCulture, base.ErrorMessageString, name, ExtensionsFormatted);
    }
 
    public override bool IsValid(object? value)
    {
        if (value != null)
        {
            if (value is string fileName)
            {
                return ValidateExtension(fileName);
            }
            return false;
        }
        return true;
    }
 
    private bool ValidateExtension(string fileName)
    {
        return ExtensionsParsed.Contains(Path.GetExtension(fileName).ToLowerInvariant());
    }
}