Helper class for working with Entity Tags (ETags) in Web APIs

ETag.cs

using System;
using System.Globalization;

namespace Common
{
	/// <summary>
	/// A value type representing an Entity Tag (ETag).
	/// </summary>
	public struct ETag : IFormattable
	{
		/// <summary>Represents an empty <see cref="ETag"/>. This field is read-only.</summary>
		public static readonly ETag Empty = new ETag(string.Empty);

		private readonly string stringValue;

		private ETag(string stringValue)
			=> this.stringValue = stringValue ?? string.Empty;

		/// <summary>Indicates whether the value of the <see cref="ETag"/> is an integer.</summary>
		public bool IsInt32 
			=> int.TryParse(stringValue, out var _);

		/// <summary>Creates an <see cref="ETag"/> from an <see cref="int"/>.</summary>
		public static ETag FromInt32(int value)
			=> new ETag(value.ToString());

		/// <summary>Creates an <see cref="ETag"/> from a <see cref="string"/>.</summary>
		public static ETag FromString(string value)
			=> new ETag(value);

		/// <summary>Creates an <see cref="ETag"/> from a header value (enclosed in quotes).</summary>
		public static ETag FromHeaderValue(string value)
			=> new ETag(value.Trim('"'));

		/// <summary>Returns a value indicating whether this instance is equal to a specified object</summary>
		public override bool Equals(object obj)
			=> obj is ETag other ? this == other : false;

		/// <summary>Returns the hash code for this <see cref="ETag"/>.</summary>
		public override int GetHashCode()
			=> stringValue.GetHashCode();

		/// <summary>Converts the <see cref="ETag"/> to an <see cref="int"/>.</summary>
		public int ToInt32()
			=> int.Parse(stringValue);

		/// <summary>Converts the <see cref="ETag"/> to a <see cref="string"/>.</summary>
		public override string ToString()
			=> ToString("G");

		/// <summary>
		/// Formats the value of the current <see cref="ETag"/> using the specified format.
		/// Format "G" yields the general string representation. Format "E" yields the HTTP
		/// ETag format.
		/// </summary>
		public string ToString(string format)
			=> ToString(format, CultureInfo.InvariantCulture);

		/// <summary>
		/// Formats the value of the current <see cref="ETag"/> using the specified format.
		/// Format "G" yields the general string representation. Format "E" yields the HTTP
		/// ETag format.
		/// </summary>
		public string ToString(string format, IFormatProvider formatProvider)
			=> Format(format);

		/// <summary>Determines whether two specified <see cref="ETag"/>s have the same value.</summary>
		public static bool operator ==(ETag x, ETag y)
			=> x.stringValue == y.stringValue;

		/// <summary>Determines whether two specified <see cref="ETag"/>s have different values.</summary>
		public static bool operator !=(ETag x, ETag y)
			=> x.stringValue != y.stringValue;

		/// <summary>Provides implicit casting from <see cref="ETag"/> to <see cref="string"/>.</summary>
		public static implicit operator string(ETag t)
			=> t.stringValue;

		/// <summary>Provides implicit casting from <see cref="string"/> to <see cref="ETag"/>.</summary>
		public static implicit operator ETag(string s)
			=> FromString(s);

		/// <summary>Provides implicit casting from <see cref="ETag"/> to <see cref="int"/>.</summary>
		public static implicit operator int(ETag t)
			=> t.ToInt32();

		/// <summary>Provides implicit casting from <see cref="int"/> to <see cref="ETag"/>.</summary>
		public static implicit operator ETag(int i)
			=> FromInt32(i);

		private string Format(string format)
		{
			if (string.IsNullOrEmpty(format)) format = "G";

			switch (format.ToUpperInvariant())
			{
				case "G":
					return stringValue;
				case "E":
					return $@"""{stringValue}""";
				default:
					throw new FormatException($"The {format} format string is not supported.");
			}
		}
	}
}