您可能想要查看此站点：http://prettycode.org/2009/11/12/short-guid/
它看起来和你现在做的很像。

public class ShortGuid
{
    private readonly Guid guid;
    private readonly string value;

    /// <summary>Create a 22-character case-sensitive short GUID.</summary>
    public ShortGuid(Guid guid)
    {
        if (guid == null)
        {
            throw new ArgumentNullException("guid");
        }

        this.guid = guid;
        this.value = Convert.ToBase64String(guid.ToByteArray())
            .Substring(0, 22)
            .Replace("/", "_")
            .Replace("+", "-");
    }

    /// <summary>Get the short GUID as a string.</summary>
    public override string ToString()
    {
        return this.value;
    }

    /// <summary>Get the Guid object from which the short GUID was created.</summary>
    public Guid ToGuid()
    {
        return this.guid;
    }

    /// <summary>Get a short GUID as a Guid object.</summary>
    /// <exception cref="System.ArgumentNullException"></exception>
    /// <exception cref="System.FormatException"></exception>
    public static ShortGuid Parse(string shortGuid)
    {
        if (shortGuid == null)
        {
            throw new ArgumentNullException("shortGuid");
        }
        else if (shortGuid.Length != 22)
        {
            throw new FormatException("Input string was not in a correct format.");
        }

        return new ShortGuid(new Guid(Convert.FromBase64String
            (shortGuid.Replace("_", "/").Replace("-", "+") + "==")));
    }

    public static implicit operator String(ShortGuid guid)
    {
        return guid.ToString();
    }

    public static implicit operator Guid(ShortGuid shortGuid)
    {
        return shortGuid.guid;
    }
}

使用该技术来格式化GUID以用于URL或文件名的一个问题是，两个不同的GUID可以产生仅大小写不同的两个值，例如：

var b1 = GuidToBase64(new Guid("c9d045f3-e21c-46d0-971d-b92ebc2ab83c"));
    var b2 = GuidToBase64(new Guid("c9d045f3-e21c-46d0-971d-b92ebc2ab8a4"));
    Console.WriteLine(b1);  // 80XQyRzi0EaXHbkuvCq4PA
    Console.WriteLine(b2);  // 80XQyRzi0EaXHbkuvCq4pA

由于URL有时被解释为不区分大小写，而在Windows中，文件路径和文件名不区分大小写，因此这可能会导致冲突。

我知道你在最后删掉==的原因是因为你可以确定对于GUID（16字节），编码后的字符串 * 总是 * 以==结尾，所以每次转换可以节省2个字符。
除了已经提到的@Skurmedal（在输入无效字符串的情况下应该抛出异常），我认为您发布的代码已经足够好了。

如果你的方法不能将传递给它的Base64转换成GUID，你不应该抛出一个异常吗？传递给方法的数据显然是错误的。

在.NET Core中，您可以使用Spans来获得更好的性能，并且无需分配内存。

using System.Buffers.Text;
using System.Runtime.InteropServices;

namespace Extensions;

public static class GuidExtensions
{
    private const char Dash = '-';
    private const char EqualsChar = '=';
    private const byte ForwardSlashByte = (byte)Slash;
    private const char Plus = '+';
    private const byte PlusByte = (byte)Plus;
    private const char Slash = '/';
    private const char Underscore = '_';
    private const int Base64LengthWithoutEquals = 22;

    public static string EncodeBase64String(this Guid guid)
    {
        Span<byte> guidBytes = stackalloc byte[16];
        Span<byte> encodedBytes = stackalloc byte[24];

        MemoryMarshal.TryWrite(guidBytes, ref guid);
        Base64.EncodeToUtf8(guidBytes, encodedBytes, out _, out _);

        Span<char> chars = stackalloc char[Base64LengthWithoutEquals];

        // Replace any characters which are not URL safe.
        // And skip the final two bytes as these will be '==' padding we don't need.
        for (int i = 0; i < Base64LengthWithoutEquals; i++)
        {
            chars[i] = encodedBytes[i] switch
            {
                ForwardSlashByte => Dash,
                PlusByte => Underscore,
                _ => (char)encodedBytes[i],
            };
        }

        return new(chars);
    }

    public static Guid DecodeBase64String(this ReadOnlySpan<char> id)
    {
        Span<char> base64Chars = stackalloc char[24];

        for (var i = 0; i < Base64LengthWithoutEquals; i++)
        {
            base64Chars[i] = id[i] switch
            {
                Dash => Slash,
                Underscore => Plus,
                _ => id[i],
            };
        }

        base64Chars[22] = EqualsChar;
        base64Chars[23] = EqualsChar;

        Span<byte> idBytes = stackalloc byte[16];
        Convert.TryFromBase64Chars(base64Chars, idBytes, out _);

        return new(idBytes);
    }
}

using AutoFixture.Xunit2;
using FluentAssertions;
using Extensions;
using Xunit;

namespace ExtensionTests;

public class GuidExtensionsTests
{
    private const int Base64LengthWithoutEquals = 22;
    private const string EmptyBase64 = "AAAAAAAAAAAAAAAAAAAAAA";

    [Theory]
    [AutoData]
    public void EncodeBase64String_DecodeBase64String_Should_ReturnInitialGuid(Guid guid)
    {
        string actualBase64 = guid.EncodeBase64String();
        actualBase64.Should().NotBe(string.Empty)
            .And.HaveLength(Base64LengthWithoutEquals);

        Guid actualGuid = ((ReadOnlySpan<char>)actualBase64).DecodeBase64String();
        actualGuid.Should().Be(guid);
    }

    [Theory]
    [InlineData(EmptyBase64)]
    public void EncodeBase64String_Should_ReturnEmptyBase64_When_GuidIsEmpty(string expected)
    {
        string actualBase64 = Guid.Empty.EncodeBase64String();
        actualBase64.Should().Be(expected);
    }

    [Theory]
    [InlineData(EmptyBase64)]
    public void DecodeBase64String_Should_ReturnEmptyGuid_When_StringIsEmptyBase64(string base64)
    {
        Guid actual = ((ReadOnlySpan<char>)base64).DecodeBase64String();
        actual.Should().Be(Guid.Empty);
    }
}

要了解更多信息，请阅读有关使用high-performance techniques to base64 encode a guid的信息，以及一个非常好的video explanation。

有一个方法，我正在使用编码以及缩短我的URL（Guid）：https://dotnetfiddle.net/iQ7nGv

public static void Main()
{
    Guid gg = Guid.NewGuid();
    string ss = Encode(gg);
    Console.WriteLine(gg);
    Console.WriteLine(ss);
    Console.WriteLine(Decode(ss));
}

public static string Encode(Guid guid)
{
    string encoded = Convert.ToBase64String(guid.ToByteArray());
    encoded = encoded.Replace("/", "_").Replace("+", "-");
    return encoded.Substring(0, 22);
}

public static Guid Decode(string value)
{
    value = value.Replace("_", "/").Replace("-", "+");
    byte[] buffer = Convert.FromBase64String(value + "==");
    return new Guid(buffer);
}