WebEncoders

Base64Encoding.UsingWebEncoders.cs

public static string UsingWebEncoders(string input)
{
    byte[] inputBytes = null;
    char[] outputChars = null;

    try
    {
        var minimumLength = Encoding.UTF8.GetByteCount(input);
        inputBytes = ArrayPool<byte>.Shared.Rent(minimumLength);
        int inputBytesCount = Encoding.UTF8.GetBytes(input, 0, input.Length, inputBytes, 0);

        var outputSize = WebEncoders.GetArraySizeRequiredToEncode(inputBytesCount);
        outputChars = ArrayPool<char>.Shared.Rent(outputSize);

        var charsSize = WebEncoders.Base64UrlEncode(inputBytes, 0, outputChars, 0, inputBytesCount);
        return new string(outputChars, 0, charsSize);
    }
    finally
    {
        if (inputBytes != null) ArrayPool<byte>.Shared.Return(inputBytes);
        if (outputChars != null) ArrayPool<char>.Shared.Return(outputChars);
    }
}