C# Online Compiler | .NET Fiddle

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace StringBuilderToUTF8
{

    public static class StringExtensions
    {
        //set for UTF-8 encoding, no errors
        private static readonly Encoding Utf8Encoder = UTF8Encoding.GetEncoding("UTF-8", new EncoderReplacementFallback(string.Empty), new DecoderExceptionFallback());
        private static readonly Encoding Utf16Encoder = UTF8Encoding.GetEncoding("UTF-16", new EncoderReplacementFallback(string.Empty), new DecoderExceptionFallback());
        private static readonly Encoding ISO88591Encoder = UTF8Encoding.GetEncoding("ISO-8859-1", new EncoderReplacementFallback(string.Empty), new DecoderExceptionFallback());
        
        public static string toUTF8String(this StringBuilder sb) //incomming string is UTF-16
        {

            byte[] bytesUTF16 = Utf16Encoder.GetBytes(sb.ToString());
            char[] charsUTF16 = Utf16Encoder.GetChars(bytesUTF16);

            byte[] bytesUTF8 = Utf8Encoder.GetBytes(sb.ToString());
            char[] charsUTF8 = Utf8Encoder.GetChars(bytesUTF8);

            byte[] bytesISO88591 = ISO88591Encoder.GetBytes(sb.ToString());
            char[] charsISO88591 = ISO88591Encoder.GetChars(bytesUTF16);


            byte[] bytes = Encoding.Default.GetBytes(sb.ToString()); //defaults to ANSI string
            char[] chars = Encoding.Default.GetChars(bytes);

            //get clean UTF-8 string from stringbuilder char[]  
            //return Utf8Encoder.GetString(Utf8Encoder.GetBytes(charsISO88591));
            //return Utf8Encoder.GetString(ISO88591Encoder..GetBytes(charsISO88591));

            //return Encoding.Default.GetString(bytesISO88591, 0, bytesISO88591.Length); 
            //return ISO88591Encoder.GetString(bytesISO88591, 0, bytesISO88591.Length); 

            //Get UTF-8 string
            return Utf8Encoder.GetString(bytesUTF8, 0, bytesUTF8.Length); 
        }

        public static string toISO88591String(this StringBuilder sb)
        {

            byte[] bytesISO88591 = ISO88591Encoder.GetBytes(sb.ToString());
            return ISO88591Encoder.GetString(bytesISO88591, 0, bytesISO88591.Length); 
            
        }

  
        public static string toUTF8StringFastCopy(this StringBuilder sb)
        {

            char[] bytes = new char[sb.Length];

            //copies StringBuilder to []
            sb.CopyTo(0, bytes, 0, sb.Length);

            //get clean UTF-8 string from stringbuilder char[]  
            return Utf8Encoder.GetString(Utf8Encoder.GetBytes(bytes));

        }

        // Based on http://www.codeproject.com/Articles/13503/Stripping-Accents-from-Latin-Characters-A-Foray-in
        // Proper Normalization //Wed 08-Feb-23 9:34am metadataconsulting.ca - faster
        public static string UnicodeToANSI(this string s)
        {
            var newStringBuilder = new StringBuilder();
            newStringBuilder.Append(s.Normalize(NormalizationForm.FormKD)
                                    .Where(x => (x > 30 && x <= 255))
                                    .ToArray());
            return newStringBuilder.ToString();
        }




    }
    
    
    public class Program
    {
        public static void Main(string[] args)
        {
            //Console.OutputEncoding = Encoding.UTF8;

            //Search for “𤭢” code point, which is “U+24B62”
            //In case of a code points is over than U+10000, UTF16 encoding requires 2 bytes of 16 bits binary. We have to subtract 0x10000 from the code points. Now the remaining is “14B62”
            //Convert “14B62” to binary numbers.

            string utf16 = "Menü \uD852\uDF62 a\u0304\u0308  你好"; //The result of  “𤭢” or “U+24B62” will be 0xD852 0xDF62 or \uD852\uDF62

            //combining characters ("a\u0304\u0308" = "ā̈"), and surrogate pairs are interpretted

            StringBuilder sb = new StringBuilder(utf16);

            Console.WriteLine("Clean UTF8 only = " + utf16);

            Console.WriteLine("Clean ISO-8859-1 only = " + sb.toISO88591String());
            Console.WriteLine("Clean UTF8 only = " + sb.toUTF8String());
            Console.WriteLine("Clean UTF8 fast copy = " + sb.toUTF8StringFastCopy());

            

        }
    }
}

            //In case of a code points is over than U+10000, UTF16 encoding requires 2 bytes of 16 bits binary. We have to subtract 0x10000 from the code points. Now the remaining is “14B62”
 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
​
namespace StringBuilderToUTF8
{
​
    public static class StringExtensions
    {
        //set for UTF-8 encoding, no errors
        private static readonly Encoding Utf8Encoder = UTF8Encoding.GetEncoding("UTF-8", new EncoderReplacementFallback(string.Empty), new DecoderExceptionFallback());
        private static readonly Encoding Utf16Encoder = UTF8Encoding.GetEncoding("UTF-16", new EncoderReplacementFallback(string.Empty), new DecoderExceptionFallback());
        private static readonly Encoding ISO88591Encoder = UTF8Encoding.GetEncoding("ISO-8859-1", new EncoderReplacementFallback(string.Empty), new DecoderExceptionFallback());
        
        public static string toUTF8String(this StringBuilder sb) //incomming string is UTF-16
        {
​
            byte[] bytesUTF16 = Utf16Encoder.GetBytes(sb.ToString());
            char[] charsUTF16 = Utf16Encoder.GetChars(bytesUTF16);
​
            byte[] bytesUTF8 = Utf8Encoder.GetBytes(sb.ToString());
            char[] charsUTF8 = Utf8Encoder.GetChars(bytesUTF8);
​