C# Online Compiler | .NET Fiddle

using System.Linq; 
using System.Diagnostics;
using System;
using System.Text;
using System.Globalization; 
					
public static class Program
{
    public static string UnicodeReplaceAt(this string str, int offset, char replaceChar)
    {
		int count=1; 
        string replaceBy = replaceChar.ToString(); 
		return new StringInfo(str).ReplaceByPosition(replaceBy, offset, count).String;
    }

    public static StringInfo ReplaceByPosition(this StringInfo str, string replaceBy, int offset, int count)
    {
        return str.RemoveByTextElements(offset, count).InsertByTextElements(offset, replaceBy);
    }

    public static StringInfo RemoveByTextElements(this StringInfo str, int offset, int count)
    {
        return new StringInfo(string.Concat(
            str.SubstringByTextElements(0, offset),
            offset + count < str.LengthInTextElements
                ? str.SubstringByTextElements(offset + count, str.LengthInTextElements - count - offset)
                : ""
            ));
    }
    public static StringInfo InsertByTextElements(this StringInfo str, int offset, string insertStr)
    {
        if (string.IsNullOrEmpty(str.String))
            return new StringInfo(insertStr);
		
        return new StringInfo(string.Concat(
            str.SubstringByTextElements(0, offset),
            insertStr,
            str.LengthInTextElements - offset > 0 ? str.SubstringByTextElements(offset, str.LengthInTextElements - offset) : ""
        ));
    }	
	
	
   public static string SubsituteStringStringBuilder(this string s, int idx, char replaceChar)
   {
       if (string.IsNullOrEmpty(s) || idx >= s.Length || idx < 0) 
                    return s; 
		 
	   return new StringBuilder(s).Remove(idx, 1).Insert(idx, replaceChar.ToString()).ToString();
   }

	public static string ReplaceAtSubstring(this string s, int idx, char replaceChar)
    {
		if (string.IsNullOrEmpty(s) || idx >= s.Length || idx < 0) 
                    return s; 
		
		return s.Substring(0,idx) + replaceChar.ToString() + s.Substring(idx+replaceChar.ToString().Length,s.Length - (idx+replaceChar.ToString().Length) );
        
    }
	
	public static string ReplaceAtStringManipulation(this string s, int idx, char replaceChar)
    {
		if (string.IsNullOrEmpty(s) || idx >= s.Length || idx < 0) 
                    return s; 
	
      	 return s.Remove(idx, 1).Insert(idx, replaceChar.ToString());
    }
	
	public static string ReplaceAtLinq(this string value, int index, char newchar)
	{
		if (value.Length <= index)
			return value;
		else
			return string.Concat(value.Select((c, i) => i == index ? newchar : c));
	}

	public static string ReplaceAtCharArray(this string input, uint index, char newChar)
	{
		if (string.IsNullOrEmpty(input) || index >= input.Length) 
			return input;

		char[] chars = input.ToCharArray();
		chars[index] = newChar;
		return new string(chars);
	}

	public static void Main()
	{
		Stopwatch sw = new Stopwatch();
		sw.Start();
		Console.WriteLine("ReplaceAtCharArraySSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSƾƾƾƾ".ReplaceAtCharArray(70,'X')); 
		sw.Stop();
		Console.WriteLine("in {0} ticks.",sw.ElapsedTicks.ToString("N0")); 
		
		sw.Restart();
		Console.WriteLine("ReplaceAtLinqSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSƾƾƾƾ".ReplaceAtLinq(70,'Y')); 
		sw.Stop();
		Console.WriteLine("in {0} ticks.",sw.ElapsedTicks.ToString("N0")); 
		
		sw.Restart();
		Console.WriteLine("ReplaceAtStringManipulationSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSƾƾƾƾ".ReplaceAtStringManipulation(70,'Z')); 
		sw.Stop();
		Console.WriteLine("in {0} ticks.",sw.ElapsedTicks.ToString("N0")); 
		
		sw.Restart();
		Console.WriteLine("ReplaceAtSubstringgggggggggSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSƾƾƾƾ".ReplaceAtSubstring(70,'R')); 
		sw.Stop();
		Console.WriteLine("in {0} ticks.",sw.ElapsedTicks.ToString("N0")); 

		sw.Restart();
		Console.WriteLine("SubsituteStringStringBuilderSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSƾƾƾƾ".SubsituteStringStringBuilder(70,'Ɯ')); 
		sw.Stop();
		Console.WriteLine("in {0} ticks.",sw.ElapsedTicks.ToString("N0")); 

		sw.Restart();
		Console.WriteLine("UnicodeReplaceAtSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSƾƾƾƾ".UnicodeReplaceAt(50,'Ɯ')); 
		sw.Stop();
		Console.WriteLine("in {0} ticks.",sw.ElapsedTicks.ToString("N0")); 

		
		
	}
}

        return s.Substring(0,idx) + replaceChar.ToString() + s.Substring(idx+replaceChar.ToString().Length,s.Length - (idx+replaceChar.ToString().Length) );
 
using System.Linq; 
using System.Diagnostics;
using System;
using System.Text;
using System.Globalization; 
                    
public static class Program
{
    public static string UnicodeReplaceAt(this string str, int offset, char replaceChar)
    {
        int count=1; 
        string replaceBy = replaceChar.ToString(); 
        return new StringInfo(str).ReplaceByPosition(replaceBy, offset, count).String;
    }
​
    public static StringInfo ReplaceByPosition(this StringInfo str, string replaceBy, int offset, int count)
    {
        return str.RemoveByTextElements(offset, count).InsertByTextElements(offset, replaceBy);
    }
​
    public static StringInfo RemoveByTextElements(this StringInfo str, int offset, int count)
    {
        return new StringInfo(string.Concat(
            str.SubstringByTextElements(0, offset),