C# Online Compiler | .NET Fiddle

using System;using System.Linq;
public static class Program
{ 
	private static string seperator = "~"; //must be single character string, tilde is a valid filename char, may want to use a non-valid char for filenames.
	private const int leadingnumbers = 5;  //number of leading characters to build up to
    /// <summary>
    /// Format filename to a length (maxFileLen), by splitting filename into 1/3rd leading and 2/3rds ending until max leadingnumbers is reached. Used for toast nofications less than 41 characters by metadataconsulting.blogspot.com
    /// </summary>
    /// <param name="filename"></param>
    /// <param name="maxFileLen"></param>
    /// <returns></returns>
	public static string TrimMidFileNameFormat(this string filename, int maxFileLen)
	{
	
		 Console.Write(string.Format("{0,5:###}", maxFileLen)+" "); //remove line when using in your library

		 string ext = filename.GetFileNameExtension(); 
		 int extLen = ext.Length;  //includes dot in length 
		 string filenamelessext = filename.Substring(0, filename.Length - extLen);
		 int filenamelessextLen = filenamelessext.Length;
		 int maxroom = Math.Min(leadingnumbers * 2 - 1, filenamelessextLen); 
		 int minLen  = 2 + 1 + extLen; //1st char + tilde.length(1) + last char + extLen(min 4) => 7 ie s~p.exe
		 int lastLen = 1 + extLen; // last char + extLen(min 4) ie p.exe

		 if (filename.Length <= maxFileLen)
			return filename;
		 else if (filename.Length < minLen)
			 return filename;
		 else if (maxFileLen < minLen) //implied && filename.Length > maxFileLen) 
			 return filename.Substring(0, 1) + seperator + filename.Substring(filename.Length-lastLen, lastLen);
		 else if (maxFileLen >= minLen && maxFileLen < (maxroom + minLen)) //implied && filename.Length > maxFileLen) { //middle
		 {
			  int room = (maxFileLen - lastLen); 		 
			  int one3rd = (3 + room - 1) / 3; //integer division with round up, old room/3 rounds down.
			  int two3rds = room - one3rd;  
			  return filename.Substring(0, one3rd) + seperator + filename.Substring(filename.Length-(two3rds + lastLen), (two3rds + lastLen));
		 }
		 else if (filename.Length > maxFileLen + leadingnumbers + 1) // > 2  + 1 + extLen (min 4)  
			 return filename.Substring(0, leadingnumbers) + seperator + filename.Substring(filename.Length - maxFileLen, maxFileLen);
		 else
			 return filename;
    }
  
	private static Random random = new Random(); //not cryptographically secure
	public static string RandomString(int length) 
	{
		const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!@#$%^*-.";
		return new string(Enumerable.Repeat(chars, length)
		  .Select(s => s[random.Next(s.Length)]).ToArray());
	}
	/// <summary>
	/// Get filename extension, not ext of . only returns empty string, mirrors Path.GetExtension not try/catch issues by metadataconsulting.blogspot.com
	/// </summary>
	/// <param name="s"></param>
	/// <returns></returns>
	public static string GetFileNameExtension(this string s)
	{
		string ext = "";
		int fileExtPos = s.LastIndexOf('.');
		if (fileExtPos > s.LastIndexOf('\\'))
		{
			ext = s.Substring(fileExtPos, s.Length - fileExtPos);
			if (ext.Length <= 1) ext = "";
		}
		else
			ext = "";
		return ext;
	}  

	public static void Main()
	{
     string filename = ""; 
		
     for (int j = 1; j < 41; j++) {
         filename = RandomString(j) + ".exe";
         Console.WriteLine(filename); Console.WriteLine("Length=" + filename.Length); Console.WriteLine();
	     for (int i = -1; i < filename.Length+3; i++)
		    Console.WriteLine(filename.TrimMidFileNameFormat(i)); //7 min length viable name for shrinked name
         Console.WriteLine();
	 }
	}
}

 
using System;using System.Linq;
public static class Program
{ 
    private static string seperator = "~"; //must be single character string, tilde is a valid filename char, may want to use a non-valid char for filenames.
    private const int leadingnumbers = 5;  //number of leading characters to build up to
    /// <summary>
    /// Format filename to a length (maxFileLen), by splitting filename into 1/3rd leading and 2/3rds ending until max leadingnumbers is reached. Used for toast nofications less than 41 characters by metadataconsulting.blogspot.com
    /// </summary>
    /// <param name="filename"></param>
    /// <param name="maxFileLen"></param>
    /// <returns></returns>
    public static string TrimMidFileNameFormat(this string filename, int maxFileLen)
    {
    
         Console.Write(string.Format("{0,5:###}", maxFileLen)+" "); //remove line when using in your library
​
         string ext = filename.GetFileNameExtension(); 
         int extLen = ext.Length;  //includes dot in length 
         string filenamelessext = filename.Substring(0, filename.Length - extLen);
         int filenamelessextLen = filenamelessext.Length;
         int maxroom = Math.Min(leadingnumbers * 2 - 1, filenamelessextLen); 
         int minLen  = 2 + 1 + extLen; //1st char + tilde.length(1) + last char + extLen(min 4) => 7 ie s~p.exe
         int lastLen = 1 + extLen; // last char + extLen(min 4) ie p.exe
​