Returns the length of the longest continuous substring of a string with no repeated character (C# &Python)

LengthOfLongestSubstring.cs

public int LengthOfLongestSubstring(string s) 
    {
        // Dictionary to keep track of valid substrings and their length
        var substringDict = new Dictionary<string, int>(); 
        for(int i = 0; i < s.Length; i++)
        {
            // Dictionary to detect if any character is being repeated
            // This should have been an HashSet😒 as the value in each pair is a dummy
            var reptDetector = new Dictionary<char, int>();            
            string x = s.Substring(i);
            for(int j = 0; j < x.Length; j++)
            {
                try
                {                  
                    reptDetector.Add(x[j],1);
                }
                catch
                {
                    // If current character is a repetition, make x the last valid substring
                    // and break out of the loop
                    x = x.Substring(0,j);
                    break;
                }

            }
            // Add substring to dictionary only when the key is new
            if(!substringDict.ContainsKey(x))
            {                
                substringDict.Add(x,x.Length);
            }
        }
         // Return 0 if dictionary is empty, else, return the max value
         return substringDict.Count > 0 ? substringDict.Select(x => x.Value).Max() : 0;

LengthOfLongestSubstring.py

def lengthOfLongestSubstring(s):
  # Dictionary to keep track of valid substrings and their length
  substrings_dict = {}
  for i in range(len(s)):
    # Dictionary to detect if any character is being repeated
    # This should have been an HashSet😒 as the value in each pair is a dummy
    rept_detector_dict = {}
    x = x[i:]
    for j in range(len(x)):
      try:
        rept_detector_dict[x[j]] = 1
      except:
        # If current character is a repetition, make x the last valid substring
        # and break out of the loop
        x = x[j:]
        break
    # Add substring to dictionary only when the key is new
    if x in substrings_dict.keys() == False:
      substrings_dict[x] = len(x)
  # Return 0 if dictionary is empty, else, return the max value
  if len(substrings_dict) == 0:
    return 0
  return max(substrings_dict.values())