How to List all Substrings in a given String using C#?

A substring is any contiguous sequence of characters within a string. In C#, you can generate all possible substrings of a given string using the Substring() method with nested loops to iterate through different starting positions and lengths.

Syntax

The Substring() method extracts a portion of a string −

string.Substring(startIndex, length)

To generate all substrings, use nested loops −

for (int length = 1; length <= str.Length; length++) {
    for (int start = 0; start <= str.Length - length; start++) {
        string substring = str.Substring(start, length);
    }
}

How It Works

The algorithm uses two nested loops: the outer loop controls the length of substrings (from 1 to the full string length), while the inner loop controls the starting position for each substring of that length.

Using Nested Loops

Example

using System;

public class Demo {
    public static void Main() {
        string myStr = "pqrz";
        Console.WriteLine("All substrings of "" + myStr + "":");
        
        for (int length = 1; length <= myStr.Length; length++) {
            for (int start = 0; start <= myStr.Length - length; start++) {
                string substr = myStr.Substring(start, length);
                Console.WriteLine(substr);
            }
        }
    }
}

The output of the above code is −

All substrings of "pqrz":
p
q
r
z
pq
qr
rz
pqr
qrz
pqrz

Using a Method to Generate Substrings

Example

using System;
using System.Collections.Generic;

public class SubstringGenerator {
    public static List<string> GetAllSubstrings(string input) {
        List<string> substrings = new List<string>();
        
        for (int length = 1; length <= input.Length; length++) {
            for (int start = 0; start <= input.Length - length; start++) {
                substrings.Add(input.Substring(start, length));
            }
        }
        
        return substrings;
    }
    
    public static void Main() {
        string text = "ABC";
        List<string> allSubstrings = GetAllSubstrings(text);
        
        Console.WriteLine("String: " + text);
        Console.WriteLine("Total substrings: " + allSubstrings.Count);
        Console.WriteLine("Substrings:");
        
        foreach (string substring in allSubstrings) {
            Console.WriteLine(""" + substring + """);
        }
    }
}

The output of the above code is −

String: ABC
Total substrings: 6
Substrings:
"A"
"B"
"C"
"AB"
"BC"
"ABC"

Performance Considerations

For a string of length n, the total number of possible substrings is n × (n + 1) / 2. This means the time complexity is O(n²) for generating all substrings, and space complexity depends on whether you store them or just process them.

Example with Count

using System;

public class SubstringCounter {
    public static void Main() {
        string[] testStrings = {"A", "AB", "ABC", "ABCD"};
        
        foreach (string str in testStrings) {
            int count = str.Length * (str.Length + 1) / 2;
            Console.WriteLine("String: "" + str + "" - Length: " + str.Length + " - Substrings: " + count);
        }
    }
}

The output of the above code is −

String: "A" - Length: 1 - Substrings: 1
String: "AB" - Length: 2 - Substrings: 3
String: "ABC" - Length: 3 - Substrings: 6
String: "ABCD" - Length: 4 - Substrings: 10

Conclusion

Generating all substrings in C# requires nested loops with the Substring() method. The outer loop controls substring length while the inner loop controls the starting position. For a string of length n, this produces n×(n+1)/2 total substrings with O(n²) time complexity.