Substring

The C# Substring method extracts a fragment of a string based on character positions. We specify a start and length (both ints) to describe this fragment.

With Substring, we must be careful to pass in arguments that are within the bounds of the string. And for performance, avoiding Substring() is usually helpful.

First example

We invoke Substring to extract parts of a string into a new string. We use 1 or 2 arguments—the first is the start index, and the second is the desired length.

using System;

string input = "abcd";

// Part 1: get substring starting at index 2.
// ... This continues until the end of the string.
string result = input.Substring(2);
Console.WriteLine($"RESULT: {result}");

// Part 2: use a second argument.
// ... Start at index 1.
// ... Continue for 2 places.
string result2 = input.Substring(1, 2);
Console.WriteLine($"RESULT: {result2}");
RESULT: cd
RESULT: bc

Get middle part

Here we take several chars in the middle of a string and place them into a new string. To take a middle substring, pass 2 arguments to Substring.

using System;

string input = "OneTwoThree";

// Take substring.
string result = input.Substring(3, 3);
Console.WriteLine("RESULT: {0}", result);
RESULT: Two

Get beginning part

We can get just the first part of a string. Here we eliminate the last 2 characters from the input string. Substring() returns a new string without them.

using System;

string input = "abcde";

// Take beginning part.
string result = input.Substring(0, input.Length - 2);
Console.WriteLine("RESULT: {0}", result);
RESULT: abc

IndexOf with Substring

The IndexOf method is made to be used with Substring (and other String methods). Here we want to parse a string by finding a separator.

using System;

string value = "Unit: 300 V";
string separator = ": ";

// Part 1: get index of separator.
int separatorIndex = value.IndexOf(separator);

// Part 2: if separator exists, get substring.
if (separatorIndex >= 0)
{
    string result = value.Substring(separatorIndex + separator.Length);
    Console.WriteLine("RESULT: {0}", result);
}
RESULT: 300 V

Exceptions

Substring() must be passed arguments within the range of the string. Exceptions can help us fix problems faster. This example triggers the ArgumentOutOfRangeException.

using System;

string input = "OneTwoThree";

// Part 1: try negative start index.
try
{
    string sub = input.Substring(-1);
}
catch (Exception ex)
{
    Console.WriteLine(ex);
}

// Part 2: try excessive length.
try
{
    string sub = input.Substring(0, 100);
}
catch (Exception ex)
{
    Console.WriteLine(ex);
}
System.ArgumentOutOfRangeException
System.String.InternalSubStringWithChecks

System.ArgumentOutOfRangeException
System.String.InternalSubStringWithChecks

Null substring

If a string may be null, we need to test it against null before using Substring on it. We cannot take a Substring of a null string.

using System;

string value = null;
// This is safe.
if (!string.IsNullOrEmpty(value))
{
    Console.WriteLine(value.Substring(1));
}
// This will cause an exception.
Console.WriteLine(value.Substring(1));
Unhandled Exception: System.NullReferenceException:
Object reference not set to an instance of an object.

One character

It is possible to take a one-character substring. But if we use the indexer to get a char, this will be faster—a char is a value, and does not require a heap allocation.

using System;

string value = "cat";
// ... In many programs, we can use a char instead of Substring.
Console.WriteLine(value[0]);
Console.WriteLine(value.Substring(0, 1));
c
c

Avoid substring

With logic, we can avoid invoking Substring. Suppose a program gets the same Substring over and over again. We can handle this case in code, and return a literal.

using System;

class Program
{
    static string SubstringFirst3(string value)
    {
        // ... Use logic to avoid creating a new string.
        if (value == "Windows")
        {
            return "Win";
        }
        else
        {
            return value.Substring(0, 3);
        }
    }

    static void Main()
    {
        Console.WriteLine(SubstringFirst3("Windows"));
        Console.WriteLine(SubstringFirst3("Computer"));
    }
}
Win
Com

Benchmark, char array

String-related allocations can be a burden. Here we see if taking characters and putting them into a char array is faster than calling Substring.

using System;
using System.Diagnostics;

const int _max = 1000000;
const string value = "onetwothree";
// Version 1: create new string from char array.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    char[] array = new char[3];
    array[0] = value[3];
    array[1] = value[4];
    array[2] = value[5];
    string result = new string(array);
    if (result == null)
    {
        return;
    }
}
s1.Stop();
// Version 2: use Substring.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    string result = value.Substring(3, 3);
    if (result == null)
    {
        return;
    }
}
s2.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
19.19 ns    new char[], new string()
13.58 ns    Substring

Benchmark, one char

Suppose we have a string and we want to get a single character from it. A 1-char substring is possible, but accessing the char directly is a better option.

using System;
using System.Diagnostics;

const int _max = 1000000;
const string value = "jounce";
// Version 1: get 1-character substring.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    string firstLetter = value.Substring(0, 1);
    if (firstLetter != "j")
    {
        return;
    }
}
s1.Stop();
// Version 2: access char directly.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    char firstLetter = value[0];
    if (firstLetter != 'j')
    {
        return;
    }
}
s2.Stop();
Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
18.07 ns    Substring, 1-char
 0.99 ns    Access char directly

Summary

Substring() allocates a new string. We invoke it with 1 or 2 arguments—the start and length. Avoiding Substring when possible is often a worthwhile optimization.