Parse, int

The C# int.Parse and TryParse methods convert strings to ints. Consider the string "100": it can be represented as an int, with no loss of information.

With TryParse, we use an out-parameter. This requires the "out" keyword. Usually calling TryParse is worthwhile—it makes a program sturdier and more reliable.

Parse example

Here is the int.Parse method. This is the simplest one. Parse() throws exceptions on invalid input. This can be slow if errors are common.

using System;

// Part 1: string containing number.
string text = "500";

// Part 2: pass string to int.Parse.
int num = int.Parse(text);
Console.WriteLine(num);
500

Errors

The int.Parse method is strict. With an invalid string, it throws a FormatException. We can catch this using a try-catch construct.

using System;

string input = "carrot";
// ... This will throw an exception.
int carrots = int.Parse(input);
Console.WriteLine(carrots);
Unhandled Exception: System.FormatException:
Input string was not in a correct format.
   at System.Number.StringToNumber(String str, NumberStyles options, ...
   at System.Number.ParseInt32(String s, NumberStyles style, ...

TryParse

A useful method for parsing integers is the int.TryParse method. The parsing logic in TryParse is the same. But the way we call it is different.

using System;

// See if we can parse the string.
string text1 = "x";
int num1;
bool res = int.TryParse(text1, out num1);
if (res == false)
{
    // String is not a number.
}

// Use int.TryParse on a valid numeric string.
string text2 = "10000";
int num2;
if (int.TryParse(text2, out num2))
{
    // It was assigned.
}

// Display both results.
Console.WriteLine(num1);
Console.WriteLine(num2);
0
10000

TryParse with no if

Usually we call int.TryParse inside an if-statement. But this is not required. And sometimes, we can just treat a failed parse as the default value (0 for int).

using System;

string error = "Welcome";
// This will leave the result variable with a value of 0.
int result;
int.TryParse(error, out result);
Console.WriteLine(result);
0

TryParse, new out syntax

We can place the "out int" keywords directly inside a method call. Older versions of C# do not allow this syntax. But this can reduce the line count of a program.

using System;

const string value = "345";
// We can place the "out int" declaration in the method call.
if (int.TryParse(value, out int result))
{
    Console.WriteLine(result + 1);
}
346

Convert

Convert.ToInt32 (along with its siblings Convert.ToInt16 and Convert.ToInt64) is a static wrapper method for the int.Parse method.

using System;

// Convert "text" string to an integer with Convert.ToInt32.
string text = "500";
int num = Convert.ToInt32(text);
Console.WriteLine(num);
500

DateTime

Parsing is fun. It is one of my passions in life. We can parse a string into a DateTime type with TryParse. Parsing methods on DateTime use a similar syntax.

using System;

string now = "1/22/2017";
// Use TryParse on the DateTime type to parse a date.
DateTime parsed;
if (DateTime.TryParse(now, out parsed))
{
    Console.WriteLine(parsed);
}
1/22/2017 12:00:00 AM

Benchmark, invalid strings

When I first wrote about int.Parse, I recommended it as the best parsing option. After many years, I have changed my mind.

using System;
using System.Diagnostics;

const int _max = 10000;
// Version 1: parse an invalid string with exception handling.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    int result;
    try
    {
        result = int.Parse("abc");
    }
    catch
    {
        result = 0;
    }
}
s1.Stop();

// Version 2: parse an invalid string with TryParse.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    int result;
    int.TryParse("abc", out result);
}
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"));
13243.34 ns    int.Parse
   15.46 ns    int.TryParse

Benchmark, IntParseFast

An optimized int.Parse method reduces computational complexity. It uses a for-loop on the characters in a string.

using System;
using System.Diagnostics;

class Program
{
    public static int IntParseFast(string value)
    {
        // An optimized int parse method.
        int result = 0;
        for (int i = 0; i < value.Length; i++)
        {
            result = 10 * result + (value[i] - 48);
        }
        return result;
    }

    const int _max = 1000000;
    static void Main()
    {
        // Test the methods.
        Console.WriteLine(IntParseFast("123456"));
        Console.WriteLine(int.Parse("123456"));

        var s1 = Stopwatch.StartNew();
        // Version 1: use custom parsing algorithm.
        for (int i = 0; i < _max; i++)
        {
            int result = IntParseFast("123456");
        }
        s1.Stop();
        var s2 = Stopwatch.StartNew();
        // Version 2: use int.Parse.
        for (int i = 0; i < _max; i++)
        {
            int result = int.Parse("123456");
        }
        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"));
    }
}
123456
123456
 3.76 ns:    IntParseFast
20.31 ns:    int.Parse

A review

Programmers can write their own integer conversion routines—but this is complicated. With Parse and TryParse, .NET helps solve the parsing problem.