int.Parse() / int.TryParse()

int.Parse(string s)

// Attempts the conversion. Returns true and the converted value on success. Does not throw an exception on failure.
int.TryParse(string s, out int result)

// Similar methods exist for other numeric types such as double and long.
double.Parse(string s)
double.TryParse(string s, out double result)

Program.cs

using System;

// Convert a string to an integer using int.Parse()
string numStr = "42";
int num = int.Parse(numStr);
Console.WriteLine(num + 10); // 52

// Convert safely using int.TryParse()
string input1 = "100";
string input2 = "abc"; // A string that cannot be converted to a number

if (int.TryParse(input1, out int result1)) {
    Console.WriteLine("Conversion succeeded: " + result1); // Conversion succeeded: 100
}

if (!int.TryParse(input2, out int result2)) {
    Console.WriteLine("Conversion failed: " + result2); // Conversion failed: 0
}

This produces the following output:

dotnet run
52
Conversion succeeded: 100
Conversion failed: 0

Using TryParse() is a common pattern when you want to use user input as an integer. Similar methods are available for other numeric types such as double and long.

UserInput.cs

using System;

// A typical pattern for converting user input to an integer
Console.Write("Enter your score: ");
string userInput = Console.ReadLine() ?? "";
if (int.TryParse(userInput, out int score)) {
    Console.WriteLine($"Your score is {score}.");
} else {
    Console.WriteLine("Please enter a valid number.");
}

// Example using double.Parse()
double ratio = double.Parse("0.75");
Console.WriteLine($"Achievement rate: {ratio:P0}"); // Achievement rate: 75%

// Converting to long
if (long.TryParse("9999999999", out long bigNum)) {
    Console.WriteLine(bigNum); // 9999999999
}

This produces the following output:

dotnet run
Enter your score: 88
Your score is 88.
Achievement rate: 75%
9999999999

Common Mistake: Passing a Non-Convertible String to int.Parse()

Passing a non-numeric string to int.Parse() throws a FormatException. Always use TryParse() for user input.

using System;

// NG: Passing a non-convertible string throws an exception
int value = int.Parse("abc"); // FormatException

dotnet run
Unhandled exception. System.FormatException: Input string was not in a correct format.

using System;

// OK: Use TryParse() to convert safely without exceptions
if (int.TryParse("abc", out int value)) {
    Console.WriteLine(value);
} else {
    Console.WriteLine("Conversion failed.");
}

This produces the following output:

dotnet run
Conversion failed.

When converting user input or external data, using int.TryParse() is safer than int.Parse(). If the input is not a valid number, int.Parse() throws an exception and halts the program, whereas int.TryParse() lets you handle the failure safely.

Convert.ToInt32() can also convert a string to an integer, but it differs in that passing null returns 0 instead of throwing an exception. For details, see Convert.ToString() / Convert.ToInt32().