C# How to Convert String to Int with Examples

In C#, you can convert a string to an int using int.Parse(stringValue) or safely with int.TryParse(stringValue, out int result) to avoid errors.

Examples

Input"123"

Output123

Input"0"

Output0

Input"-456"

Output-456

How to Think About It

To convert a string to an int, first check if the string contains only digits (and possibly a sign). Use int.Parse if you are sure the string is valid, or int.TryParse to safely attempt conversion without crashing if the string is invalid.

Algorithm

1

Get the input string.

2

Try to convert the string to an integer using a built-in method.

3

If conversion succeeds, return the integer value.

4

If conversion fails, handle the error or return a default value.

Code

csharp

using System;

class Program {
    static void Main() {
        string str = "123";
        int number = int.Parse(str);
        Console.WriteLine(number);
    }
}

Output

123

Dry Run

Let's trace converting the string "123" to an int using int.Parse.

1

Input string

str = "123"

2

Convert string to int

number = int.Parse("123") results in 123

3

Print result

Console.WriteLine(number) outputs 123

Step	Value
Input string	"123"
Converted int	123
Output	123

Why This Works

Step 1: Using int.Parse

int.Parse converts a valid numeric string directly to an integer but throws an exception if the string is invalid.

Step 2: Using int.TryParse

int.TryParse attempts conversion and returns true or false, preventing exceptions and allowing safe error handling.

Alternative Approaches

int.TryParse

csharp

using System;

class Program {
    static void Main() {
        string str = "123";
        if (int.TryParse(str, out int number)) {
            Console.WriteLine(number);
        } else {
            Console.WriteLine("Invalid number");
        }
    }
}

This method is safer because it does not throw an exception if the string is not a valid number.

Convert.ToInt32

csharp

using System;

class Program {
    static void Main() {
        string str = "123";
        int number = Convert.ToInt32(str);
        Console.WriteLine(number);
    }
}

This method converts the string to int and returns 0 if the string is null, but throws an exception if the string is invalid.

Complexity: O(n) time, O(1) space

Time Complexity

Conversion checks each character once, so it takes linear time relative to the string length.

Space Complexity

Conversion uses constant extra space as it only stores the integer result.

Which Approach is Fastest?

int.Parse and Convert.ToInt32 are similar in speed, but int.TryParse adds a small overhead for safety checks.

Approach	Time	Space	Best For
int.Parse	O(n)	O(1)	When input is guaranteed valid
int.TryParse	O(n)	O(1)	Safe conversion with error handling
Convert.ToInt32	O(n)	O(1)	Handles null but throws on invalid strings