What is the main difference between int.Parse() and Convert.ToInt32 in C#?

Both int.Parse() and Convert.ToInt32() methods in C# are used to convert string representations of numbers to integers. However, they handle null values and certain edge cases differently.

The key difference is that Convert.ToInt32() handles null values gracefully by returning 0, while int.Parse() throws an ArgumentNullException when encountering null.

Syntax

Following is the syntax for int.Parse() method −

int result = int.Parse(stringValue);

Following is the syntax for Convert.ToInt32() method −

int result = Convert.ToInt32(stringValue);

Using int.Parse() with Valid String

The int.Parse() method converts a valid numeric string to an integer −

using System;

class Program {
   static void Main() {
      int res;
      string myStr = "5000";
      res = int.Parse(myStr);
      Console.WriteLine("Converting String is a numeric representation: " + res);
   }
}

The output of the above code is −

Converting String is a numeric representation: 5000

Using Convert.ToInt32() with Null Value

The Convert.ToInt32() method returns 0 when the input is null −

using System;

class Program {
   static void Main() {
      int res;
      string myStr = null;
      res = Convert.ToInt32(myStr);
      Console.WriteLine("Converting String is a numeric representation: " + res);
   }
}

The output of the above code is −

Converting String is a numeric representation: 0

Using int.Parse() with Null Value

The int.Parse() method throws an exception when the input is null −

using System;

class Program {
   static void Main() {
      try {
         int res;
         string myStr = null;
         res = int.Parse(myStr);
         Console.WriteLine("Converting String is a numeric representation: " + res);
      }
      catch (ArgumentNullException ex) {
         Console.WriteLine("Error: " + ex.Message);
      }
   }
}

The output of the above code is −

Error: Value cannot be null. (Parameter 's')

Comparison

Using Both Methods with Exception Handling

Here's a practical comparison showing both methods handling different scenarios −

using System;

class Program {
   static void Main() {
      string[] testValues = { "123", null, "456" };
      
      Console.WriteLine("Testing int.Parse():");
      foreach (string value in testValues) {
         try {
            int result = int.Parse(value);
            Console.WriteLine($"Parsed '{value}': {result}");
         }
         catch (ArgumentNullException) {
            Console.WriteLine($"int.Parse() failed: null value");
         }
      }
      
      Console.WriteLine("\nTesting Convert.ToInt32():");
      foreach (string value in testValues) {
         try {
            int result = Convert.ToInt32(value);
            Console.WriteLine($"Converted '{value}': {result}");
         }
         catch (Exception ex) {
            Console.WriteLine($"Convert.ToInt32() failed: {ex.Message}");
         }
      }
   }
}

The output of the above code is −

Testing int.Parse():
Parsed '123': 123
int.Parse() failed: null value
Parsed '456': 456

Testing Convert.ToInt32():
Converted '123': 123
Converted '': 0
Converted '456': 456

Conclusion

Use int.Parse() when you're certain the input is not null and want better performance. Use Convert.ToInt32() when the input might be null and you want it to default to 0. Both methods throw exceptions for invalid numeric strings, but only Convert.ToInt32() gracefully handles null values.