FormatException in C#

A FormatException is thrown when the format of an argument is invalid or cannot be converted to the expected data type. This commonly occurs when attempting to parse a string that doesn't match the expected format for numeric or date types.

Common Scenarios

FormatException typically occurs in these situations −

• Parsing a non-numeric string as a number using int.Parse(), double.Parse(), etc.

• Converting a string with decimal points to an integer

• Parsing invalid date formats using DateTime.Parse()

• Using incorrect format specifiers in string formatting

Using int.Parse() with Invalid Format

When we pass a value that cannot be converted to an integer to the int.Parse() method, a FormatException is thrown −

using System;

class Demo {
   static void Main() {
      try {
         string str = "3.5";
         int res = int.Parse(str);
         Console.WriteLine("Parsed value: " + res);
      }
      catch (FormatException ex) {
         Console.WriteLine("FormatException caught: " + ex.Message);
      }
   }
}

The output of the above code is −

FormatException caught: Input string was not in a correct format.

Using DateTime.Parse() with Invalid Format

FormatException also occurs when parsing invalid date strings −

using System;

class Demo {
   static void Main() {
      try {
         string dateStr = "invalid-date";
         DateTime date = DateTime.Parse(dateStr);
         Console.WriteLine("Parsed date: " + date);
      }
      catch (FormatException ex) {
         Console.WriteLine("FormatException caught: " + ex.Message);
      }
   }
}

The output of the above code is −

FormatException caught: String was not recognized as a valid DateTime.

Preventing FormatException

Use TryParse() methods to safely convert strings without throwing exceptions −

using System;

class Demo {
   static void Main() {
      string str1 = "123";
      string str2 = "3.5";
      
      if (int.TryParse(str1, out int result1)) {
         Console.WriteLine("Successfully parsed: " + result1);
      } else {
         Console.WriteLine("Failed to parse: " + str1);
      }
      
      if (int.TryParse(str2, out int result2)) {
         Console.WriteLine("Successfully parsed: " + result2);
      } else {
         Console.WriteLine("Failed to parse: " + str2);
      }
   }
}

The output of the above code is −

Successfully parsed: 123
Failed to parse: 3.5

Conclusion

FormatException occurs when attempting to convert or parse data with an invalid format. Use try-catch blocks to handle these exceptions gracefully, or prefer TryParse() methods to avoid exceptions when the input format might be invalid.