System.FormatException: Input string was not in a correct format.
System.FormatException: String '12.5' was not recognized as a valid DateTime.You’re trying to convert a string to a number, date, or other type, but the string doesn’t match the expected format.
Fix 1: Use TryParse Instead of Parse
// ❌ Parse throws on invalid input
int number = int.Parse("abc"); // 💥
// ✅ TryParse returns false instead of throwing
if (int.TryParse("abc", out int number)) {
Console.WriteLine(number);
} else {
Console.WriteLine("Not a valid number");
}Fix 2: Decimal Separator Mismatch
// ❌ Culture uses comma but string has dot
double price = double.Parse("12.99"); // 💥 In European locale
// ✅ Specify culture
double price = double.Parse("12.99", CultureInfo.InvariantCulture);Fix 3: DateTime Format
// ❌ Wrong date format
DateTime date = DateTime.Parse("20-03-2026"); // 💥 Ambiguous
// ✅ Use ParseExact
DateTime date = DateTime.ParseExact("20-03-2026", "dd-MM-yyyy", CultureInfo.InvariantCulture);
// ✅ Or TryParseExact
DateTime.TryParseExact("20-03-2026", "dd-MM-yyyy", CultureInfo.InvariantCulture, DateTimeStyles.None, out DateTime date);Fix 4: String.Format Placeholder Mismatch
// ❌ More placeholders than arguments
string msg = string.Format("Hello {0}, you have {1} items in {2}", name, count); // 💥 Missing {2}
// ✅ Match placeholders to arguments
string msg = string.Format("Hello {0}, you have {1} items", name, count);
// Or use string interpolation
string msg = $"Hello {name}, you have {count} items";Fix 5: Empty or Whitespace String
// ❌ Parsing empty string
int n = int.Parse(""); // 💥
// ✅ Check first
if (!string.IsNullOrWhiteSpace(input) && int.TryParse(input, out int n)) {
// Use n
}