This error message usually occurs when you’re trying to convert a string into a number but the format of the string is inappropriate for such conversion. Let’s say you’re using the `int.Parse()` or `Convert.ToInt32()` functions in C# to convert a string to an integer.

Here’s a step-by-step guide to understand and rectify the error:

1. Check what value you’re trying to parse or convert, make sure that it’s a numeric string. For example, int.Parse(“123”) is correct, but int.Parse(“123abc”) or int.Parse(“abc”) will throw the “input string was not in a correct format” error because they are not entirely composed of numbers.

2. Make sure there are no leading or trailing spaces in the string. These can also cause the “input string was not in a correct format” error. For example, int.Parse(” 123 “) will throw an error.

3. If you’re unsure about the format of the input string, I would recommend using `int.TryParse()` method instead. This method returns a Boolean value that indicates whether the conversion succeeded or failed; it doesn’t throw an exception if the conversion fails.

Example:

“`
string input = “123abc”;
int number;
bool isConversionSuccessful = int.TryParse(input, out number);

if(isConversionSuccessful)
{
Console.WriteLine(“Conversion was successful. Converted number: ” + number);
}
else
{
Console.WriteLine(“Conversion failed. Input string was not in a correct format.”);
}
“`

4. Finally, make sure that the value you’re trying to convert doesn’t exceed the boundary of the target data type. For example, trying to convert a string “123456789123456789” to an `int` will also throw the error.

By following these steps you should be able to fix the “input string was not in a correct format” error.