C# - is there any difference between x.ToString("D") and x.ToString(), where x is type int?

The main point of .ToString("D") is that it allows you to specify the preceding number of digits.

e.g,

var i = 123;

var stringed = i.ToString("D5");//stringed = 00123

Take a look at what the "D" actually means here:

https://docs.microsoft.com/en-us/dotnet/standard/base-types/standard-numeric-format-strings

"D" or "d" Decimal Result: Integer digits with optional negative sign.

Supported by: Integral types only.

Precision specifier: Minimum number of digits.

Default precision specifier: Minimum number of digits required.

More information: The Decimal("D") Format Specifier.

1234 ("D") -> 1234

-1234 ("D6") -> -001234

So, depending on the format of the output you're looking for, that's what the different format (or precision) specifiers can help dictate your output.

From the Microsoft documentation:

To format an Int32 value as an integral string with no leading zeros, you can call the parameterless ToString() method. By using the "D" format specifier, you can also include a specified number of leading zeros in the string representation.

It would appear that "D" without digits is what Int32.ToString() uses by default.

There is then, no difference in output between the two.

Look at this article

The "D" (or decimal) format specifier converts a number to a string of decimal digits (0-9), prefixed by a minus sign if the number is negative. This format is supported only for integral types.

It shows the same result for the case "D".

But it will pad with zeros to the left of your int if you "Dx" (x is a number)

ToString() is enough for you to convert to string