the resultString value should be a single space " "

No, it shouldn't.

The end of a range is exclusive, not inclusive. So for example, x[0..x.Length] will always return the whole string.

In your case, you're saying "everything before index 0" which is obviously "the empty string".

This is documented in (aside from other places) the documentation for Range.End:

Gets an Index that represents the exclusive end index of the range.

The .. is a syntax for System.Range and the form which you are using

string resultString = myString[..0];

is translated into,

string resultString = myString[Range.EndAt(new Index(0, fromEnd: true))] 

In your case, you are trying ..0 which means all elements which are before 0th index and that is the reason you are getting result as string.Empty

MSDN documentation for Index.End Property

Gets an Index that points beyond the last element.

It might be easier to consider the indexes in a range as being between the characters, not pointing to a character

If the characters in the string were sprayed on fence panels, then the indexes in a range refer to the fence posts, not the fence panels

When you cut a string with ranges you get all the chars between the indexes. When you uproot fence posts from X to Y, the fence panels between the posts fall down - you get the characters on the fallen panels

var hw = "Hello world";

hw[0..11]  //Hello world
hw[..5]    //Hello
hw[0..0]   //(empty string - there are no characters between index 0 and index 0)

The reason I recommend to think about indexes in a range as between characters is it makes using the "from the right" indexer ^n easy to reason about. Taking [^7..^2] will take these panels:

..so you get o wor

If you were to think of the indexes as pointing to characters or to panels then then have to remember a) "the end of the range is exclusive" and then also b) that there's an imaginary character added onto the end of the string that is ^0, just so you can exclude it again. Or you have to think "the ^x indexes are from the right, and if you use them it flips the exclusive around", and then you have to come up with some way of remembering or working out which is the exclusive end if you're mixing non-^ indexes with ^ indexes.

All in, it's most consistent to just remember "the indexes are between the characters" and "^ is from the right" then you can mix and match whatever you like:

hw[^5..^1] //worl
hw[^5..10] //worl
hw[6..10]  //worl
hw[6..^1]  //worl