How would I separate thousands with space in C#

Pass in a custom NumberFormatInfo with a custom NumberGroupSeparator property, and use the #,# format to tell it to do number groups. This example uses the invariant culture’s number format as its basis.

var nfi = (NumberFormatInfo)CultureInfo.InvariantCulture.NumberFormat.Clone();
nfi.NumberGroupSeparator = " ";
string formatted = 1234897.11m.ToString("#,0.00", nfi); // "1 234 897.11"