Why does NumberStyles.AllowThousands work for int.Parse but not for double.Parse in this example?

Question

To parse a string representing a number, with a comma separating thousand digits from the rest, I tried

int tmp1 = int.Parse("1,234", NumberStyles.AllowThousands);
double tmp2 = double.Parse("1,000.01", NumberStyles.AllowThousands);

The first statement is executed without issues, while the second fails with an exception:

An unhandled exception of type 'System.FormatException' occurred in mscorlib.dll

Additional information: Input string was not in a correct format.

Why do not both succeed?

Answer 1

You should pass AllowDecimalPoint, Float, or Number style (latter two styles are just a combination of several number styles which include AllowDecimalPoint flag):

double.Parse("1,000.01", NumberStyles.AllowThousands | NumberStyles.AllowDecimalPoint)

When you provide some number style to parsing method, you specify exact style of elements which can present in the string. Styles which are not included considered as not allowed. Default combination of flags (when you don't specify style explicitly) for parsing double value is NumberStyles.Float and NumberStyles.AllowThousands flags.

Consider your first example with parsing integer - you haven't passed AllowLeadingSign flag. Thus the following code will throw an exception:

int.Parse("-1,234", NumberStyles.AllowThousands)

For such numbers, AllowLeadingSign flag should be added:

int.Parse("-1,234", NumberStyles.AllowThousands | NumberStyles.AllowLeadingSign)