When I tried to convert something like 0.1 (from user in textbox), My value b is always false.
bool b = Decimal.TryParse(\"0.1\", out value);
Specify the culture for the parsing. Your current culture uses some different number format, probably 0,1.
0,1
This will successfully parse the string:
bool b = Decimal.TryParse("0.1", NumberStyles.Any, CultureInfo.InvariantCulture, out value);