c# - Long value showing default as 0 -

i have created wpf textbox , set binding long value phonenumber.

<textbox text="{binding phonenumber,updatesourcetrigger=propertychanged,stringformat={}{0:000-000-0000},targetnullvalue=''}" maxlength="10"></textbox> 

while running textbox shows defaultvalue 000-000-0000. need empty. how that?

two solutions:

1)instead of using long type, can use long? type, means long variable can null.

2) can use string , convert long string. when don't have data in variable can put string.empty.