How to convert TextBox result to Unicode characters?

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C#

SQL-Server

WinForms

so i have a text box that i have to put some Arabic characters into this text box and save it to sqlserver, my problem is when i saved changes in Sql Server it saved as "?", i already defined the column as nvarchar in SqlServer, and i use Entity Framework to save my data, i found this code but it use a simple Sql cmd:

insert into [YourTableNameHere] values(N'" + yourValue+ "')

my code is :

Veh v = new Veh();//Entity Framework
VehEntities context=new VehEntities();
v.car=TxBoxCarNumber.Text;//here which i have add something to convert the result to Arabic letters
context.Vehs.Add(v);
context.SaveChanges();

So if you have any idea i will be very appreciate