Help Me Plz About Backup .mdf File With C#

I rigth click my project , i choose add -> new item -> sql database
and i write

SqlConnection cc = new SqlConnection(@"Data Source=.SQLEXPRESS;AttachDbFilename=|DataDirectory|DataData.mdf;Integrated Security=True;User Instance=True");




when i code

cmd.CommandText = "Backup database Data to disk = 'C:\123.bak'";



i recieve Error
Could not locate entry in sysdatabases for database 'Data'. No entry found with that name. Make sure that the name is entered correctly.
BACKUP DATABASE is terminating abnormally.


help me to solve this problem please!!!

Thanks.