In all above cases @ is used to allow escape character.
If you write this
string SoftwareKey = "SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall";
It will give you syntax error.
So you can use this
string SoftwareKey = "SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Uninstall";
This is difficult way so instead
string SoftwareKey = @"SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall";
Now overall use of @
You can use reserverd keyword
1.
string for;
is not allowed but below code is allowed
string @for;
2.To allow escape character as above example.
3.To use multiline text
Label1.Text="First line \n Second Line ";
Insted of this use this
Label1.Text= @"First line
Second Line ";
So here @ sign take line how it is written.