Compiled time error in .net framework 4.6.1 ?

Question

I started work on Console application which is already completed in .net framework 4.6.1 but I have .net framework 4.5,so I upgrade my framework to 4.6.1.

After that on following code I got "Syntax error, ':' expected" this error.

 

Disconnected?.Invoke(this);

 

The following is working on my collegues system.

 

So how to resolve this issue ?