For compiling unsafe code, you have to specify the /unsafe command-line switch with command-line compiler.

For example, to compile a program named prog1.cs containing unsafe code, from command line, give the command:

csc /unsafe prog1.cs

If you are using Visual Studio IDE then you need to enable use of unsafe code in the project properties.

https://i.stack.imgur.com/2aPFY.png

To do this: