Unsafe code may only appear if compiling with /unsafe

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When you add a old C++ method in your C# code, you might get the following error when you attempt to build your project:

Unsafe code may only appear if compiling with /unsafe

The resolution is:

1: If you are using command line to compile, just include the /unsafe switch followed the compile command;

2: If you are using Visual studio, go to the property interface of your project, under Build tab, check on the 'Allow unsafe code' check box, see below: