Rule Definition
Applications having methods exposed that use Platform Invocation Services to access unmanaged code can have security issues. These methods must not be exposed as they are accessing unmanaged code. By keeping these methods private or internal, you make sure that your library cannot be used to breach security by allowing callers access to unmanaged APIs that they could not call otherwise.

Remediation
Change access modifier to "private", "internal"or "private protected"

Violation Code Sample

Will flag a violation if "public" or "protected" or "protected internal" access modifiers are used for P/Invoke declarations

using System;
using System.Runtime.InteropServices;

namespace InteroperabilityLibrary
{ 
    public class NativeMethods
    {
        [DllImport("kernel32.dll", CharSet = CharSet.Unicode)]
        public static extern bool RemoveDirectory(string name);
    }
}

Fixed Code Sample

using System;
using System.Runtime.InteropServices;

namespace InteroperabilityLibrary
{
    public class NativeMethods
    {
        [DllImport("kernel32.dll", CharSet = CharSet.Unicode)]
        private static extern bool RemoveDirectory(string name);
    }
}

https://docs.microsoft.com/en-us/visualstudio/code-quality/ca1401-p-invokes-should-not-be-visible?view=vs-2015