FunctionPointerExample.cs

using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;

public unsafe class NetStandardPlus
{
    [UnmanagedFunctionPointer(CallingConvention.StdCall)]
    public delegate void MyStdcallDelegate();
    
    public static void Call(delegate*<void> action) => action();
    
    // Same as `delegate*<void>
    // public static void Call(delegate* managed<void> action) => action();
    
    // error CS8889: The target runtime doesn't support extensible or runtime-environment default calling conventions.
    // public static void Call(delegate* unmanaged<void> action) => action();
    
    public static void Call(delegate* unmanaged[Stdcall]<void> action) => action();
    
    public static delegate*<void> GetManagedAction() => &MyManagedAction;
    
    public static delegate* unmanaged[Stdcall]<void> GetStdcallAction()
        => (delegate* unmanaged[Stdcall]<void>)Marshal.GetFunctionPointerForDelegate(MyStdcallAction);
    
    // error CS8786: Calling convention of 'Net5PlusOnly.MyDefaultAction()' is not compatible with 'Standard'.
    // public static delegate* unmanaged[Stdcall]<void> GetStdcallAction2() => &MyDefaultAction;
    
    public static void MyManagedAction() { }
    
    // error CS0246: The type or namespace name 'UnmanagedCallersOnly' could not be found (are you missing a using directive or an assembly reference?)
    //    -- This requires runtime support, it's not as simple as just defining the attribute
    // [UnmanagedCallersOnly]
    // public static void MyDefaultAction() { }
    
    // This needs to be kept alive while native has a reference
    public static MyStdcallDelegate MyStdcallAction = MyManagedAction;
}

public unsafe class Net5PlusOnly
{
    public static void Call(delegate* unmanaged<void> action) => action();
    
    public static delegate* unmanaged<void> GetDefaultAction() => &MyDefaultAction;
    
    // error CS8786: Calling convention of 'Net5PlusOnly.MyStdcallAction()' is not compatible with 'Unmanaged'.
    // public static delegate* unmanaged<void> GetDefaultAction2() => &MyStdcallAction;
    
    public static delegate* unmanaged[Stdcall]<void> GetStdcallAction() => &MyStdcallAction;
    
    // error CS8786: Calling convention of 'Net5PlusOnly.MyDefaultAction()' is not compatible with 'Standard'.
    // public static delegate* unmanaged[Stdcall]<void> GetStdcallAction2() => &MyDefaultAction;
    
    [UnmanagedCallersOnly]
    public static void MyDefaultAction() { }
    
    [UnmanagedCallersOnly(CallConvs = new Type[] { typeof(CallConvStdcall) })]
    public static void MyStdcallAction() { }
}