NativeLibHelper.cs

using System.Runtime.InteropServices;

namespace M64RPFW.Models.Helpers;

/// <summary>
/// Marks a pseudo-<see cref="DllImportAttribute"/>ed function resolved at runtime.
///
/// <para>
///     To register correctly, a <c>RuntimeDllImport</c>ed delegate must either
///     set the <see cref="Name"/> property or start with a capital <c>D</c>.
/// </para>
/// </summary>
[AttributeUsage(AttributeTargets.Delegate)]
public class RuntimeDllImportAttribute : Attribute
{
    /// <summary>
    /// The name of the function to import. If it is not set and the
    /// delegate's name begins with a capital <c>D</c>, then an automatic
    /// name will be generated by removing the first character.
    /// </summary>
    public string? Name { get; init; }
}

public static class NativeLibHelper
{
    /// <summary>
    /// Tries to get a function from a library.
    /// </summary>
    /// <param name="lib">The library</param>
    /// <param name="name">The name of the function</param>
    /// <typeparam name="T">A delegate type to call the function</typeparam>
    /// <exception cref="EntryPointNotFoundException">If the function can't be found.</exception>
    /// <returns></returns>
    public static T? GetFunction<T>(IntPtr lib, string name)
    {
        IntPtr res = NativeLibrary.GetExport(lib, name);
        return (res == IntPtr.Zero)? default : Marshal.GetDelegateForFunctionPointer<T>(res);
    }
    
    /// <summary>
    /// Resolve a delegate tagged with <see cref="RuntimeDllImportAttribute"/>.
    /// </summary>
    /// <param name="lib">the library to resolve symbols from</param>
    /// <param name="del">the delegate to set</param>
    /// <typeparam name="T">the delegate to resolve to</typeparam>
    /// <returns>the delegate</returns>
    /// <exception cref="ArgumentException">If the delegate type is not tagged correctly</exception>
    /// <seealso cref="RuntimeDllImportAttribute"/>
    public static void ResolveDelegate<T>(IntPtr lib, out T del) where T : Delegate
    {
        Attribute[] attrs = Attribute.GetCustomAttributes(typeof(T));
        var rtDllImport = (RuntimeDllImportAttribute?)
            attrs.FirstOrDefault(a => a is RuntimeDllImportAttribute, null);
        if (rtDllImport is null)
        {
            throw new ArgumentException(
                $"Type T ({typeof(T).FullName}) is not tagged with {typeof(RuntimeDllImportAttribute).FullName}");
        }

        string symbol;
        if (rtDllImport.Name is null)
        {
            string name = typeof(T).Name;
            if (name[0] != 'D')
            {
                throw new ArgumentException("Type does not match automatic delegate typename");
            }
            symbol = name.Substring(1);
        }
        else
        {
            symbol = rtDllImport.Name;
        }
        T? val = GetFunction<T>(lib, symbol);
        if (val == null)
            throw new ApplicationException("Failed to load function");
        del = val;
    }

    public static string AsDLL(string name)
    {
        return $"{name}{LibraryExtension}";
    }

    private static string? _platformLibExtension;
    public static string LibraryExtension
    {
        get
        {
            if (_platformLibExtension != null)
                return _platformLibExtension;
            if (OperatingSystem.IsWindows())
                return _platformLibExtension = ".dll";
            if (OperatingSystem.IsLinux())
                return _platformLibExtension = ".so";
            throw new PlatformNotSupportedException("Only Windows and Linux are supported at the moment");
        }
    }
}