From 4fa2755085ed75ea12f6e497951f8250a3b644c4 Mon Sep 17 00:00:00 2001
From: dahall <dahall@byu.net>
Date: Mon, 14 Dec 2020 15:57:23 -0700
Subject: [PATCH] Added IArrayStruct<T> interface to identify two-field
 structures where the first field is an UInt32 and the second is a pointer to
 an array of structures along with supporting marshaler and extension method.

---
 Core/Collections/IArrayStruct.cs | 61 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)
 create mode 100644 Core/Collections/IArrayStruct.cs

diff --git a/Core/Collections/IArrayStruct.cs b/Core/Collections/IArrayStruct.cs
new file mode 100644
index 00000000..b4a35c96
--- /dev/null
+++ b/Core/Collections/IArrayStruct.cs
@@ -0,0 +1,61 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Vanara.Extensions;
+using Vanara.InteropServices;
+
+namespace Vanara.PInvoke
+{
+	/// <summary>
+	/// Interface that identifies a structure containing only a 4-byte size field followed by a pointer to an array of <typeparamref name="T"/>.
+	/// </summary>
+	/// <typeparam name="T">A marshalable structure.</typeparam>
+	public interface IArrayStruct<T> where T : struct { }
+
+	/// <summary>Extension methods for <see cref="IArrayStruct{T}"/>.</summary>
+	public static class IArrayStructExtensions
+	{
+		/// <summary>Gets the array from an <see cref="IArrayStruct{T}"/> instance.</summary>
+		/// <typeparam name="T">The type of the array.</typeparam>
+		/// <param name="ias">The <see cref="IArrayStruct{T}"/> instance.</param>
+		/// <returns>The array contained in the instance.</returns>
+		public static T[] GetArray<T>(this IArrayStruct<T> ias) where T : struct
+		{
+			using var pin = new PinnedObject(ias);
+			return ((IntPtr)pin).ToArray<T>((int)((IntPtr)pin).ToStructure<uint>(), sizeof(uint));
+		}
+	}
+
+	/// <summary>Allows marshaling of arrays in place of a structure supporting <see cref="IArrayStruct{T}"/>.</summary>
+	/// <typeparam name="T">The type of the array element.</typeparam>
+	/// <seealso cref="Vanara.InteropServices.IVanaraMarshaler"/>
+	public class IArrayStructMarshaler<T> : IVanaraMarshaler where T : struct
+	{
+		public static readonly int hSz = sizeof(uint);
+
+		/// <inheritdoc/>
+		public SizeT GetNativeSize() => hSz + IntPtr.Size;
+
+		/// <inheritdoc/>
+		public SafeAllocatedMemoryHandle MarshalManagedToNative(object managedObject)
+		{
+			if (managedObject is IEnumerable<T> ias)
+			{
+				var cnt = ias.Count();
+				var mem = new SafeHGlobalHandle(hSz + InteropExtensions.SizeOf<T>() * cnt);
+				mem.Write(cnt);
+				mem.Write(ias, true, hSz);
+				return mem;
+			}
+			throw new ArgumentException("Unexpected type. Value must be of IEnumerable<T>.");
+		}
+
+		/// <inheritdoc/>
+		public object MarshalNativeToManaged(IntPtr pNativeData, SizeT allocatedBytes)
+		{
+			if (pNativeData == default) return new T[0];
+			var cnt = pNativeData.ToStructure<uint>();
+			return pNativeData.ToArray<T>((int)cnt, hSz, allocatedBytes);
+		}
+	}
+}
\ No newline at end of file