Fix .NET Standard support

Author: endeavour

nng.NET.Shared/AssemblyLoadContext.cs

@@ -46,7 +46,18 @@ protected override Assembly Load(AssemblyName assemblyName)
             if (assemblyName.Name == managedAssemblyName)
             {
                 // Try framework-specific managed assembly path
-                var path = Path.Combine(assemblyPath, "runtimes", "any", "lib", "net6.0"
+                var path = Path.Combine(assemblyPath, "runtimes", "any", "lib", 
+                    #if NETSTANDARD1_5
+                    "netstandard1.5"
+                    #elif NETSTANDARD2_0
+                    "netstandard2.0"
+                    #elif NET5_0
+                    "net5.0"
+                    #elif NET6_0
+                    "net6.0"                    
+                    #else
+                    #error "Unsupported framework?"
+                    #endif
                     , managedAssemblyName + ".dll");
                 if (!File.Exists(path))
                 {
@@ -63,7 +74,11 @@ protected override IntPtr LoadUnmanagedDll(string unmanagedDllName)
         {
             if (unmanagedDllName == "nng")
             {
+#if NETSTANDARD2_0
+                bool is64bit = Environment.Is64BitProcess;
+#else
                 bool is64bit = (IntPtr.Size == 8);
+#endif
                 string arch = string.Empty;
                 switch (RuntimeInformation.ProcessArchitecture)
                 {
@@ -91,8 +106,7 @@ protected override IntPtr LoadUnmanagedDll(string unmanagedDllName)
                     throw new Exception("Unexpected runtime OS platform: " + RuntimeInformation.OSDescription);
                 }
             }
-
-            return base.LoadUnmanagedDll(unmanagedDllName);
+            return IntPtr.Zero;
         }
 
         const string managedAssemblyName = "nng.NET";