fixes #1646 - netcoreapp uses wrong mscorlib (#1649)

* fixes #1646 - netcoreapp uses wrong mscorlib

* optimize framework detection

* refactor decompiler runtime to a enum

ICSharpCode.Decompiler/Metadata/UniversalAssemblyResolver.cs

@@ -29,6 +29,18 @@ namespace ICSharpCode.Decompiler.Metadata
 	// This inspired by Mono.Cecil's BaseAssemblyResolver/DefaultAssemblyResolver.
 	public class UniversalAssemblyResolver : IAssemblyResolver
 	{
+		static UniversalAssemblyResolver()
+		{
+			// TODO : test whether this works with Mono on *Windows*, not sure if we'll
+			// ever need this...
+			if (Type.GetType("Mono.Runtime") != null)
+				decompilerRuntime = DecompilerRuntime.Mono;
+			else if (Environment.OSVersion.Platform == PlatformID.Unix)
+				decompilerRuntime = DecompilerRuntime.Mono;
+			else if (typeof(object).Assembly.GetName().Name == "System.Private.CoreLib")
+				decompilerRuntime = DecompilerRuntime.NETCoreApp;
+		}
+
 		DotNetCorePathFinder dotNetCorePathFinder;
 		readonly bool throwOnError;
 		readonly PEStreamOptions streamOptions;
@@ -38,21 +50,7 @@ namespace ICSharpCode.Decompiler.Metadata
 		readonly List<string> directories = new List<string>();
 		readonly List<string> gac_paths = GetGacPaths();
 		HashSet<string> targetFrameworkSearchPaths;
-
-		/// <summary>
-		/// Detect whether we're in a Mono environment.
-		/// </summary>
-		/// <remarks>This is used whenever we're trying to decompile a plain old .NET framework assembly on Unix.</remarks>
-		static bool DetectMono()
-		{
-			// TODO : test whether this works with Mono on *Windows*, not sure if we'll
-			// ever need this...
-			if (Type.GetType("Mono.Runtime") != null)
-				return true;
-			if (Environment.OSVersion.Platform == PlatformID.Unix)
-				return true;
-			return false;
-		}
+		static readonly DecompilerRuntime decompilerRuntime;
 
 		public void AddSearchDirectory(string directory)
 		{
@@ -77,6 +75,13 @@ namespace ICSharpCode.Decompiler.Metadata
 			Silverlight
 		}
 
+		enum DecompilerRuntime
+		{
+			NETFramework,
+			NETCoreApp,
+			Mono
+		}
+
 		string targetFramework;
 		TargetFrameworkIdentifier targetFrameworkIdentifier;
 		Version targetFrameworkVersion;
@@ -291,7 +296,7 @@ namespace ICSharpCode.Decompiler.Metadata
 				return assembly;
 
 			var framework_dir = Path.GetDirectoryName(typeof(object).Module.FullyQualifiedName);
-			var framework_dirs = DetectMono()
+			var framework_dirs = decompilerRuntime == DecompilerRuntime.Mono
 				? new[] { framework_dir, Path.Combine(framework_dir, "Facades") }
 				: new[] { framework_dir };
 
@@ -367,11 +372,13 @@ namespace ICSharpCode.Decompiler.Metadata
 			var version = reference.Version;
 			var corlib = typeof(object).Assembly.GetName();
 
-			if (corlib.Version == version || IsSpecialVersionOrRetargetable(reference))
-				return typeof(object).Module.FullyQualifiedName;
+			if (decompilerRuntime != DecompilerRuntime.NETCoreApp) {
+				if (corlib.Version == version || IsSpecialVersionOrRetargetable(reference))
+					return typeof(object).Module.FullyQualifiedName;
+			}
 
 			string path;
-			if (DetectMono()) {
+			if (decompilerRuntime == DecompilerRuntime.Mono) {
 				path = GetMonoMscorlibBasePath(version);
 			} else {
 				path = GetMscorlibBasePath(version, reference.PublicKeyToken.ToHexString(8));
@@ -460,7 +467,7 @@ namespace ICSharpCode.Decompiler.Metadata
 
 		static List<string> GetGacPaths()
 		{
-			if (DetectMono())
+			if (decompilerRuntime == DecompilerRuntime.Mono)
 				return GetDefaultMonoGacPaths();
 
 			var paths = new List<string>(2);
@@ -510,7 +517,7 @@ namespace ICSharpCode.Decompiler.Metadata
 			if (reference.PublicKeyToken == null || reference.PublicKeyToken.Length == 0)
 				return null;
 
-			if (DetectMono())
+			if (decompilerRuntime == DecompilerRuntime.Mono)
 				return GetAssemblyInMonoGac(reference);
 
 			return GetAssemblyInNetGac(reference);