Fix #2037: Try to detect framework type when TargetFrameworkAttribute is missing

ICSharpCode.Decompiler/Metadata/DotNetCorePathFinderExtensions.cs

@@ -58,6 +58,38 @@ namespace ICSharpCode.Decompiler.Metadata
 				}
 			}
 
+			foreach (var h in reader.AssemblyReferences) {
+				var r = reader.GetAssemblyReference(h);
+				if (r.PublicKeyOrToken.IsNil)
+					continue;
+				string version;
+				switch (reader.GetString(r.Name)) {
+					case "netstandard":
+						version = r.Version.ToString(3);
+						return $".NETStandard,Version=v{version}";
+					case "System.Runtime":
+						// System.Runtime.dll uses the following scheme:
+						// 4.2.0 => .NET Core 2.0
+						// 4.2.1 => .NET Core 2.1 / 3.0
+						// 4.2.2 => .NET Core 3.1
+						if (r.Version >= new Version(4, 2, 0)) {
+							version = "2.0";
+							if (r.Version >= new Version(4, 2, 1)) {
+								version = "3.0";
+							}
+							if (r.Version >= new Version(4, 2, 2)) {
+								version = "3.1";
+							}
+							return $".NETCoreApp,Version=v{version}";
+						} else {
+							continue;
+						}
+					case "mscorlib":
+						version = r.Version.ToString(2);
+						return $".NETFramework,Version=v{version}";
+				}
+			}
+
 			// Optionally try to detect target version through assembly path as a fallback (use case: reference assemblies)
 			if (assemblyPath != null) {
 				/*