added unit test. made if condition easier to understand.

TestAssets/TestProjects/TestLibraryWithMultipleFrameworks/Program.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace ConsoleApplication
+{
+    public class Program
+    {
+        public static void Main()
+        {
+            Console.WriteLine("Hello World!");
+        }
+    }
+}

TestAssets/TestProjects/TestLibraryWithMultipleFrameworks/project.json

@@ -0,0 +1,20 @@
+{
+    "version": "1.0.0-*",
+    "compilationOptions": {
+        "emitEntryPoint": false
+    },
+
+    "dependencies": { },
+
+    "frameworks": {
+        "net20": { },
+        "net35": { },
+        "net40": { },
+        "net461": { },
+        "dnxcore50": { 
+            "dependencies": {
+                "NETStandard.Library": "1.0.0-rc2-23811"
+            }
+        }
+    }
+}

src/Microsoft.DotNet.Compiler.Common/AssemblyInfoFileGenerator.cs

@@ -72,14 +72,32 @@ namespace Microsoft.DotNet.Cli.Compiler.Common
                 [typeof(NeutralResourcesLanguageAttribute)] = EscapeCharacters(metadata.NeutralLanguage)
             };
 
-            NuGetFramework targetFramework = string.IsNullOrEmpty(metadata.TargetFramework) ? null : NuGetFramework.Parse(metadata.TargetFramework);
+            if (IsAllowV4Attributes(metadata))
-            if (targetFramework != null && !(targetFramework.IsDesktop() && targetFramework.Version < new Version(4, 0)))
             {
                 attributes[typeof(TargetFrameworkAttribute)] = EscapeCharacters(metadata.TargetFramework); // TargetFrameworkAttribute only exists since .NET 4.0
             };
+
             return attributes;
         }
 
+        private static bool IsAllowV4Attributes(AssemblyInfoOptions metadata)
+        {
+            if (string.IsNullOrEmpty(metadata.TargetFramework))
+            {
+                // target framework is unknown. to be on the safe side, return false.
+                return false;
+            }
+
+            NuGetFramework targetFramework = NuGetFramework.Parse(metadata.TargetFramework);
+            if (!targetFramework.IsDesktop())
+            {
+                // assuming .NET Core, which should support .NET 4.0 attributes
+                return true;
+            }
+
+            return targetFramework.Version >= new Version(4, 0);
+        }
+
         private static bool IsSameAttribute(Type attributeType, AttributeSyntax attributeSyntax)
         {
             var name = attributeSyntax.Name.ToString();

test/dotnet-build.Tests/BuildOutputTests.cs

@@ -7,6 +7,7 @@ using FluentAssertions;
 using Microsoft.DotNet.ProjectModel;
 using Microsoft.DotNet.Tools.Test.Utilities;
 using Microsoft.Extensions.PlatformAbstractions;
+using NuGet.Frameworks;
 using Xunit;
 
 namespace Microsoft.DotNet.Tools.Builder.Tests
@@ -128,6 +129,35 @@ namespace Microsoft.DotNet.Tools.Builder.Tests
             informationalVersion.Should().BeEquivalentTo("1.0.0-85");
         }
 
+        [Theory]
+        [InlineData("net20", false)]
+        [InlineData("net40", true)]
+        [InlineData("net461", true)]
+        [InlineData("dnxcore50", true)]
+        public void MultipleFrameworks_ShouldHaveValidTargetFrameworkAttribute(string frameworkName, bool shouldHaveTargetFrameworkAttribute)
+        {
+            var framework = NuGetFramework.Parse(frameworkName);
+
+            var testInstance = TestAssetsManager.CreateTestInstance("TestLibraryWithMultipleFrameworks")
+                                                .WithLockFiles();
+
+            var cmd = new BuildCommand(Path.Combine(testInstance.TestRoot, Project.FileName), framework: framework.GetShortFolderName());
+            cmd.ExecuteWithCapturedOutput().Should().Pass();
+
+            var output = Path.Combine(testInstance.TestRoot, "bin", "Debug", framework.GetShortFolderName(), "TestLibraryWithMultipleFrameworks.dll");
+            var targetFramework = PeReaderUtils.GetAssemblyAttributeValue(output, "TargetFrameworkAttribute");
+
+            if (shouldHaveTargetFrameworkAttribute)
+            {
+                targetFramework.Should().NotBeNull();
+                targetFramework.Should().BeEquivalentTo(framework.DotNetFrameworkName);
+            }
+            else
+            {
+                targetFramework.Should().BeNull();
+            }
+        }
+
         [Fact]
         public void ResourceTest()
         {