Generate error message for duplicate items

src/Tasks/Microsoft.NET.Build.Tasks/CheckForDuplicateItems.cs

@@ -0,0 +1,50 @@
+using Microsoft.Build.Framework;
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using System.Text;
+
+namespace Microsoft.NET.Build.Tasks
+{
+    public class CheckForDuplicateItems : TaskBase
+    {
+        [Required]
+        public ITaskItem [] Items { get; set; }
+
+        [Required]
+        public string ItemName { get; set; }
+
+        public bool DefaultItemsEnabled { get; set; }
+
+        public bool DefaultItemsOfThisTypeEnabled { get; set; }
+
+        [Required]
+        public string PropertyNameToDisableDefaultItems { get; set; }
+
+        public string PropertyValueToDisableDefaultItems { get; set; } = "false";
+
+        [Required]
+        public string MoreInformationLink { get; set; }
+
+        protected override void ExecuteCore()
+        {
+            if (DefaultItemsEnabled && DefaultItemsOfThisTypeEnabled)
+            {
+                var duplicateItems = Items.GroupBy(i => i.ItemSpec).Where(g => g.Count() > 1).ToList();
+                if (duplicateItems.Any())
+                {
+                    string duplicateItemsFormatted = string.Join("; ", duplicateItems.Select(d => $"'{d.Key}'"));
+
+                    string message = string.Format(CultureInfo.CurrentCulture, Strings.DuplicateItemsError,
+                        ItemName,
+                        PropertyNameToDisableDefaultItems,
+                        PropertyValueToDisableDefaultItems,
+                        duplicateItemsFormatted);
+
+                    Log.LogError(message);
+                }
+            }
+        }
+    }
+}

src/Tasks/Microsoft.NET.Build.Tasks/Microsoft.NET.Build.Tasks.csproj

@@ -35,6 +35,7 @@
     </PackageReference>
   </ItemGroup>
   <ItemGroup>
+    <Compile Include="CheckForDuplicateItems.cs" />
     <Compile Include="DiagnosticMessageSeverity.cs" />
     <Compile Include="DiagnosticsHelper.cs" />
     <Compile Include="NETSdkError.cs" />

src/Tasks/Microsoft.NET.Build.Tasks/Resources/Strings.resx

@@ -225,4 +225,7 @@
   <data name="NU1012" xml:space="preserve">
     <value>Dependency conflict. '{0}' expected '{1}' but received '{2}'</value>
   </data>
+  <data name="DuplicateItemsError" xml:space="preserve">
+    <value>Duplicate '{0}' items were included. The .NET SDK includes '{0}' items from your project directory by default. You can either remove these items from your project file, or set the '{1}' property to '{2}' if you want to explicitly include them in your project file. The duplicate items were: {3}</value>
+  </data>
 </root>

src/Tasks/Microsoft.NET.Build.Tasks/build/Microsoft.NET.Sdk.DefaultItems.targets

@@ -52,4 +52,45 @@ Copyright (c) .NET Foundation. All rights reserved.
     <Content Remove="$(DefaultRemoves)" />
   </ItemGroup>
 
+  <UsingTask TaskName="CheckForDuplicateItems" AssemblyFile="$(MicrosoftNETBuildTasksAssembly)" />
+
+  <Target Name="CheckForDuplicateItems" BeforeTargets="_CheckForInvalidConfigurationAndPlatform">
+
+    <PropertyGroup>
+      <!-- TODO: Create a fwlink for this -->
+      <DefaultItemsMoreInformationLink>https://github.com/dotnet/sdk</DefaultItemsMoreInformationLink>
+
+      <DefaultPackageReferencesEnabled Condition="'$(DisableImplicitFrameworkReferences)' != 'true'">true</DefaultPackageReferencesEnabled>
+    </PropertyGroup>
+
+    <CheckForDuplicateItems
+      Items="@(Compile)"
+      ItemName="Compile"
+      DefaultItemsEnabled="$(EnableDefaultItems)"
+      DefaultItemsOfThisTypeEnabled="$(EnableDefaultCompileItems)"
+      PropertyNameToDisableDefaultItems="EnableDefaultCompileItems"
+      MoreInformationLink="$(DefaultItemsMoreInformationLink)"
+      />
+
+    <CheckForDuplicateItems
+      Items="@(EmbeddedResource)"
+      ItemName="EmbeddedResource"
+      DefaultItemsEnabled="$(EnableDefaultItems)"
+      DefaultItemsOfThisTypeEnabled="$(EnableDefaultEmbeddedResourceItems)"
+      PropertyNameToDisableDefaultItems="EnableDefaultEmbeddedResourceItems"
+      MoreInformationLink="$(DefaultItemsMoreInformationLink)"
+      />
+
+    <!-- TODO: Do we check for duplicate content items here, or in the Web SDK? EnableDefaultContentItems isn't defined or used at all by the .NET SDK -->
+    <CheckForDuplicateItems
+      Items="@(Content)"
+      ItemName="Content"
+      DefaultItemsEnabled="$(EnableDefaultItems)"
+      DefaultItemsOfThisTypeEnabled="$(EnableDefaultContentItems)"
+      PropertyNameToDisableDefaultItems="EnableDefaultContentItems"
+      MoreInformationLink="$(DefaultItemsMoreInformationLink)"
+      />
+
+  </Target>
+
 </Project>

test/Microsoft.NET.Build.Tests/GivenThereAreDefaultItems.cs

@@ -14,6 +14,7 @@
 using System.Xml.Linq;
 using Xunit;
 using static Microsoft.NET.TestFramework.Commands.MSBuildTest;
+using Microsoft.NET.TestFramework.ProjectConstruction;
 
 namespace Microsoft.NET.Build.Tests
 {
@@ -352,6 +353,82 @@ public void Compile_items_can_be_explicitly_specified_while_default_EmbeddedReso
             GivenThatWeWantAllResourcesInSatellite.TestSatelliteResources(_testAssetsManager, projectChanges, setup, "ExplicitCompileDefaultEmbeddedResource");
         }
 
+        [Fact]
+        public void It_gives_an_error_message_if_duplicate_compile_items_are_included()
+        {
+            var testProject = new TestProject()
+            {
+                Name = "DuplicateCompileItems",
+                TargetFrameworks = "netstandard1.6",
+                IsSdkProject = true
+            };
+
+            var testAsset = _testAssetsManager.CreateTestProject(testProject)
+                .WithProjectChanges(project =>
+                {
+                    var ns = project.Root.Name.Namespace;
+                    var itemGroup = new XElement(ns + "ItemGroup");
+                    project.Root.Add(itemGroup);
+                    itemGroup.Add(new XElement(ns + "Compile", new XAttribute("Include", @"**\*.cs")));
+                })
+                .Restore(testProject.Name);
+
+            var buildCommand = new BuildCommand(Stage0MSBuild, Path.Combine(testAsset.TestRoot, testProject.Name));
+
+            WriteFile(Path.Combine(buildCommand.ProjectRootPath, "Class1.cs"), "public class Class1 {}");
+
+            buildCommand
+                .CaptureStdOut()
+                .Execute()
+                .Should()
+                .Fail()
+                .And.HaveStdOutContaining("DuplicateCompileItems.cs")
+                .And.HaveStdOutContaining("Class1.cs")
+                .And.HaveStdOutContaining("EnableDefaultCompileItems");
+        }
+
+        [Fact]
+        public void It_gives_the_correct_error_if_duplicate_compile_items_are_included_and_default_items_are_disabled()
+        {
+            var testProject = new TestProject()
+            {
+                Name = "DuplicateCompileItems",
+                TargetFrameworks = "netstandard1.6",
+                IsSdkProject = true
+            };
+
+            var testAsset = _testAssetsManager.CreateTestProject(testProject, "DuplicateCompileItemsWithDefaultItemsDisabled")
+                .WithProjectChanges(project =>
+                {
+                    var ns = project.Root.Name.Namespace;
+
+                    project.Root.Element(ns + "PropertyGroup").Add(
+                        new XElement(ns + "EnableDefaultCompileItems", "false"));
+
+                    var itemGroup = new XElement(ns + "ItemGroup");
+                    project.Root.Add(itemGroup);
+                    itemGroup.Add(new XElement(ns + "Compile", new XAttribute("Include", @"**\*.cs")));
+                    itemGroup.Add(new XElement(ns + "Compile", new XAttribute("Include", @"DuplicateCompileItems.cs")));
+                })
+                .Restore(testProject.Name);
+
+            var buildCommand = new BuildCommand(Stage0MSBuild, Path.Combine(testAsset.TestRoot, testProject.Name));
+
+            WriteFile(Path.Combine(buildCommand.ProjectRootPath, "Class1.cs"), "public class Class1 {}");
+
+            buildCommand
+                .CaptureStdOut()
+                .Execute()
+                .Should()
+                .Fail()
+                .And.HaveStdOutContaining("DuplicateCompileItems.cs")
+                //  Class1.cs wasn't included multiple times, so it shouldn't be mentioned
+                .And.NotHaveStdOutMatching("Class1.cs")
+                //  Default items weren't enabled, so the error message should come from the C# compiler and shouldn't include the information about default compile items
+                .And.HaveStdOutContaining("MSB3105")
+                .And.NotHaveStdOutMatching("EnableDefaultCompileItems");
+        }
+
         void RemoveGeneratedCompileItems(List<string> compileItems)
         {
             //  Remove auto-generated compile items.

test/Microsoft.NET.TestFramework/TestAsset.cs

@@ -42,8 +42,7 @@ internal void FindProjectFiles()
         {
             _projectFiles = new List<string>();
 
-            var files = Directory.GetFiles(base.Path, "*.*", SearchOption.AllDirectories)
-                .Where(file => !IsInBinOrObjFolder(file));
+            var files = Directory.GetFiles(base.Path, "*.*", SearchOption.AllDirectories);
 
             foreach (string file in files)
             {