Merge pull request dotnet#675 from dsplaisted/design-time-duplicate-item-error-664

Show duplicate item errors due to implicit items in design-time builds

Author: srivatsn

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

@@ -27,11 +27,18 @@ public class CheckForDuplicateItems : TaskBase
         [Required]
         public string MoreInformationLink { get; set; }
 
+        [Output]
+        public ITaskItem [] DeduplicatedItems { get; set; }
+
         protected override void ExecuteCore()
         {
+            DeduplicatedItems = Array.Empty<ITaskItem>();
+
             if (DefaultItemsEnabled && DefaultItemsOfThisTypeEnabled)
             {
-                var duplicateItems = Items.GroupBy(i => i.ItemSpec).Where(g => g.Count() > 1).ToList();
+                var itemGroups = Items.GroupBy(i => i.ItemSpec);
+
+                var duplicateItems = itemGroups.Where(g => g.Count() > 1).ToList();
                 if (duplicateItems.Any())
                 {
                     string duplicateItemsFormatted = string.Join("; ", duplicateItems.Select(d => $"'{d.Key}'"));
@@ -43,6 +50,8 @@ protected override void ExecuteCore()
                         duplicateItemsFormatted);
 
                     Log.LogError(message);
+
+                    DeduplicatedItems = itemGroups.Select(g => g.First()).ToArray();
                 }
             }
         }

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

@@ -114,10 +114,17 @@ Copyright (c) .NET Foundation. All rights reserved.
 
   <UsingTask TaskName="CheckForDuplicateItems" AssemblyFile="$(MicrosoftNETBuildTasksAssembly)" />
 
-  <Target Name="CheckForDuplicateItems" BeforeTargets="_CheckForInvalidConfigurationAndPlatform">
+  <Target Name="CheckForDuplicateItems" BeforeTargets="_CheckForInvalidConfigurationAndPlatform;CoreCompile">
 
     <PropertyGroup>
       <DefaultItemsMoreInformationLink>https://aka.ms/sdkimplicititems</DefaultItemsMoreInformationLink>
+
+      <!-- For the design-time build, we will continue on error and remove the duplicate items.
+           This is because otherwise there won't be any references to pass to the compiler, leading to design-time
+           compilation errors for every API that is used in the project.  Amidst all the compile errors, it would
+           be easy to miss the duplicate items error which is the real source of the problem. -->
+      <CheckForDuplicateItemsContinueOnError>false</CheckForDuplicateItemsContinueOnError>
+      <CheckForDuplicateItemsContinueOnError Condition="'$(DesignTimeBuild)' == 'true'">ErrorAndContinue</CheckForDuplicateItemsContinueOnError>
     </PropertyGroup>
 
     <CheckForDuplicateItems
@@ -127,7 +134,9 @@ Copyright (c) .NET Foundation. All rights reserved.
       DefaultItemsOfThisTypeEnabled="$(EnableDefaultCompileItems)"
       PropertyNameToDisableDefaultItems="EnableDefaultCompileItems"
       MoreInformationLink="$(DefaultItemsMoreInformationLink)"
-      />
+      ContinueOnError="$(CheckForDuplicateItemsContinueOnError)">
+      <Output TaskParameter="DeduplicatedItems" ItemName="DeduplicatedCompileItems" />
+    </CheckForDuplicateItems>
 
     <CheckForDuplicateItems
       Items="@(EmbeddedResource)"
@@ -136,17 +145,36 @@ Copyright (c) .NET Foundation. All rights reserved.
       DefaultItemsOfThisTypeEnabled="$(EnableDefaultEmbeddedResourceItems)"
       PropertyNameToDisableDefaultItems="EnableDefaultEmbeddedResourceItems"
       MoreInformationLink="$(DefaultItemsMoreInformationLink)"
-      />
+      ContinueOnError="$(CheckForDuplicateItemsContinueOnError)">
+      <Output TaskParameter="DeduplicatedItems" ItemName="DeduplicatedEmbeddedResourceItems" />
+    </CheckForDuplicateItems>
 
-    <!-- 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 -->
+    <!-- Default content items are enabled by the Web SDK, not the .NET SDK, but we check it here for simplicity -->
     <CheckForDuplicateItems
       Items="@(Content)"
       ItemName="Content"
       DefaultItemsEnabled="$(EnableDefaultItems)"
       DefaultItemsOfThisTypeEnabled="$(EnableDefaultContentItems)"
       PropertyNameToDisableDefaultItems="EnableDefaultContentItems"
       MoreInformationLink="$(DefaultItemsMoreInformationLink)"
-      />
+      ContinueOnError="$(CheckForDuplicateItemsContinueOnError)">
+      <Output TaskParameter="DeduplicatedItems" ItemName="DeduplicatedContentItems" />
+    </CheckForDuplicateItems>
+
+    <ItemGroup Condition="'$(DesignTimeBuild)' == 'true' And '@(DeduplicatedCompileItems)' != ''">
+      <Compile Remove="@(Compile)" />
+      <Compile Include="@(DeduplicatedCompileItems)" />
+    </ItemGroup>
+
+    <ItemGroup Condition="'$(DesignTimeBuild)' == 'true' And '@(DeduplicatedEmbeddedResourceItems)' != ''">
+      <EmbeddedResource Remove="@(EmbeddedResource)" />
+      <EmbeddedResource Include="@(DeduplicatedEmbeddedResourceItems)" />
+    </ItemGroup>
+
+    <ItemGroup Condition="'$(DesignTimeBuild)' == 'true' And '@(DeduplicatedContentItems)' != ''">
+      <Content Remove="@(Content)" />
+      <Content Include="@(DeduplicatedContentItems)" />
+    </ItemGroup>
 
   </Target>
 </Project>