Msbuild ItemGroup unexpectedly accumulates files to copy

Question

I am writing an msbuild script to deploy multiple targets. I am trying to reuse some elements and am getting an unexpected behavior.

When I run this proj file:

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Target Name="All" DependsOnTargets="DeployTarget1;DeployTarget2">
  </Target>
  <Target Name="DeployTarget1">
    <PropertyGroup>
      <RootDir>.\source1</RootDir>
    </PropertyGroup>
    <ItemGroup>
      <DeployFiles Include="$(RootDir)\**\*.dll" Exclude="$(RootDir)\bin\C_*.xml" />
    </ItemGroup>
    <Copy SourceFiles="@(DeployFiles)" DestinationFiles="@(DeployFiles->'D:\deploytest\dest1\%(RecursiveDir)%(Filename)%(Extension)')" />
  </Target>
  <Target Name="DeployTarget2">
    <PropertyGroup>
      <RootDir>.\source2</RootDir>
    </PropertyGroup>
    <ItemGroup>
      <DeployFiles Include="$(RootDir)\**\*.dll" Exclude="$(RootDir)\bin\C_*.xml" />
    </ItemGroup>
    <Copy SourceFiles="@(DeployFiles)" DestinationFiles="@(DeployFiles->'D:\deploytest\dest2\%(RecursiveDir)%(Filename)%(Extension)')" />
  </Target>
</Project>

DeployTarget1 copies the files from the source1 directory recursively to dest1 as I expect.

DeployTarget2 copies both source1 and source2 to dest2, this is not what I expected.

D:\deploytest>msbuild /t:All test.proj
Microsoft (R) Build Engine version 4.0.30319.17929
[Microsoft .NET Framework, version 4.0.30319.18052]
Copyright (C) Microsoft Corporation. All rights reserved.

Build started 8/20/2013 2:37:17 PM.
Project "D:\deploytest\test.proj" on node 1 (All target(s)).
DeployTarget1:
  Copying file from ".\source1\test1.dll" to "D:\deploytest\dest1\test1.dll".
  copy /y ".\source1\test1.dll" "D:\deploytest\dest1\test1.dll"
DeployTarget2:
  Copying file from ".\source1\test1.dll" to "D:\deploytest\dest2\test1.dll".
  copy /y ".\source1\test1.dll" "D:\deploytest\dest2\test1.dll"
  Copying file from ".\source2\test2.dll" to "D:\deploytest\dest2\test2.dll".
  copy /y ".\source2\test2.dll" "D:\deploytest\dest2\test2.dll"
Done Building Project "D:\deploytest\test.proj" (All target(s)).


Build succeeded.
    0 Warning(s)
    0 Error(s)

Time Elapsed 00:00:00.06

Does anyone know why this happens?
Can you point me to documentation on this "feature"?

Answer 1

ItemGroups are cummulative. Each time an item group if referenced, items are added to the existing group. If you want to have explicit groups, you either need to give them a unique name or you need to clear them before usage.

Use <DeployFiles Remove="**/*">

See also: https://stackoverflow.com/a/7915992/736079