Any way to update MSBuild item metadata value? - msbuild

Question

I need to update item metadata values. It's easy to add to the value:
<ItemDefinitionGroup>
<ClCompile>
<PreprocessorDefinitions>FOO;BAR;%(PreprocessorDefinitions)</PreprocessorDefinitions>
</ClCompile>
</ItemDefinitionGroup>
However, what I need to do is remove part of the value. Ideally something like this would work, but it doesn't:
<ItemDefinitionGroup>
<ClCompile>
<PreprocessorDefinitions>%(PreprocessorDefinitions.Replace('FOO;',''))</PreprocessorDefinitions>
</ClCompile>
</ItemDefinitionGroup>
Is there any way to accomplish this in MSBuild 4?

Answer 1

I was trying to do the same thing, and while I couldn't figure out how to strip definitions out of the string, I did discover an additional property: UndefinePreprocessorDefinitions.
<ItemDefinitionGroup>
<ClCompile>
<UndefinePreprocessorDefinitions>FOO</UndefinePreprocessorDefinitions>
</ClCompile>
</ItemDefinitionGroup>
This will cancel out a previous definition of FOO. It might look a bit silly to pass -DFOO -UFOO to the compiler instead of nothing at all, but it works just as well.

Answer 2

In a subsequent ItemDefinitionGroup, you can create a copy of the current metadata then call Replace on that:
<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="12.0" DefaultTargets="Dump" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<ItemDefinitionGroup>
<SomeItem>
<SomeMetaData>foo,bar,baz</SomeMetaData>
</SomeItem>
</ItemDefinitionGroup>
<ItemGroup>
<SomeItem Include="one;two" />
</ItemGroup>
<ItemDefinitionGroup>
<SomeItem>
<!-- Remove "bar" -->
<SomeMetaData>$([System.String]::Copy('%(SomeMetaData)').Replace('bar',''))</SomeMetaData>
</SomeItem>
</ItemDefinitionGroup>
<Target Name="Dump">
<Message Text="SomeItem.SomeMetaData: #(SomeItem -> '%(Identity)=%(SomeMetaData)') " />
</Target>
</Project>
Here's the output when run with MSBuild 14:
> MSBuild .\foo.proj
Microsoft (R) Build Engine version 14.0.25420.1
Copyright (C) Microsoft Corporation. All rights reserved.
Build started 2/17/2017 7:09:48 PM.
Project "D:\temp\mb\foo.proj" on node 1 (default targets).
Dump:
SomeItem.SomeMetaData: one=foo,,baz;two=foo,,baz
Done Building Project "D:\temp\mb\foo.proj" (default targets).
Build succeeded.
0 Warning(s)
0 Error(s)
Time Elapsed 00:00:00.03