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AlexVallat/msbuild-itemgroup-intersection.md

Created March 4, 2021 18:20
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MSBuild ItemGroup Intersection
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msbuild-itemgroup-intersection.md

It's not obvious how to produce an intersection of two itemgroups in MSBuild. There's this brainteaser solution based on tricky metadata behaviour, but it only works inside a <target> due to it's use of metadata.

There is another way. Algebra tells us that A ∩ B = A ∖ (A∖B): intersection of A and B is A excluding (A excluding B). This can be expressed in MSBuild as:

<ItemGroup>
	<A Include="1" />
	<A Include="2" />
	<A Include="3" />
</ItemGroup>
<ItemGroup>
	<B Include="2" />
	<B Include="3" />
	<B Include="4" />
</ItemGroup>
<ItemGroup>
	<A_not_B Include="@(A)" Exclude="@(B)" />
	<A_intersect_B Include="@(A)" Exclude="@(A_not_B)" />
</ItemGroup>

<Message Text="A∩B: @(A_intersect_B)"/>: A∩B: 2;3

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