How do I update one item with the other item's metadata?

[Nirmal4G]

The more elaborate question would be...

How do I update one item with the other item's metadata when their identities are different and somehow I need to compare different metadata for equality?

  <Target Name="Default">
    <ItemGroup>
      <A Update="@(B->'%(Meta0)')" Meta0="%(B.Identity)" Meta1="%(B.Meta1)%(B.Meta2)" />
    </ItemGroup>
  </Target>

For all the B.Meta0 items present in A items, update A.Meta0 with B's Identity and A.Meta1 with B.Meta{1,2}

[KalleOlaviNiemitalo]

According to Item element (MSBuild), the Update attribute is not valid in an Item in an ItemGroup in a Target. I wish MSBuild warned about that.

So, if you need to do this in a Target and cannot use Update, I think a custom task (inline or otherwise) will be the easiest solution.

[Nirmal4G]

I know but I also know there's another way using batching but I myself don't know how to write it correctly. That's why I asked here.

I wish MSBuild warned about that (not supporting the Update on the item in the target).

Me Too!

[rainersigwald]

Me three, but it is a breaking change that we have to consider fairly carefully: #2835

[Nirmal4G]

Let me think about this out loud, previously specifying Update is ignored and processed all items, meaning that anyone who used Update should know that their code wasn't working, like me and must have used batching to solve this. So, even if we provide Update filter does the batching still happen or do we go through different route for Update as in regular item groups?

For Example

<!-- V1 -->
<Item1 Condition="'%(Identity)' != '' AND '@(Item1)' != '' AND '@(Item2)' != ''" Metadata="Updated" />

<!-- V2 -->
<Item1 Update="@(Item2)" Condition="'%(Identity)' != '' AND '@(Item1)' != '' AND '@(Item2)' != ''" Metadata="Updated" />

<!-- V3 -->
<Item1 Update="@(Item2)" Metadata="Updated" />

Users could've written V1, V2 but unlikely V3 when Update doesn't work like they intended. V1 and V2 are similar now but that'll change when Update is supported. Hence, the breaking change. But does it also mean that actual user code still produces the same output even after Update is supported since both filters are same?

So, what's the actual difference between the above three variants which is preventing us from supporting Update in items inside targets?

[KalleOlaviNiemitalo]

For batching, this seems to work:

<?xml version="1.0" encoding="utf-8"?>
<!-- https://github.com/dotnet/msbuild/discussions/7753 -->
<Project>
  <Target Name="Default">
    <ItemGroup>
      <A Include="Blue" Meta0="Carrot" Meta1="Happy" Meta2="Pitch" />
      <A Include="Red" Meta0="Bilberry" Meta1="Anxious" Meta2="Yaw" />
      <A Include="Cyan" Meta0="Rhubarb" Meta1="Tired" Meta2="Roll" />
    </ItemGroup>
    <ItemGroup>
      <B Include="Garlic" Meta0="Blue" Meta1="Curious" Meta2="Desk" />
      <B Include="Cucumber" Meta0="Red" Meta1="Flabbergasted" Meta2="Chair" />
      <B Include="Olive" Meta0="Purple" Meta1="Clever" Meta2="Bookshelf" />
    </ItemGroup>

    <ItemGroup>
      <_SwapB Include="%(B.Meta0)">
        <Meta0>%(Identity)</Meta0>
        <Meta1>%(Meta1)%(Meta2)</Meta1>
      </_SwapB>
      <_ModifiedA Include="@(A)" Condition="%(Identity) != '' and '@(_SwapB)' != ''">
        <Meta0>@(_SwapB->'%(Meta0)')</Meta0>
        <Meta1>@(_SwapB->'%(Meta1)')</Meta1>
      </_ModifiedA>
      <A Remove="@(_ModifiedA)" />
      <A Include="@(_ModifiedA)" />
    </ItemGroup>

    <Message Importance="high" Text="A: Identity=%(A.Identity) Meta0=%(A.Meta0) Meta1=%(A.Meta1) Meta2=%(A.Meta2)" />
  </Target>
</Project>

Output:

  A: Identity=Cyan Meta0=Rhubarb Meta1=Tired Meta2=Roll
  A: Identity=Blue Meta0=Garlic Meta1=CuriousDesk Meta2=Pitch
  A: Identity=Red Meta0=Cucumber Meta1=FlabbergastedChair Meta2=Yaw

[KalleOlaviNiemitalo]

However, if you need to preserve the order of A items, then I don't know how to do that.

[Nirmal4G]

While looking into your solution, I also found a way to make this even simpler and as a side effect, it seems to preserve order at lease in my tests but do please test and confirm before using it!

[Nirmal4G]

Here's another (simpler) variant of the solution proposed by @KalleOlaviNiemitalo

<ItemGroup>
	<_SwapB Include="@(B->'%(Meta0)')">
		<Meta0>%(B.Identity)</Meta0>
		<Meta1>%(B.Meta1)%(B.Meta2)</Meta1>
		<Meta2/> <!-- this not needed it seems but I don't know why! -->
	</_SwapB>

	<A Condition="'%(Identity)' != '' AND '@(A)' != '' AND '@(_SwapB)' != ''">
		<Meta0>@(_SwapB->'%(Meta0)')</Meta0>
		<Meta1>@(_SwapB->'%(Meta1)')</Meta1>
	</A>

	<_SwapB Remove="@(_SwapB)"/>
</ItemGroup>

This is almost the same except we don't need to Modify A items separately. I still don't like the extra _SwapB but I don't know how to optimize any more than this!

But a request to MSBuild team... @Forgind @rainersigwald
I would certainly like my proposed code better than this.
Please provide Update support for items within targets!