Item metadata in task batching
MSBuild has the ability to divide item lists into different categories, or batches, based on item metadata, and run a task one time with each batch. It can be confusing to understand exactly what items are being passed with which batch. This topic covers the following common scenarios that involve batching.
Dividing an item list into batches
Dividing several item lists into batches
Batching one item at a time
Filtering item lists
For more information on batching with MSBuild, see Batching.
Divide an item list into batches
Batching allows you to divide an item list into different batches based on item metadata, and pass each of the batches into a task separately. This is useful for building satellite assemblies.
The following example shows how to divide an item list into batches based on item metadata. The ExampColl item list is divided into three batches based on the Number item metadata. The presence of %(ExampColl.Number)in the Text attribute notifies MSBuild that batching should be performed. The ExampColl item list is divided into three batches based on the Number metadata, and each batch is passed separately into the task.
<Project
xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<ItemGroup>
<ExampColl Include="Item1">
<Number>1</Number>
</ExampColl>
<ExampColl Include="Item2">
<Number>2</Number>
</ExampColl>
<ExampColl Include="Item3">
<Number>3</Number>
</ExampColl>
<ExampColl Include="Item4">
<Number>1</Number>
</ExampColl>
<ExampColl Include="Item5">
<Number>2</Number>
</ExampColl>
<ExampColl Include="Item6">
<Number>3</Number>
</ExampColl>
</ItemGroup>
<Target Name="ShowMessage">
<Message
Text = "Number: %(ExampColl.Number) -- Items in ExampColl: @(ExampColl)"/>
</Target>
</Project>
The Message task displays the following information:
Number: 1 -- Items in ExampColl: Item1;Item4
Number: 2 -- Items in ExampColl: Item2;Item5
Number: 3 -- Items in ExampColl: Item3;Item6
Divide several item lists into batches
MSBuild can divide multiple item lists into batches based on the same metadata. This makes it easy to divide different item lists into batches to build multiple assemblies. For example, you could have an item list of .cs files divided into an application batch and an assembly batch, and an item list of resource files divided into an application batch and an assembly batch. You could then use batching to pass these item lists into one task and build both the application and the assembly.
Note
If an item list being passed into a task contains no items with the referenced metadata, every item in that item list is passed into every batch.
The following example shows how to divide multiple item lists into batches based on item metadata. The ExampColl and ExampColl2 item lists are each divided into three batches based on the Number item metadata. The presence of %(Number)in the Text attribute notifies MSBuild that batching should be performed. The ExampColl and ExampColl2 item lists are divided into three batches based on the Number metadata, and each batch is passed separately into the task.
<Project
xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<ItemGroup>
<ExampColl Include="Item1">
<Number>1</Number>
</ExampColl>
<ExampColl Include="Item2">
<Number>2</Number>
</ExampColl>
<ExampColl Include="Item3">
<Number>3</Number>
</ExampColl>
<ExampColl2 Include="Item4">
<Number>1</Number>
</ExampColl2>
<ExampColl2 Include="Item5">
<Number>2</Number>
</ExampColl2>
<ExampColl2 Include="Item6">
<Number>3</Number>
</ExampColl2>
</ItemGroup>
<Target Name="ShowMessage">
<Message
Text = "Number: %(Number) -- Items in ExampColl: @(ExampColl) ExampColl2: @(ExampColl2)"/>
</Target>
</Project>
The Message task displays the following information:
Number: 1 -- Items in ExampColl: Item1 ExampColl2: Item4
Number: 2 -- Items in ExampColl: Item2 ExampColl2: Item5
Number: 3 -- Items in ExampColl: Item3 ExampColl2: Item6
Batch one item at a time
Batching can also be performed on well-known item metadata that is assigned to every item upon creation. This guarantees that every item in a collection will have some metadata to use for batching. The Identity metadata value is useful for dividing every item in an item list into a separate batch. For a complete list of well-known item metadata, see Well-known item metadata.
The following example shows how to batch each item in an item list one at a time. The ExampColl item list is divided into six batches, each batch containing one item of the item list. The presence of %(Identity)in the Text attribute notifies MSBuild that batching should be performed.
<Project
xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<ItemGroup>
<ExampColl Include="Item1"/>
<ExampColl Include="Item2"/>
<ExampColl Include="Item3"/>
<ExampColl Include="Item4"/>
<ExampColl Include="Item5"/>
<ExampColl Include="Item6"/>
</ItemGroup>
<Target Name="ShowMessage">
<Message
Text = "Identity: '%(Identity)' -- Items in ExampColl: @(ExampColl)"/>
</Target>
</Project>
The Message task displays the following information:
Identity: 'Item1' -- Items in ExampColl: Item1
Identity: 'Item2' -- Items in ExampColl: Item2
Identity: 'Item3' -- Items in ExampColl: Item3
Identity: 'Item4' -- Items in ExampColl: Item4
Identity: 'Item5' -- Items in ExampColl: Item5
Identity: 'Item6' -- Items in ExampColl: Item6
However, Identity is not guaranteed to be unique; its value is the evaluated final value of the Include attribute. Therefore, if any Include attributes are used multiple times, they are batched together. As the following example shows, this technique requires the Include attributes to be unique for each item in the group. To illustrate this point, consider the following code:
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<ItemGroup>
<Item Include="1">
<M>1</M>
</Item>
<Item Include="1">
<M>2</M>
</Item>
<Item Include="2">
<M>3</M>
</Item>
</ItemGroup>
<Target Name="Batching">
<Warning Text="@(Item->'%(Identity): %(M)')" Condition=" '%(Identity)' != '' "/>
</Target>
</Project>
The output shows that the first two items are in the same batch, because the Include attribute is the same for them:
test.proj(15,5): warning : 1: 1;1: 2
test.proj(15,5): warning : 2: 3
Filter item lists
Batching can be used to filter out certain items from an item list before passing it to a task. For example, filtering on the Extension well-known item metadata value allows you to run a task on only files with a specific extension.
The following example shows how to divide an item list into batches based on item metadata, and then filter those batches when they are passed into a task. The ExampColl item list is divided into three batches based on the Number item metadata. The Condition attribute of the task specifies that only batches with a Number item metadata value of 2 will be passed into the task
<Project
xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<ItemGroup>
<ExampColl Include="Item1">
<Number>1</Number>
</ExampColl>
<ExampColl Include="Item2">
<Number>2</Number>
</ExampColl>
<ExampColl Include="Item3">
<Number>3</Number>
</ExampColl>
<ExampColl Include="Item4">
<Number>1</Number>
</ExampColl>
<ExampColl Include="Item5">
<Number>2</Number>
</ExampColl>
<ExampColl Include="Item6">
<Number>3</Number>
</ExampColl>
</ItemGroup>
<Target Name="Exec">
<Message
Text = "Items in ExampColl: @(ExampColl)"
Condition="'%(Number)'=='2'"/>
</Target>
</Project>
The Message task displays the following information:
Items in ExampColl: Item2;Item5