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How To: Batch Tasks with Item Metadata

MSBuild has the ability to divide item collections 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 collection into batches

  • Dividing several item collections into batches

  • Batching one item at a time

  • Filtering item collections

For more information on batching with MSBuild, see MSBuild Batching.

Batching allows you to divide an item collection 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 collection into batches based on item metadata. The ExampColl item collection 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 collection 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 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

MSBuild can divide multiple item collections into batches based on the same metadata. This makes it easy to divide different item collections into batches to build multiple assemblies. For example, you could have an item collection of .cs files divided into an application batch and an assembly batch, and an item collection of resource files divided into an application batch and an assembly batch. You could then use batching to pass these item collections into one task and build both the application and the assembly.

NoteNote

If an item collection being passed into a task contains no items with the referenced metadata, every item in that item collection is passed into every batch.

The following example shows how to divide multiple item collection into batches based on item metadata. The ExampColl and ExampColl2 item collections 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 collections 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 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

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 unique for every item, and is useful for dividing every item in an item collection into a separate batch. For a complete list of well-known item metadata, see MSBuild Well-known Item Metadata.

The following example shows how to batch each item in an item collection one at a time. Because the Identity metadata value of every item is unique, the ExampColl item collection is divided into six batches, each batch containing one item of the item collection. 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: &quot;%(Identity)&quot; -- Items in ExampColl: @(ExampColl)"/>
    </Target>

</Project>

The Message Task 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

Batching can be used to filter out certain items from an item collection 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 collection into batches based on item metadata, and then filter those batches when they are passed into a task. The ExampColl item collection 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 task displays the following information:

Items in ExampColl: Item2;Item5

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