Target Build Order

Articolo
04/28/2015

Targets must be ordered if the input to one target depends on the output of another target. You can use these attributes to specify the order in which targets are run:

InitialTargets. This Project attribute specifies the targets that will run first, even if targets are specified on the command line or in the DefaultTargets attribute.
DefaultTargets. This Project atttribute specifies which targets are run if a target is not specified explicitly on the command line.
DependsOnTargets. This Target attribute specifies targets that must run before this target can run.
BeforeTargets and AfterTargets. These Target attributes specify that this target should run before or after the specified targets (MSBuild 4.0).

A target is never run twice during a build, even if a subsequent target in the build depends on it. Once a target has been run, its contribution to the build is complete.

Targets may have a Condition attribute. If the specified condition evaluates to false, the target isn't executed and has no effect on the build. For more information about conditions, see MSBuild Conditions.

Initial Targets

The InitialTargets attribute of the Project element specifies targets that will run first, even if targets are specified on the command line or in the DefaultTargets attribute. Initial targets are typically used for error checking.

The value of the InitialTargets attribute can be a semicolon-delimited, ordered list of targets. The following example specifies that the Warm target runs, and then the Eject target runs.

<Project InitialTargets="Warm;Eject" xmlns="https://schemas.microsoft.com/developer/msbuild/2003">

Imported projects may have their own InitialTargets attributes. All initial targets are aggregated together and run in order.

For more information, see How to: Specify Which Target to Build First.

Default Targets

The DefaultTargets attribute of the Project element specifies which target or targets are built if a target isn't specified explicitly in a command line.

The value of the DefaultTargets attribute can be a semicolon-delimited, ordered list of default targets. The following example specifies that the Clean target runs, and then the Build target runs.

<Project DefaultTargets="Clean;Build" xmlns="https://schemas.microsoft.com/developer/msbuild/2003">

You can override the default targets by using the /target switch on the command line. The following example specifies that the Build target runs, and then the Report target runs. When you specify targets in this way, any default targets are ignored.

msbuild /target:Build;Report

If both initial targets and default targets are specified, and if no command-line targets are specified, MSBuild runs the initial targets first, and then runs the default targets.

Imported projects may have their own DefaultTargets attributes. The first DefaultTargets attribute encountered determines which default targets will run.

For more information, see How to: Specify Which Target to Build First.

First Target

If there are no initial targets, default targets, or command-line targets, then MSBuild runs the first target it encounters in the project file or any imported project files.

Target Dependencies

Targets can describe dependency relationships with each other. The DependsOnTargets attribute indicates that a target depends on other targets. For example,

<Target Name="Serve" DependsOnTargets="Chop;Cook" />

tells MSBuild that the Serve target depends on the Chop target and the Cook target. MSBuild runs the Chop target, and then runs the Cook target before it runs the Serve target.

Before Targets and After Targets

In MSBuild 4.0, you can specify target order by using the BeforeTargets and AfterTargets attributes.

Consider the following script.

<Project DefaultTargets="Compile;Link" xmlns="https://schemas.microsoft.com/developer/msbuild/2003">
    <Target Name="Compile">
        <Message Text="Compiling" />
    </Target>
    <Target Name="Link">
        <Message Text="Linking" />
    </Target>
</Project>

To create an intermediate target Optimize that runs after the Compile target, but before the Link target, add the following target anywhere in the Project element.

    <Target Name="Optimize" 
        AfterTargets="Compile" BeforeTargets="Link">
        <Message Text="Optimizing" />
    </Target>

Determining the Target Build Order

MSBuild determines the target build order as follows:

InitialTargets targets are run.
Targets specified on the command line by the /target switch are run. If you specify no targets on the command line, then the DefaultTargets targets are run. If neither is present, then the first target encountered is run.
The Condition attribute of the target is evaluated. If the Condition attribute is present and evaluates to false, the target isn't executed and has no further effect on the build.
Before a target is executed, its DependsOnTargets targets are run.
Before a target is executed, any target that lists it in a BeforeTargets attribute is run.
Before a target is executed, its Inputs attribute and Outputs attribute are compared. If MSBuild determines that any output files are out of date with respect to the corresponding input file or files, then MSBuild executes the target. Otherwise, MSBuild skips the target.
After a target is executed or skipped, any target that lists it in an AfterTargets attribute is run.

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