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Task Writing

Tasks provide the code that runs during the build process. Tasks are contained in targets. A library of typical tasks is included with MSBuild, and you can also create your own tasks. For more information about the library of tasks that are included with MSBuild, see MSBuild Task Reference.

Examples of tasks include Copy, which copies one or more files, MakeDir, which creates a directory, and Csc, which compiles Visual C# source code files. Each task is implemented as a .NET class that implements the ITask interface, which is defined in the Microsoft.Build.Framework.dll assembly.

There are two approaches you can use when implementing a task:

  • Implement the ITask interface directly.

  • Derive your class from the helper class, Task, which is defined in the Microsoft.Build.Utilities.dll assembly. Task implements ITask and provides default implementations of some ITask members. Additionally, logging is easier.

In both cases, you must add to your class a method named Execute, which is the method that is called when the task runs. This method takes no parameters and returns a Boolean value: true if the task succeeded or false if it failed. The following example shows a task that performs no action and returns true.

using System;
using Microsoft.Build.Framework;
using Microsoft.Build.Utilities;

namespace MyTasks
{
    public class SimpleTask : Task
    {
        public override bool Execute()
        {
            return true;
        }
    }
}

The following project file runs this task:

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
    <Target Name="MyTarget">
        <SimpleTask />
    </Target>
</Project>

When tasks run, they can also receive inputs from the project file if you create .NET properties on the task class. MSBuild sets these properties immediately before calling the task's Execute method. To create a string property, use task code such as:

using System;
using Microsoft.Build.Framework;
using Microsoft.Build.Utilities;

namespace MyTasks
{
    public class SimpleTask : Task
    {
        public override bool Execute()
        {
            return true;
         }

        private string myProperty;
        public string MyProperty
        {
            get { return myProperty; }
            set { myProperty = value; }
        }
    }
}

The following project file runs this task and sets MyProperty to the given value:

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <Target Name="MyTarget">
      <SimpleTask MyProperty="Value for MyProperty" />
   </Target>
</Project>

If a project is going to run a task, MSBuild must know how to locate the assembly that contains the task class. Tasks are registered using the UsingTask Element (MSBuild).

The MSBuild file Microsoft.Common.Tasks is a project file that contains a list of UsingTask elements that register all the tasks that are supplied with MSBuild. This file is automatically included when building every project. If a task that is registered in Microsoft.Common.Tasks is also registered in the current project file, the current project file takes precedence; that is, you can override a default task with your own task that has the same name.

Tip Tip

You can see a list of the tasks that are supplied with MSBuild by viewing the contents of Microsoft.Common.Tasks.

If your task derives from the Task helper class, you can use any of the following helper methods on the Task class to raise events that will be caught and displayed by any registered loggers:

public override bool Execute()
{
    Log.LogError("messageResource1", "1", "2", "3");
    Log.LogWarning("messageResource2");
    Log.LogMessage(MessageImportance.High, "messageResource3");
    ...
}

If your task implements ITask directly, you can still raise such events but you must use the IBuildEngine interface. The following example shows a task that implements ITask and raises a custom event:

public class SimpleTask : ITask
{
    private IBuildEngine buildEngine;
    public IBuildEngine BuildEngine
    {
        get{ return buildEngine; }
        set{ buildEngine = value; }
    }

    public override bool Execute()
    {
        TaskEventArgs taskEvent =
            new TaskEventArgs(BuildEventCategory.Custom,
            BuildEventImportance.High, "Important Message",
           "SimpleTask");
        BuildEngine.LogBuildEvent(taskEvent);
        return true;
    }
}

You can mark certain task properties as "required" so that any project file that runs the task must set values for these properties or the build fails. Apply the [Required] attribute to the .NET property in your task as follows:

private string requiredProperty;

[Required]
public string RequiredProperty
{
    get { return requiredProperty; }
    set { requiredProperty = value; }
}

The [Required] attribute is defined by RequiredAttribute in the Microsoft.Build.Framework namespace.

This following Visual C# class demonstrates a task deriving from the Task helper class. This task returns true, indicating that it succeeded.

using System;
using Microsoft.Build.Utilities;

namespace SimpleTask1
{
    public class SimpleTask1: Task
    {
        public override bool Execute()
        {
            // This is where the task would presumably do its work.
            return true;
        }
    }
}

This following Visual C# class demonstrates a task implementing the ITask interface. This task returns true, indicating that it succeeded.

using System;
using Microsoft.Build.Framework;

namespace SimpleTask2
{
    public class SimpleTask2: ITask
    {
        //When implementing the ITask interface, it is necessary to
        //implement a BuildEngine property of type
        //Microsoft.Build.Framework.IBuildEngine. This is done for
        //you if you derive from the Task class.
        private IBuildEngine buildEngine;
        public IBuildEngine BuildEngine
        {
            get
            {
                return buildEngine;
            }
            set
            {
                buildEngine = value;
            }
         }

        // When implementing the ITask interface, it is necessary to
        // implement a HostObject property of type Object.
        // This is done for you if you derive from the Task class.
        private Object hostObject;
        public Object HostObject
        {
            get
            {
                return hostObject;
            }

            set
            {
                hostObject = value;
            }
        }

        public bool Execute()
        {
            // This is where the task would presumably do its work.
            return true;
        }
    }
}

This Visual C# class demonstrates a task that derives from the Task helper class. It has a required string property, and raises an event that is displayed by all registered loggers.

using System;
using Microsoft.Build.Framework;
using Microsoft.Build.Utilities;

namespace SimpleTask3
{
	public class SimpleTask3 : Task
	{
		private string myProperty;

		// The [Required] attribute indicates a required property. 
		// If a project file invokes this task without passing a value 
		// to this property, the build will fail immediately.
		[Required]
		public string MyProperty
		{
			get
			{
				return myProperty;
			}
			set
			{
				myProperty = value;
			}
		}

		public override bool Execute()
		{
			// Log a high-importance comment
			Log.LogMessage(MessageImportance.High,
				"The task was passed \"" + myProperty + "\".");
			return true;
		}
	}
}

The following example shows a project file invoking the previous example task, SimpleTask3.

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
    <UsingTask TaskName="SimpleTask3.SimpleTask3" 
        AssemblyFile="SimpleTask3\bin\debug\simpletask3.dll"/>

    <Target Name="MyTarget">
        <SimpleTask3 MyProperty="Hello!"/>
    </Target>
</Project>
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