Create an MSBuild inline task with RoslynCodeTaskFactory

RoslynCodeTaskFactory uses the cross-platform Roslyn compilers to generate in-memory task assemblies for use as inline tasks. RoslynCodeTaskFactory tasks target .NET Standard and can work on .NET Framework and .NET Core runtimes as well as other platforms such as Linux and macOS.

The structure of an inline task with RoslynCodeTaskFactory

RoslynCodeTaskFactory inline tasks are declared by using the UsingTask element. The inline task and the UsingTask element that contains it are typically included in a .targets file and imported into other project files as required. Here's a basic inline task. Notice that it does nothing.

<Project>
  <!-- This simple inline task does nothing. -->
  <UsingTask
    TaskName="DoNothing"
    TaskFactory="RoslynCodeTaskFactory"
    AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.Core.dll" >
    <ParameterGroup />
    <Task>
      <Reference Include="" />
      <Using Namespace="" />
      <Code Type="Fragment" Language="cs">
      </Code>
    </Task>
  </UsingTask>
</Project>

The UsingTask element in the example has three attributes that describe the task and the inline task factory that compiles it.

• The TaskName attribute names the task, in this case, DoNothing.

• The TaskFactory attribute names the class that implements the inline task factory.

• The AssemblyFile attribute gives the location of the inline task factory. Alternatively, you can use the AssemblyName attribute to specify the fully qualified name of the inline task factory class, which is typically located in the global assembly cache (GAC).

The remaining elements of the DoNothing task are empty and are provided to illustrate the order and structure of an inline task. A more robust example is presented later in this article.

• The ParameterGroup element is optional. When specified, it declares the parameters for the task. For more information about input and output parameters, see Input and Output Parameters later in this article.

• The Task element describes and contains the task source code.

• The Reference element specifies references to the .NET assemblies that you're using in your code. This is equivalent to adding a reference to a project in Visual Studio. The Include attribute specifies the path of the referenced assembly.

• The Using element lists the namespaces that you want to access. This element resembles the using directive in Visual C#. The Namespace attribute specifies the namespace to include.

Reference and Using elements are language-agnostic. Inline tasks can be written in any one of the supported .NET CodeDom languages, for example, Visual Basic or Visual C#.

Code element

The last child element to appear within the Task element is the Code element. The Code element contains or locates the code that you want to be compiled into a task. What you put in the Code element depends on how you want to write the task.

The Language attribute specifies the language in which your code is written. Acceptable values are cs for C#, vb for Visual Basic.

The Type attribute specifies the type of code that is found in the Code element.

• If the value of Type is Class, then the Code element contains code for a class that derives from the ITask interface.

• If the value of Type is Method, then the code defines an override of the Execute method of the ITask interface.

• If the value of Type is Fragment, then the code defines the contents of the Execute method, but not the signature or the return statement.

The code itself typically appears between a <![CDATA[ marker and a ]]> marker. Because the code is in a CDATA section, you do not have to worry about escaping reserved characters, for example, "<" or ">".

Alternatively, you can use the Source attribute of the Code element to specify the location of a file that contains the code for your task. The code in the source file must be of the type that is specified by the Type attribute. If the Source attribute is present, the default value of Type is Class. If Source is not present, the default value is Fragment.

Hello World

Here is a more robust inline task with RoslynCodeTaskFactory. The HelloWorld task displays "Hello, world!" on the default error logging device, which is typically the system console or the Visual Studio Output window. The Reference element in the example is included just for illustration.

<Project>
  <!-- This simple inline task displays "Hello, world!" -->
  <UsingTask
    TaskName="HelloWorld"
    TaskFactory="RoslynCodeTaskFactory"
    AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.Core.dll" >
    <ParameterGroup />
    <Task>
      <Reference Include="System.Xml"/>
      <Using Namespace="System"/>
      <Using Namespace="System.IO"/>
      <Code Type="Fragment" Language="cs">
<![CDATA[
// Display "Hello, world!"
Log.LogError("Hello, world!");
]]>
      </Code>
    </Task>
  </UsingTask>
</Project>

You could save the HelloWorld task in a file that is named HelloWorld.targets, and then invoke it from a project as follows.

<Project>
  <Import Project="HelloWorld.targets" />
  <Target Name="Hello">
    <HelloWorld />
  </Target>
</Project>

Input and output parameters

Inline task parameters are child elements of a ParameterGroup element. Every parameter takes the name of the element that defines it. The following code defines the parameter Text.

<ParameterGroup>
    <Text />
</ParameterGroup>

Parameters may have one or more of these attributes:

• Required is an optional attribute that is false by default. If true, then the parameter is required and must be given a value before calling the task.

• ParameterType is an optional attribute that is System.String by default. It may be set to any fully qualified type that is either an item or a value that can be converted to and from a string by using System.Convert.ChangeType. (In other words, any type that can be passed to and from an external task.)

• Output is an optional attribute that is false by default. If true, then the parameter must be given a value before returning from the Execute method.

For example,

<ParameterGroup>
    <Expression Required="true" />
    <Files ParameterType="Microsoft.Build.Framework.ITaskItem[]" Required="true" />
    <Tally ParameterType="System.Int32" Output="true" />
</ParameterGroup>

defines these three parameters:

• Expression is a required input parameter of type System.String.

• Files is a required item list input parameter.

• Tally is an output parameter of type System.Int32.

If the Code element has the Type attribute of Fragment or Method, then properties are automatically created for every parameter. In RoslynCodeTaskFactory, if the Code element has the Type attribute of Class, then you do not have to specify the ParameterGroup, since it is inferred from the source code (this is a difference from CodeTaskFactory). Otherwise, properties must be explicitly declared in the task source code, and must exactly match their parameter definitions.

Example

The following inline task logs some messages and returns a string.

<Project>

    <UsingTask TaskName="MySample"
               TaskFactory="RoslynCodeTaskFactory"
               AssemblyFile="$(MSBuildBinPath)\Microsoft.Build.Tasks.Core.dll">
        <ParameterGroup>
            <Parameter1 ParameterType="System.String" Required="true" />
            <Parameter2 ParameterType="System.String" />
            <Parameter3 ParameterType="System.String" Output="true" />
        </ParameterGroup>
        <Task>
            <Using Namespace="System" />
            <Code Type="Fragment" Language="cs">
              <![CDATA[
              Log.LogMessage(MessageImportance.High, "Hello from an inline task created by Roslyn!");
              Log.LogMessageFromText($"Parameter1: '{Parameter1}'", MessageImportance.High);
              Log.LogMessageFromText($"Parameter2: '{Parameter2}'", MessageImportance.High);
              Parameter3 = "A value from the Roslyn CodeTaskFactory";
            ]]>
            </Code>
        </Task>
    </UsingTask>

    <Target Name="Demo">
      <MySample Parameter1="A value for parameter 1" Parameter2="A value for parameter 2">
          <Output TaskParameter="Parameter3" PropertyName="NewProperty" />
      </MySample>

      <Message Text="NewProperty: '$(NewProperty)'" />
    </Target>
</Project>

These inline tasks can combine paths and get the file name.

<Project>

    <UsingTask TaskName="PathCombine"
               TaskFactory="RoslynCodeTaskFactory"
               AssemblyFile="$(MSBuildBinPath)\Microsoft.Build.Tasks.Core.dll">
        <ParameterGroup>
            <Paths ParameterType="System.String[]" Required="true" />
            <Combined ParameterType="System.String" Output="true" />
        </ParameterGroup>
        <Task>
            <Using Namespace="System" />
            <Code Type="Fragment" Language="cs">
            <![CDATA[
            Combined = Path.Combine(Paths);
            ]]>
            </Code>
        </Task>
    </UsingTask>

    <UsingTask TaskName="PathGetFileName"
             TaskFactory="RoslynCodeTaskFactory"
             AssemblyFile="$(MSBuildBinPath)\Microsoft.Build.Tasks.Core.dll">
        <ParameterGroup>
            <Path ParameterType="System.String" Required="true" />
            <FileName ParameterType="System.String" Output="true" />
        </ParameterGroup>
        <Task>
            <Using Namespace="System" />
            <Code Type="Fragment" Language="cs">
            <![CDATA[
            FileName = System.IO.Path.GetFileName(Path);
            ]]>
            </Code>
        </Task>
    </UsingTask>

    <Target Name="Demo">
        <PathCombine Paths="$(Temp);MyFolder;$([System.Guid]::NewGuid()).txt">
            <Output TaskParameter="Combined" PropertyName="MyCombinedPaths" />
        </PathCombine>

        <Message Text="Combined Paths: '$(MyCombinedPaths)'" />

        <PathGetFileName Path="$(MyCombinedPaths)">
            <Output TaskParameter="FileName" PropertyName="MyFileName" />
        </PathGetFileName>

        <Message Text="File name: '$(MyFileName)'" />
    </Target>
</Project>

Provide backward compatibility

RoslynCodeTaskFactory first became available in MSBuild version 15.8. Suppose you want to support previous versions of Visual Studio and MSBuild, when RoslynCodeTaskFactory was not available, but CodeTaskFactory was, but you want to use the same build script. You can use a Choose construct that uses the $(MSBuildVersion) property to decide at build time whether to use the RoslynCodeTaskFactory or fall back to CodeTaskFactory, as in the following example:

<Project Sdk="Microsoft.NET.Sdk" DefaultTargets="RunTask">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net8.0</TargetFramework>
  </PropertyGroup>

  <Choose>
    <When Condition=" '$(MSBuildVersion.Substring(0,2))' >= 16 Or
    ('$(MSBuildVersion.Substring(0,2))' == 15 And '$(MSBuildVersion.Substring(3,1))' >= 8)">
      <PropertyGroup>
        <TaskFactory>RoslynCodeTaskFactory</TaskFactory>
      </PropertyGroup>
    </When>
    <Otherwise>
      <PropertyGroup>
        <TaskFactory>CodeTaskFactory</TaskFactory>
      </PropertyGroup>
    </Otherwise>
  </Choose>
  
  <UsingTask
    TaskName="HelloWorld"
    TaskFactory="$(TaskFactory)"
    AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.Core.dll">
    <ParameterGroup />
    <Task>
      <Using Namespace="System"/>
      <Using Namespace="System.IO"/>
      <Code Type="Fragment" Language="cs">
        <![CDATA[
         Log.LogError("Using RoslynCodeTaskFactory");
      ]]>
      </Code>
    </Task>
  </UsingTask>

  <Target Name="RunTask" AfterTargets="Build">
    <Message Text="MSBuildVersion: $(MSBuildVersion)"/>
    <Message Text="TaskFactory: $(TaskFactory)"/>
    <HelloWorld />
  </Target>

</Project>

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• Tasks