MSBuild: Factorial!

I've been doing some exploring of MSBuild as a programming language. There are some interesting results regarding mutability/immutability, but that's for another post.

This post is about functions. In particular, a Target may be invoked using the MSBuild task, so I'm exploring using Targets as functions. MSBuild can pass parameters to a Target by sending it Properties. Property changes are not propagated back to the caller, though, so getting a return value is a bit trickier.

It turns out that MSBuild does return one bit of information from a Target: its Outputs. It's possible to set the Outputs of a Target to a Property, and have that Target depend on another Target that sets that Property. In this way, it is possible to create a pair of Targets that can "calculate" the outer Target's Outputs.

By combining these approaches (setting Properties for arguments, and using the Target's Outputs as a return value), it is possible to treat a Target as a function.

To demonstrate, I wrote this program, which uses MSBuild to recursively calculate the factorial of the $(Input) property. Have fun playing!

 <Project ToolsVersion="3.5" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
    <Import Project="$(MSBuildExtensionsPath)\ExtensionPack\MSBuild.ExtensionPack.tasks"/>

    <!-- Factorial program using MSBuild recursively -->
    <Target Name="Default">

        <!-- Display usage -->
        <Error Condition="'$(Input)' == ''" 
        Text="Usage: msbuild factorial.proj [/nologo] [/clp:v=minimal] /p:Input=nnn"/>

        <!-- Argument error checking -->
        <MSBuild.ExtensionPack.Science.Maths TaskAction="Compare" 
        P1="$(Input)" P2="1" Comparison="LessThan">
            <Output TaskParameter="LogicalResult" PropertyName="InputCheck"/>
        </MSBuild.ExtensionPack.Science.Maths>
        <Error Condition="'$(InputCheck)' != 'False'" 
        Text="Input cannot be less than 1."/>

        <!-- Invoke the Factorial target with the current Input property -->
        <MSBuild Projects="$(MSBuildProjectFile)" 
        Targets="Factorial" Properties="Input=$(Input)">
            <Output TaskParameter="TargetOutputs" 
            ItemName="FactorialResult"/>
        </MSBuild>

        <!-- Display the result -->
        <Message Importance="high" Text="Result: @(FactorialResult)"/>
    </Target>

    <!-- The Factorial target uses FactorialCore to do the calculation, 
    storing the result in FactorialResult -->
    <Target Name="Factorial" 
    DependsOnTargets="FactorialCore" Outputs="$(FactorialResult)" />
    <Target Name="FactorialCore">

        <!-- If the input is 1, then the factorial is 1 -->
        <PropertyGroup Condition="'$(Input)' == '1'">
            <FactorialResult>1</FactorialResult>
        </PropertyGroup>

        <!-- If we don't know the result yet 
             (i.e., the input is not 1), then calculate the factorial -->
        <CallTarget Condition="'$(FactorialResult)' == 
            ''" Targets="CalculateFactorial"/>
    </Target>
    <Target Name="CalculateFactorial">

        <!-- Subtract 1 from $(Input) -->
        <MSBuild.ExtensionPack.Science.Maths TaskAction="Subtract" Numbers="$(Input);1">
            <Output TaskParameter="Result" PropertyName="InputMinus1"/>
        </MSBuild.ExtensionPack.Science.Maths>

        <!-- Determine the factorial of $(Input) - 1 -->
        <MSBuild Projects="$(MSBuildProjectFile)" 
            Targets="Factorial" Properties="Input=$(InputMinus1)">
            <Output TaskParameter="TargetOutputs" ItemName="SubResult"/>
        </MSBuild>

        <!-- Multiply !($(Input) - 1) by $(Input) to get the result-->
        <MSBuild.ExtensionPack.Science.Maths TaskAction="Multiply" 
            Numbers="@(SubResult);$(Input)">
            <Output TaskParameter="Result" PropertyName="FactorialResult"/>
        </MSBuild.ExtensionPack.Science.Maths>
    </Target>

    <!-- Maybe I just have way too much time on my hands... -->
</Project>

msbuild factorial.proj /nologo /clp:v=minimal /p:Input=5

Default:
Result: 120

msbuild factorial.proj /nologo /clp:v=minimal /p:Input=7

Default:
Result: 5040

Useless, but cool nonetheless.