Property overwrite behaviour

Posted by jeremyj on Geeks with Blogs See other posts from Geeks with Blogs or by jeremyj
Published on Wed, 22 Aug 2012 14:06:21 GMT Indexed on 2012/08/27 21:40 UTC
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I thought it worth sharing about property overwrite behaviour because i found it confusing at first in the hope of preventing some learning pain for the uninitiated with MSBuild :-)

The confusion for me came because of the redundancy of using a Condition statement in a _project_ level property to test that a property has not been previously set. What i mean is that the following two statements are always identical in behaviour, regardless if the property has been supplied on the command line -

  <PropertyGroup>
    <PropA Condition=" '$(PropA)' == '' ">PropA set at project level</PropA>
  </PropertyGroup>

has the same behaviour regardless of command line override as -

  <PropertyGroup>
    <PropA>PropA set at project level</PropA>
  </PropertyGroup>

 i.e. the two above property declarations have the same result whether the property is overridden on the command line or not.

To prove this experiment with the following .proj file -

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" >

  <PropertyGroup>
    <PropA Condition=" '$(PropA)' == '' ">PropA set at project level</PropA>
  </PropertyGroup>

  <Target Name="Target1">
    <Message Text="PropA: $(PropA)"/>
  </Target>

  <Target Name="Target2">
    <PropertyGroup>
      <PropA>PropA set in Target2</PropA>
    </PropertyGroup>
    <Message Text="PropA: $(PropA)"/>
  </Target>

  <Target Name="Target3">
    <PropertyGroup>
      <PropA Condition=" '$(PropA)' == '' ">PropA set in Target3</PropA>
    </PropertyGroup>
    <Message Text="PropA: $(PropA)"/>
  </Target>

  <Target Name="Target4">
    <PropertyGroup>
      <PropA Condition=" '$(PropA)' != '' ">PropA set in Target4</PropA>
    </PropertyGroup>
    <Message Text="PropA: $(PropA)"/>
  </Target>

</Project>

Try invoking it using both of the following invocations and observe its output -

1)
>msbuild blog.proj /t:Target1;Target2;Target3;Target4

2)
>msbuild blog.proj /t:Target1;Target2;Target3;Target4 "/p:PropA=PropA set on command line"

Then try those two invocations with the following three variations of specifying PropA at the project level -

1)
  <PropertyGroup>
    <PropA Condition=" '$(PropA)' == '' ">PropA set at project level</PropA>
  </PropertyGroup>

2)
  <PropertyGroup>
    <PropA>PropA set at project level</PropA>
  </PropertyGroup>

3)
  <PropertyGroup>
    <PropA Condition=" '$(PropA)' != '' ">PropA set at project level</PropA>
  </PropertyGroup>

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