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: $(PropA)"/>  </Target></Project>Try invoking it using both of the following invocations and observe its output -1)>msbuild blog.proj /t:Target1;Target2;Target3;Target42)>msbuild blog.proj /t:Target1;Target2;Target3;Target4 "/p:PropA=PropA set on command line"Then try those two invocations with the following three variations of specifying PropA at the project level -1)  <PropertyGroup>     <PropA Condition=" '$(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>