Gael Fraiteur reported differing behavior with the Exists condition in MSBuild 3.5, which has been verified to be a bug by Microsoft.

Here is the setup. Have one file import a file from a different folder. Have that imported file import another relative to that file's path. Add a condition to check if the file exists to the Import task.

File 1 (foo.proj):

<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
    <Import Project="..\Bar\bar.targets" />
    <Target Name="Build">
        <Message Text="$(FooBar)"/>
    </Target>
</Project>

File 2 (bar.targets):

<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
    <Import Project="message.targets" Condition="Exists('message.targets')"/>
</Project>

File 3 (message.targets):

<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
    <PropertyGroup>
        <FooBar>Hello World!</FooBar>
    </PropertyGroup>
</Project>

If you run C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\msbuild.exe foo.proj, "Hello World" will be displayed. If you run C:\WINDOWS\Microsoft.NET\Framework\v3.5\msbuild.exe foo.proj, the text will be empty and nothing will display.

You can use absolute paths to get around this relative pathing issue in Exists. However, in the case of imports, I recommend removing the condition. In most cases your build should fail if the script failed to import a file.