How does msbuild determine whether a project should be built?

MSBuild will always build a project. However it may not build all of the targets inside of the project. This will depend on whether or not the outputs produced by the targets are up to date with respect to the inputs.

When MSBuild builds a project it will compare the inputs of the targets with the outputs of the targets based on the timestamp of these inputs and outputs. If the inputs are newer then the outputs then the target will build.

This partial building of a project is called incremental build. Here is a link to the msdn site which explains how incremental build works:

http://msdn2.microsoft.com/en-us/library/ms171483.aspx

Do any of you know how to determine whether a given file needs to be built using the extensibility model (without compiling first). I've been searching high and low for this info and am starting to think it can't be done using conventional means. If anyone can shed some light on the situation it would be greately appreciated. Thanks very much.