Question

MSBuild emits the following message for up to date targets:

Skipping target "MyTarget" because all output files are up-to-date with respect to the input files.

How is the actual check performed?

Answer 1

Check flow of Incremental Build:

A target element can have both an Inputs attribute, which indicates
what items the target > expects as input, and an Outputs attribute,
which indicates what items it produces as output
MSBuild attempts to find a 1-to-1 mapping
between the values of these attributes. If a 1-to-1 mapping exists,
MSBuild compares the time stamp of every input item to the time stamp
of its corresponding output item. Output files that have no 1-to-1
mapping are compared to all input files. An item is considered
up-to-date if its output file is the same age or newer than its input
file or files.

If all output items are up-to-date, MSBuild skips the target. This
incremental build of the target can significantly improve the build
speed. If only some files are up-to-date, MSBuild executes the target
but skips the up-to-date items, and thereby brings all items
up-to-date. This is known as a partial incremental build.