Programming by Contract in MsBuild

One of the great ways to prevent bugs creeping into your code during development is to use contracts and assertions for what you know is true or what should be true at particular points in your code. Using this technique allows you to automatically catch the kinds of errors that are easy to cause (e.g. by forgetting to set a required property correctly) and are often difficult to track down.

Spending a while developing MsBuild scripts that are used by non-technical team mates leaves open the possiblity that they may be misused or and fail to build. The only clue to what has gone wrong can be a cryptic error message.

The Error task in MsBuild can be used to allow a sort of programming by contract if you make use of the Condition attribute.

For example, say the script relies on a file path existing and having the value loaded into a specific property before a target uses it.

I can embed an Error task in the script to make sure the value has been defined:

<Error 
	Text="The FooBar property has not been set." 
	Condition=" '$(FooBar)' == '' " 
	/>

and I can embed another Error task to make sure that wherever the value is pointing actually exists:

<Error 
	Text="The file $(FooBar) does not exist" 
	Condition=" !Exists($(FooBar)) " 
	/>

I have found using this approach over the last couple of months has saved time tracking down silly bugs and made my scripts much more reliable.