How to pass argument to NAnt exec task conditionally based on the property existence?

I dag out an old thread at NAnt mailing list, which actually gives the best suggestion for a workaround (to my case). It's based on the fact that <property> task works as expected. The workaround introduces an extra property to set depending on whether the original property is defined or not.

Like this:

<target name="example">  <property name="arg.value" value="${arg}" if="${property::exists('arg')}" />  <property name="arg.value" value="" unless="${property::exists('arg')}" />  <exec program="program.exe">    <arg value="${arg.value}" />  </exec></target>

In my case it turns out to be easier to always set a property with default value, and even don't introduce an extra property - just pass the original property to <arg/>:

<target name="example">  <property name="arg.value" value="default value" overwrite="false" />  <exec program="program.exe">    <arg value="${arg.value}" />  </exec></target>

It can (and will) be overwritten in other include files.

I have also found another thread where the same question was asked, and a person involved into the NAnt development suggested to indicate whether people are interested in a patch to fix this for <arg> element. As far as I can see, no one demonstrated the interest :), hence the behavior wasn't changed.

I managed to put a little time investigating what a fix could be, and it seems that it's only about adding ExpandProperties=false to the TaskAttribute for e.g. the line property of the <arg> task. The same is done for <property value="..."> attribute where it works as expected. I didn't try it out, though - if I have some more time one of these days, I will and post an update here.

For that case, you can just use a fileset to pass as the arguments. By default, fileset does not include items that did not match actual files on disk.

I also ran into this issue (to be honest even more than once). The if-attribute in some cases doesn't work as you might expect it to. This is my ugly workaround:

<choose>  <when test="${property::exists('file')}">    <exec program="notepad.exe">      <arg line="${file}" />    </exec>  </when>  <otherwise>    <exec program="notepad.exe" />  </otherwise></choose>