Execute alternate set of tasks depending on the configuration being built.

    <choose>
    <when test="${build.config == 'Debug'}">
        <!-- compile app in debug configuration -->
        ...
    </when>
    <when test="${build.config == 'Release'}">
        <!-- compile app in release configuration -->
        ...
    </when>
    <otherwise>
        <fail>Build configuration '${build.config}' is not supported!</fail>
    </otherwise>
</choose>

Define a sources patternset holding an alternate set of patterns depending on the configuration being built.

    <choose>
    <when test="${build.config == 'Debug'}">
        <patternset id="sources">
            <include name="**/*.cs" />
        </patternset>
    </when>
    <when test="${build.config == 'Release'}">
        <patternset id="sources">
            <include name="**/*.cs" />
            <exclude name="**/Instrumentation/*.cs" />
        </patternset>
    </when>
    <otherwise>
        <fail>Build configuration '${build.config}' is not supported!</fail>
    </otherwise>
</choose>

The NAnt.Core.Tasks.ChooseTask selects one among a number of possible alternatives. It consists of a sequence of <when> elements followed by an optional <otherwise> element.

Each <when> element has a single attribute, test, which specifies an expression. The content of the <when> and <otherwise> elements is a set of nested tasks.

The content of the first, and only the first, <when> element whose test is true is executed. If no <when> element is true, the content of the <otherwise> element is executed. If no <when> element is true, and no <otherwise> element is present, nothing is done.