<p>I have a bunch of <strong>.NET</strong> frameworks installed on my machine.<br>
I know that with the <strong>Java JDK</strong>, I can use the <strong>6.0</strong> version to target <strong>5.0</strong> and earlier.  </p>

<p>Can I do something similar with the <strong>.NET</strong> framework - target <strong>1.0</strong> and <strong>2.0</strong> with the <strong>3.0</strong> framework?</p>

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You need to compile with the 1.0 compilers. These are only available with the 1.0 release of the runtime/SDK.
The 2.0/3.5 compilers won't emit 1.0-compatible assemblies.
Visual Studio 2008 can generate 2.0 assemblies, but 1.0 was left off.