I just tried to use NUnit 2.6.0 in a .NET 4.0 solution, but it failed to recognize my test assembly.
No matter which runner I used, NUnit always said that it couldn’t find any tests:

Console runner:

GUI runner:

I spent a few minutes staring at the screen and wondering if I made a mistake somethere.

It couldn’t be the test class, because the test class was very simple:

[TestFixture]
public class SomeTest
{
    [Test]
    public void JustATest()
    {
        Assert.IsTrue(true);
    }
}

After a while, I opened the config files of one of the runners (nunit.exe.config for nunit.exe) and found this at the beginning:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
    <!--
        The .NET 2.0 build of the console runner only
        runs under .NET 2.0 or higher. The setting
        useLegacyV2RuntimeActivationPolicy only applies
        under .NET 4.0 and permits use of mixed mode
        assemblies, which would otherwise not load
        correctly.
    -->
    <startup useLegacyV2RuntimeActivationPolicy="true">
        <!-- Comment out the next line to force use of .NET 4.0 -->
        <supportedRuntime version="v2.0.50727" />
        <supportedRuntime version="v4.0.30319" />
    </startup>

Apparently NUnit was running on .NET 2.0 (or 3.5, see the screenshot from the console runner above), so it wasn’t able to open my .NET 4.0 test assembly.

So the solution is to comment out the first supportedRuntime line (line 3 in this example) in the config files of every runner you need:

<startup useLegacyV2RuntimeActivationPolicy="true">
    <!-- Comment out the next line to force use of .NET 4.0 -->
    <!-- <supportedRuntime version="v2.0.50727" /> -->
    <supportedRuntime version="v4.0.30319" />
</startup>

In my case, I did that in:

• nunit.exe.config (for the GUI, which I use during development)
• nunit-console.exe.config (for the console runner, which I use in my build script).

Apparently, this setting was added in NUnit 2.6.0, because I still had a working example solution from a few months ago where I used NUnit 2.5.10.

The example solution had very simple test classes similar to the one above, and I didn’t have to change the config files there – in fact, this whole <startup> section didn’t even exist.