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Editorial Team
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Asked: 2026-05-31T17:10:42+00:00

I have a .NET 4.0 C# solution with a tests project which runs unit

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I have a .NET 4.0 C# solution with a tests project which runs unit tests under NUnit. The NUnit binaries are v3.5.

I can run the tests perfectly well, but I can’t set breakpoints and single step in Visual Studio. I’m guessing this is caused by the mismatch in .NET versions. Is there a way to single step through a v4.0 tests assembly using NUnit for v3.5?

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  1. Editorial Team

    The problem is that unless you tell it otherwise, NUnit will spawn a subprocess to run tests when it determines it necessary. If you watch it in Process Explorer, you can see that "nunit-console.exe"* spawns "nunit-agent.exe"*. The Visual Studio debugger doesn’t automatically attach to child processes.

    In this case, I believe the version mismatch is why it chooses to start a subprocess. The easiest way to work around this is to edit "nunit-console.exe.config"* to change the set of <supportedRuntime> values. There should already be a comment there marking the line that you should comment out in order to force it to run as .NET 4.0:

    <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>

    Once you change that, the first NUnit process will already be .NET 4.0 and it shouldn’t need to spawn a subprocess. If you want to be sure, specify /process=Single and NUnit will either run in a single process or fail immediately if it cannot.

    * – If you need to use the x86 versions, substitute:

    nunit-console.exe        -> nunit-console-x86.exe
nunit-agent.exe          -> nunit-agent-x86.exe
nunit-console.exe.config -> nunit-console-x86.exe.config
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