NSubstitute keeps reference to GC collectable objects while detached from events about nsubstitute

[alexandrnikitin]

Hi @Jekops,
Thank you for the great repro! I copied the relevant parts below

public class Parent : IDisposable
{
    private IChild m_Child;

    public Parent(IChild child)
    {
        m_Child = child;
        m_Child.Changed += Child_Changed;
    }

    public void Dispose()
    {
        m_Child.Changed -= Child_Changed;
        m_Child = null;
    }

    private void Child_Changed(object sender, EventArgs args)
    {
    }
}

public class ParentTest : IDisposable
{
    // this field holds a reference to the substitute created below
   // the object is still available in the Dispose method and not reclaimed by GC yet
    // because this instance of ParentTest is still alive
    private IChild m_Child;
    private Parent m_SUT;

    public ParentTest()
    {
        m_Child = Substitute.For<IChild>();
        m_SUT = new Parent(m_Child);
    }

    public void Dispose()
    {
        m_SUT.Dispose();
        m_SUT = null;
        // 
    }
}

NSubstitute keeps the state and setup in its instances. It's an instance of IChild substitute in this case. When you subscribe to the event with m_Child.Changed += Child_Changed; it also keeps a reference to the current instance of the Parent class.

The observed behavior happens because an instance of the ParentTest test class keep a reference to the substitute and its state in its field m_Child. So it looks like this xUnit -> ParentTest -> Substitute.For<IChild> -> Parent. The field will be reclaimed by GC after the ParentTest (note: oversimplified, GC is a complex beast).

Or you can assign the field to null in your Dispose method then you will get what you want:

public void Dispose()
{
    m_Child = null;
    m_SUT.Dispose();
    m_SUT = null;
}

I hope it helps.