Finally got the hang of partial s(t)ubs with nsubstitute. The key is that the original method is called when setting up the sub. To avoid this use argument matchers and the DoNotCallBase() method.

// sample class to test nsubstitute
public class TestMe
{
    // property
    public virtual string Text { 
        get { throw new Exception(); } 
    }
    // parameterless returning method
    public virtual TestMe Copy() { 
        throw new Exception(); 
    }
    // void method with parameter
    public virtual void DoSomething(int index) { 
        throw new Exception(); 
    }
    // returning method with parameter
    public virtual object ReturnSomething(int index) { 
        throw new Exception(); 
    }
}

Property

To test/setup the property to return something and not throw an exception you can use this code.

[Fact]
public void SubStitute_TestMe_Text() {
    var ignoreMe = default(string);
    var expectedString = "hello world";
    var fakeTestMe = Substitute.ForPartsOf<TestMe>();
    // call the property get by assigning the value to some variable
    fakeTestMe.When(x => ignoreMe = x.Text).DoNotCallBase();
    fakeTestMe.Text.Returns<string>(expectedString);
    Assert.Equal(expectedString, fakeTestMe.Text);
}

Parameterless returning method

This looks like the get property, except it is a (real) method. See code below for setup.

[Fact]
public void SubStitute_TestMe_Copy() {
    var fakeTestMe = Substitute.ForPartsOf<TestMe>();
    fakeTestMe.When(x => x.Copy()).DoNotCallBase();
    fakeTestMe.Copy().Returns(fakeTestMe);
    Assert.Same(fakeTestMe, fakeTestMe.Copy());
}

Void method with parameter

For the setup use a parameter matcher. This prevents the real code from executing in the When setup.

[Fact]
public void SubStitute_TestMe_DoSomething() {
    var someRandomNumber = 1234;
    var fakeTestMe = Substitute.ForPartsOf<TestMe>();
    fakeTestMe.When(x => x.DoSomething(Arg.Any<int>()))
              .DoNotCallBase();
    fakeTestMe.DoSomething(someRandomNumber);
}

Returning method with parameter

Last flavor. Setup the return for the method.

[Fact]
public void SubStitute_TestMe_ReturnSomething() {
    var someRandomNumber = 98273;
    var fakeTestMe = Substitute.ForPartsOf<TestMe>();
    fakeTestMe.When(x => x.ReturnSomething(Arg.Any<int>()))
              .DoNotCallBase();
    fakeTestMe.ReturnSomething(Arg.Any<int>())
              .ReturnsForAnyArgs(DateTime.Now); // boxing of struct
    Assert.NotNull(fakeTestMe.ReturnSomething(someRandomNumber));
}

References

Partial subs and test spies on github