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Mocking indexed property
public Node GetNode(IMyInterface interface, string itemName)
{
    return interface.Items[itemName];
}

var expected = new Node();
var itemName = "TestName";
var mock = new Mock<IMyInterface>();
mock.Setup(f => f.Items[itemName]).Returns(expected);
var target = new MyClass();

var actual = target.GetNode(mock.Object, itemName);
Assert.AreEqual(expected, actual);

mock.Setup(f => f.Items[itemName]).Returns(expected);

mock.Setup(f => f.get_Items(itemName)).Returns(expected);