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Asked: 2026-05-13T06:34:49+00:00

I’m trying to mock MouseButtonEventArgs.GetPosition() with Moq, but I keep receiving this error: System.ArgumentException:

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I’m trying to mock MouseButtonEventArgs.GetPosition() with Moq, but I keep receiving this error:

System.ArgumentException: Invalid setup on a non-overridable member:
m => m.GetPosition(It.IsAny<IInputElement>())

at Moq.Mock.ThrowIfCantOverride(Expression setup, MethodInfo methodInfo)  
at Moq.Mock.<>c__DisplayClass12`2.<Setup>b__11()
at Moq.PexProtector.Invoke<T>(Func`1 function)
at Moq.Mock.Setup<T1,TResult>(Mock mock, Expression`1 expression)
at Moq.Mock`1.Setup<TResult>(Expression`1 expression)

Here is the code where I setup my mock:

    var mockMbEventArgs = new Mock<MouseButtonEventArgs>();
    mockMbEventArgs.Setup(m => m.GetPosition(It.IsAny<IInputElement>())).Returns(new Point(10.0, 10.0));

I’m not sure what I’m doing wrong, does anyone have any suggestions for how to do this?

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1 Answer

  1. Editorial Team

    This error means that you’re trying to fake a method which is not declared as virtual.

    Moq generates a type at runtime, in order to be able to fake it the generated type inherits the from original type and overrides its virtual methods. Since non-virtual methods cannot be overridden (this is the spec of the language, it’s not a limitation of Moq) it is not possible to fake these methods.

    As a solution you can wrap the class that raises the event that sends MouseButtonEventArgs and pass a class of your own that declares the relevant methods as virtual. I think it might be a little challenge in your case but it’s worth trying.

    Another solution can be to use isolation framework that enables faking non-virtual methods. Typemock Isolator for example is a framework that can do this. Isolator uses different mechanism so it allows faking this kind of methods.

    Disclaimer – I work at Typemock

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