How to verify extension method via Moq

user_7803911 0
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In the Configure method I'm calling my extension method

app.UseJsonResponse();

My extension method 

public static class JsonResponseExtension
{
    public static IApplicationBuilder UseJsonResponse(this IApplicationBuilder app)
    {
        app.UseMiddleware<JsonResponseMiddleware>();
        return app;
    }
}

I want to test UseJsonResponse method. I wrote

public class JsonResponseExtensionTest
{
    [Fact]
    public void UseJsonResponse()
    {
        var iApplicationBuilderMock = new Mock<IApplicationBuilder>();
        iApplicationBuilderMock
            .Verify(b => b.UseMiddleware<JsonResponseMiddleware>(), Times.Never);

    }
}

And i'm getting an exception

Extension methods (here: UseMiddlewareExtensions.UseMiddleware) may not be used in setup / verification expressions.

How to resolve this problem?

asp.net-core xunit moq mocking
1 Answers

UseMiddleware is an extension which can't be mocked, you've got to mock what the extension invokes on the interface. In this case the method is probably

IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware);

Source

I would need to know more about JsonResponseMiddleware as the UseMiddleware extension, and the extensions that it too invokes, has quite a bit going on, some of it to do with the interfaces JsonResponseMiddleware implements. For reference, the UseMiddleware extensions can be found here.

With the information at hand, after invoking your UseJsonResponse extension the following will verify that Use was invoked once.

[Test]
public void Test1()
{
    var applicationBuilderMock = new Mock<IApplicationBuilder>();
    var mockedApplicationBuilder = applicationBuilderMock.Object;

    mockedApplicationBuilder.UseJsonResponse();

    applicationBuilderMock.Verify(x => x.Use(It.IsAny<Func<RequestDelegate, RequestDelegate>>()), Times.Once);
}

To take it further I would need to know more about JsonResponseMiddleware as mentioned. It also looks like a lot more effort to include a check for the JsonResponseMiddleware type.

user_29758100
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