var client = container.Resolve<MyService>();
        A.CallTo(() => client.GetUserProfile(userName)).Returns(null);

[Test]
    public void MyTest(string sitecollectionGuid, string customerName)
    {
        var mockHttpContext = SetupHttpContext(sitecollectionGuid, customerName);

        var client = container.Resolve<MyService>();
        A.CallTo(() => client.GetUserProfile(userName)).Returns(null);

        var controllerContext = new ControllerContext(mockHttpContext, new RouteData(), A.Fake<ControllerBase>());
        controller.ControllerContext = controllerContext;

        var result = controller.CheckUsername(userName);
        Assert.IsNotNull(result, "Result is not as expected");
    }

public UserDAO GetUserProfile(string userName)
    {
        UserDAO objUserProfile = new UserDAO();
        IUsers objUsers = (IUsers)Global.Container["Users"];
        IUser objUser = objUsers.GetByUserName(userName);
        if (objUser == null)
        {

            return null;
        }
        else
        {
            objUserProfile = AutoMapper.Mapper.Map<IUser, UserDAO>(objUser);
            objUserProfile.FirstName = objUser.FirstName;
            objUserProfile.MiddleName = objUser.MiddleName;
            objUserProfile.LastName = objUser.LastName;
            ....................
            ....................
            <setting other properties>
            ....................
            ....................

            return objUserProfile;
        }
    }

public UserDAO GetUserProfile(string userName)

UserDAO returnValue = null;
var client = container.Resolve<MyService>();
        A.CallTo(() => client.GetUserProfile(userName)).Returns(returnValue);