Unit testing controller methods which return IActionResult

Assuming something like the

public IActionResult GetOrders() {
    var orders = repository.All();
    return Ok(orders);
}

the controller in this case is returning an OkObjectResult class.

Cast the result to the type of what you are returning in the method and perform your assert on that

[Fact]
public void GetOrders_WithOrdersInRepo_ReturnsOk() {
    // arrange
    var controller = new OrdersController(new MockRepository());

    // act
    var result = controller.GetOrders();
    var okResult = result as OkObjectResult;

    // assert
    Assert.IsNotNull(okResult);
    Assert.AreEqual(200, okResult.StatusCode);
}