Insure assertions execute on the same rendered state

[Jan-IT-Minds]

I'm experiencing an issue with the way bUnit behaves when a test contains multiple assertion statements, is there any way to ensure that all assertion statements in a test are run on the same rendered state, ie. no rerenders can occur between the assertion statements?

As an example I have this test:

public void When_Set_Then_TheTextIsDisplayedAsTheColumnHeader(string headerText)
 {
  // Arrange
  var testData = new List<Dto>() { new() { Value = null, }, };

  // Act
  var render = TestContext.RenderComponent<EnDataGridExcel<Dto>>(builder =>
  {
    builder.Add(component => component.DataSource, testData);

    builder.AddChildContent<EnGridColumnInteger<Dto>>(column =>
    {
      column.Add(component => component.Property, (d) => d.Value);
      column.Add(component => component.HeaderText, headerText);
    });
  });

  // Assert
  var columnHeader = render.Find(TableHeaderCellSelector);
  columnHeader.TextContent.Should().Be(headerText);
}

This test has failed with the following exception on the last line:
Bunit.ElementRemovedFromDomException : The DOM element you tried to access, which you previously found with the CSS selector "th", is no longer available in the DOM tree. It has probably been removed after a render.

As far as I can tell this can only occur if the state of render changes in an other thread between the execution of the find statement and the assertion. Is there any way to ensure all the statements in the Assert section of this test are run without the other thread changing the state of render between the execution of these statements?

[egil]

Hi @Jan-IT-Minds,

Take a look at https://bunit.dev/docs/verification/async-assertion.html. You basically want to wrap your assertions in a cut.WaitForAssertion(() => { ... assertions goes here });

[Jan-IT-Minds]

Thanks for your quick response @egil.

Maybe I should have expanded more on my inquiry, I had already considered that but found it to be insufficient since the nature of the issue is that I can't be certain that all the assertions I'm making are true at the same time if the state of what I'm asserting against can change between my assertion statements. Using cut.WaitForAssertion it's possible for a test that should fail to pass just based on the luck of when a rerender occurs in relation to the assertion statements.
As an example, we might have a table with two columns but it only renders one of the columns at a time, and every time it rerenders it switches which column is rendered, now suppose i have a test to ensure that both columns are rendered:

public void When_Rendered_Then_TwoColumnsAreShown()
 {
  // Arrange
  var testData = new List<Dto>() { new() { Foo = 1, Bar = 2, }, };

  // Act
  var render = TestContext.RenderComponent<BadTable<Dto>>(builder =>
  {
    builder.Add(component => component.DataSource, testData);

    builder.AddChildContent<ColumnInteger<Dto>>(column =>
    {
      column.Add(component => component.Property, (d) => d.Foo);
    });

    builder.AddChildContent<ColumnInteger<Dto>>(column =>
    {
      column.Add(component => component.Property, (d) => d.Bar);
    });
  });

  // Assert
  var cells = render.FindAll("td");
  cells[0]!.TextContent.Should().Be(1);
  cells[1]!.TextContent.Should().Be(2);
}

If a rerender occurs between the first and second assertion this test would pass. Putting the assertions in a cut.WaitForAssertion makes this more likely to occur despite the fact that the test should fail. Granted in this case you could add an other assertion statement about the cell count, but there are cases where no single assertion statement can encompass the entirety of what you're trying to test.

Is there some way to await the rerendering to stop?

[egil]

While the lambda you pass to WaitForAssertion is running, no renders should be happening. It will block rendering until the WaitForAssertion has finished running.

What testing framework are you using?

[Jan-IT-Minds]

I'm using FluentAssertions.
I had missed that WaitForAssertion actually blocks and runs after every render, I'll try converting my tests to use this method. Thanks for the help.