Error while testing my async function using XUNIT in c# .net core

I have convereted my IEnumerable function into a (public async Task<List>) however im having issues fixing my unit test for that specific part. Im using fakeiteasy to mock my data, and assert that my result.count == 1 after fetching my data.

However i get this specific error

Unable to cast object of type ‘System.Threading.Tasks.Task1[System.Collections.Generic.List1[WebAPI__CodeFirst.Classes.Customer]]’ to type ‘System.Collections.Generic.IEnumerable`1[WebAPI__CodeFirst.Classes.Customer]’.

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This is my ICustomerRepository

public interface ICustomerRepository
{
    Task<List<Customer>> GetAllCustomers();
}

This is my Test for my getAll

private readonly ICustomerRepository _repo;
private readonly CustomersController _controller;

public CustomerControllerTests()
{
    _repo = A.Fake<ICustomerRepository>();
    _controller = new CustomersController(_repo);
}


[Fact]
public void GetAll_Returns_All_Customers()
{
    // Arrange
    var customers = new List<Customer>() { new Customer { customer_ID = 1 } };
    A.CallTo(() => _repo.GetAllCustomers()).Returns(customers);

    // Act
    var result = _controller.GetAll();

    // Assert
    Assert.Single(result);
}

>Solution :

Your code is asynchronous, and your test is for code that executes synchronously. Make your unit test asynchronous as well. Also, your method signature returns a Task wrapped result, not just the result.

As a side note it is considered best practice to add the word Async to your method name as a suffix if it returns a Task or Task<T>. It becomes apparent to the caller that this is an asynchronous call without having to check the return type. So GetAllCustomersAsync instead of GetAllCustomers.

[Fact]
public async Task GetAll_Returns_All_Customers()
{
    // Arrange
    var customers = new List<Customer>() { new Customer { customer_ID = 1 } };
    A.CallTo(() => _repo.GetAllCustomers()).Returns(Task.FromResult(customers));

    // Act
    var result = await _controller.GetAll();

    // Assert
    Assert.Single(result);
}