Troubleshooting Async Method Issues in Unit Tests: Resolving FirstOrDefaultAsync() Failures

Problem with Testing FirstOrDefaultAsync() in My Example Code

I have the method below where I'm fetching the user ID by username, and I've written tests for it. However, the tests are not working when using FirstOrDefaultAsync(). If I switch to FirstOrDefault(), the tests work fine. Here's the code:

Method Being Tested:

public async Task<int> GetUserId(string userName)
{
   return await _unitofWork.Repository<UserEntity>().Entities
                              .Where(u => u.UserName == userName)
                              .Select(i => i.Id)
                              .FirstOrDefaultAsync();
}

Test Code:

[Fact]
public async Task GetUserIdTests()
{
    //Arrange
    var userEntity= new List<UserEntity>
    {
        new UserEntity
        {
            UserName = "testuser", Id = 1
        }
    };
    var repositoryMock = new Mock<IGenericRepository<UserEntity>>();
    repositoryMock.Setup(r => r.Entities).Returns(new TestAsyncEnumerable<UserEntity>(systemUsers).AsQueryable());
    _unitOfWorkMock.Setup(u => u.Repository<UserEntity>()).Returns(repositoryMock.Object);

    var service= new service(_userContextMock.Object,_vaultServiceMock.Object);
    //Act
    var result = await service.GetUserId("testuser");
    //Assert
    Assert.NotEqual(0, result);

}

Issue: This is just an example I created to test the FirstOrDefaultAsync() method, but it is not working. The test works correctly when using FirstOrDefault() but fails with FirstOrDefaultAsync().

Supporting Code: Here is the TestAsyncEnumerable class that I'm using to support async operations in the test:

public class TestAsyncEnumerable<T> : EnumerableQuery<T>, IAsyncEnumerable<T>, IQueryable<T>
 {
     public TestAsyncEnumerable(IEnumerable<T> enumerable) : base(enumerable) { }
     public TestAsyncEnumerable(Expression expression) : base(expression) { }

     public IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken cancellationToken = default)
     {
         return new TestAsyncEnumerator<T>(this.AsEnumerable().GetEnumerator());
     }

     IAsyncEnumerator<T> IAsyncEnumerable<T>.GetAsyncEnumerator(CancellationToken cancellationToken)
     {
         return GetAsyncEnumerator(cancellationToken);
     }

     IQueryProvider IQueryable.Provider => new TestAsyncQueryProvider<T>(this);
 }

 public class TestAsyncEnumerator<T> : IAsyncEnumerator<T>
 {
     private readonly IEnumerator<T> _inner;

     public TestAsyncEnumerator(IEnumerator<T> inner)
     {
         _inner = inner;
     }

     public ValueTask DisposeAsync()
     {
         _inner.Dispose();
         return ValueTask.CompletedTask;
     }

     public ValueTask<bool> MoveNextAsync()
     {
         return new ValueTask<bool>(_inner.MoveNext());
     }

     public T Current => _inner.Current;
 }

 public class TestAsyncQueryProvider<T> : IAsyncQueryProvider
 {
     private readonly IQueryProvider _inner;

     public TestAsyncQueryProvider(IQueryProvider inner)
     {
         _inner = inner;
     }

     public IQueryable CreateQuery(Expression expression)
     {
         return new TestAsyncEnumerable<T>(expression);
     }

     public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
     {
         return new TestAsyncEnumerable<TElement>(expression);
     }

     public object Execute(Expression expression)
     {
         return _inner.Execute(expression);
     }

     public TResult Execute<TResult>(Expression expression)
     {
         return _inner.Execute<TResult>(expression);
     }

     public TResult ExecuteAsync<TResult>(Expression expression, CancellationToken cancellationToken)
     {
         return Execute<TResult>(expression);
     }

     public IAsyncEnumerable<TResult> ExecuteAsync<TResult>(Expression expression)
     {
         return new TestAsyncEnumerable<TResult>(expression);
     }
 }

Question: What might be causing the test to fail when using FirstOrDefaultAsync()? Is there something wrong with how I've implemented the TestAsyncEnumerable class or how the async test is set up?

**Error:**
Message: 
System.ArgumentException : Argument expression is not valid

  Stack Trace: 
IQueryProvider.Execute[TElement](Expression expression)
TestAsyncQueryProvider`1.Execute[TResult](Expression expression) line 72
TestAsyncQueryProvider`1.ExecuteAsync[TResult](Expression expression, CancellationToken cancellationToken) line 78
EntityFrameworkQueryableExtensions.ExecuteAsync[TSource,TResult](MethodInfo operatorMethodInfo, IQueryable`1 source, Expression expression, CancellationToken cancellationToken)
EntityFrameworkQueryableExtensions.ExecuteAsync[TSource,TResult](MethodInfo operatorMethodInfo, IQueryable`1 source, CancellationToken cancellationToken)
EntityFrameworkQueryableExtensions.FirstOrDefaultAsync[TSource](IQueryable`1 source, CancellationToken cancellationToken)