Unit Test for Handler Method

Question

Details: I Have tried a few different solutions, i don't use an interface as it is unnecessary for the service, I also have a few other methods for creating and retrieving/ updating. The error im getting is

invalid operation connection string must be initialised"System.InvalidOperationException: The ConnectionString property has not been Initialised",

i get this error when await connection.openasync() is hit, since the connection string is invalid this throws. I want to know what the best way for writing test cases for these are: Thanks.

All I'm looking to achieve is write test for my handler method, i keep getting errors with connection string not set, below is my code example. I want to write a test to confirm that the method creates an item.

My Handler Method

private readonly IMapper _mapper 
private readonly ILogger < CreateDogTrailItemCommandHandler >  _dogTrailService;
private readonly DogTrailService.DogTrailService _dogTrailService;
public CreateDogTrailCommandHandler(Imapper mapper,DogTrailService.DogTrailService dogTrailService,ILogger<CreateDogTrailItemCommandHandler> dogLogger) {
    _dogTrailService = dogTrailService;
    _mapper = mapper;
    _logger = dogLogger
}

public async Task < Unit > Handle(CreateDogTrailItemCommand request,CancellationToken    cancellationToken) {
        var dogTrailItem = _mapper.Map < DogTrail > (request.DogTrailDto);
        await _dogTrailService.AddAsync(dogTrailItem);
        return Unit.value;

My Service:

public class DogTrailService {
    private readonly string _dogTrailConnectionString;

    public DogTrailServices(IConfiguration configuration) {
    _dogTrailConnectionString = configuration.GetConnectionString("DogTrailSqlDb");
    }

    public async Task < DogTrail > AddAsync(DogTrail entity) {
     using(SqlConnection connection = new SqLConnection(_dogTrailConnectionString)) {
           await connection.OpenAsync();
           using(SqlTransaction transaction = connection.BeginTransaction()) 
    {
              string query = "INSERT INTO DogTrail ( dognumb, dogIndex) 
    VALUES ( @dognumb,   @dogindex): SELECT SCOPE_IDENTITY();";
              using(Sqlcommand command = new Sqlcommand(query, connection)) {
                 command.Parameters.AddWithValue("@dognumb, entity.dognumb); 
                    command.Parameters.AddWithValue("@dogindex, entity.dogindex);
                       entity.Id = (int)(decimal) await command.ExecuteScalarAsync();  
    transaction.Commit();
                       return entity;
            }
        }
    }
}

My Test:

[Test]
public async Task Handle_ValidRequest_AddsNewDogTrailItem() {
   // Arrange
   var mockMapper = new Mock < IMapper > ();
   var mockDogTrailService = new Mock < DogTrailService > ();
   var mockLogger = new Mock < ILogger < CreateDogTrailItemCommandHandler >> ();
   var handler = new CreateDogTrailItemCommandHandler(mockMapper.Object, 
   mockDogTrailService.Object,  mockLogger.Object);
   var request = new CreateDogTrailItemCommand(new DogTrailDto());

   var dogTrail = new DogTrail();

   mockMapper.Setup(m => m.Map < DogTrail > (It.IsAny < DogTrailDto > ())).Returns(dogTrail);

   // Act
   await handler.Handle(request, CancellationToken.None);

   // Assert
   mockMapper.Verify(m => m.Map < DogTrail > (request.DogTrailDto), Times.Once);
   mockDogTrailService.Verify(m => m.AddAsync(dogTrail), Times.Once);
}

I have tried passing in a fake connection string, but my problem is when the AddAsync Method is called it fails as openconnection is throwing as its a dummy connection.