NpgsqlOperationInProgressException while querying postgres database

Whenever i try to run query i get this exception: Npgsql.NpgsqlOperationInProgressException (0x80004005): A command is already in progress. I'm using Dapper and PostgreSQL. Here is my code for data access in CarService:

public class CarsService : ICarsService
{
    private readonly IDbConnection _connection;

    public CarsService(IDbConnection connection) => _connection = connection;

    public Task<IEnumerable<int>> GetAvailableCarsIdsAsync(int carModelId, DateTime startDate, DateTime endDate)
    {
        return _connection.QueryAsync<int>("""
            select c.id
            from cars c
            where c.car_model_id = @CarModelId
                and c.id not in (
                    select r.car_id
                    from rentals r
                    where r.car_id = c.id
                        and r.start_date <= @EndDate
                        and r.end_date >= @StartDate
            )
            """, new { CarModelId = carModelId, StartDate = startDate, EndDate = endDate });
    }

    public Task<IEnumerable<int>> GetCarIdsAsync(int carModelId)
    {
        return _connection.QueryAsync<int>("""
            select c.id
            from cars c
            where c.car_model_id = @CarModelId
            """, new { CarModelId = carModelId });
    }
}

Here is the method, that calls service:

public async Task<IEnumerable<GetCarModelResponse>> Handle(GetCarModelsQuery query, CancellationToken cancellationToken)
{
    var carModels = await _carModelsService.GetAsync(query);

    return await Task.WhenAll(carModels.Select
    (
        async carModel =>
        {
            var availableCarIds = await _carsService.GetAvailableCarsIdsAsync(carModel.Id, query.StartDate, query.EndDate);
                    
            return new GetCarModelResponse
            (
                carModel.Id,
                carModel.Model,
                carModel.Color,
                availableCarIds.Count(),
                availableCarIds,
                await _carsService.GetCarIdsAsync(carModel.Id)
            );
        }));
}

IDbConnection is injected as transient, injecting it as scoped or singleton didn't solve the issue. (builder.Services.AddTransient<IDbConnection>(_ => new NpgsqlConnection("connection string"));)

How can I fix this?

Thanks

As I wrote in the comment: Don't reuse connections!

A working CarsService could look like this:

public class CarsService : ICarsService
{
    private readonly string connectionString;

    public CarsService(string connectionString) => this.connectionString = connectionString;

    public Task<IEnumerable<int>> GetAvailableCarsIdsAsync(int carModelId, DateTime startDate, DateTime endDate)
    {
        using (var connection = new NpgsqlConnection(connectionString))
        {   
            return connection.QueryAsync<int>("""
                select c.id
                from cars c
                where c.car_model_id = @CarModelId
                    and c.id not in (
                        select r.car_id
                        from rentals r
                        where r.car_id = c.id
                            and r.start_date <= @EndDate
                            and r.end_date >= @StartDate
                )
                """, new { CarModelId = carModelId, StartDate = startDate, EndDate = endDate });
        }
    }

    public Task<IEnumerable<int>> GetCarIdsAsync(int carModelId)
    {
        using (var connection = new NpgsqlConnection(connectionString))
        {   
            return _connection.QueryAsync<int>("""
                select c.id
                from cars c
                where c.car_model_id = @CarModelId
                """, new { CarModelId = carModelId });
        }
    }
}

This is tightly coupled to Postgres, so you might want to make a factory for creating the connection. You can then choose to inject the factory and have it independent of the database implementation and have the conenction string handled there.

Connections are put back in the connection pool and there is very little overhead in creating a connection, so always create and dispose it around your query code.