public class ApiExceptionHandler : IExceptionHandler
{
    public async Task HandleAsync(ExceptionHandlerContext context, CancellationToken cancellationToken)
    {

        var exceptionType = context.Exception.GetType();

        if (exceptionType == typeof(ResourceNotFoundException))
        {
            context.Result = new ResponseMessageResult(
                context.Request.CreateResponse(HttpStatusCode.NotFound, context.Exception.Message));
        }

        else if (exceptionType == typeof(UserNotFoundException))
        {
            context.Result = new ResponseMessageResult(
                context.Request.CreateResponse(HttpStatusCode.Unauthorized, context.Exception.Message));
        }

        else if (exceptionType == typeof(UserAlreadyExistsExeption))
        {
            context.Result = new ResponseMessageResult(
                context.Request.CreateResponse(HttpStatusCode.Conflict, context.Exception.Message));
        }

        ...

        else
        {
            context.Result = new ResponseMessageResult(
                context.Request.CreateResponse(HttpStatusCode.InternalServerError, "An unexpected error occured"));
        }
    }
}

public class UserAlreadyExistsExeption: Exception
{
    public UserAlreadyExistsExeption()
    {
    }

    public UserAlreadyExistsExeption(string message) : base(message)
    {
    }

    public UserAlreadyExistsExeption(string message, Exception inner) : base(message, inner)
    {

    }
}

//If user exists
throw new UserAlreadyExistsExeption("User already exists");

public interface IHasHttpErrorCode
{
    HttpStatusCode GetHttpStatusCode();
}

public class UserAlreadyExistsExeption: Exception, IHasHttpErrorCode
{
    public UserAlreadyExistsExeption()
    {
    }

    public UserAlreadyExistsExeption(string message) : base(message)
    {
    }

    public UserAlreadyExistsExeption(string message, Exception inner) : base(message, inner)
    {

    }

    public HttpStatusCode GetHttpStatusCode() {
        return HttpStatusCode.Conflict;
    }
}

public class ApiExceptionHandler : IExceptionHandler
{
    public async Task HandleAsync(ExceptionHandlerContext context, CancellationToken cancellationToken)
    {

        var exceptionType = context.Exception as IHasHttpErrorCode;


        if (customException != null) {
            context.Result = new ResponseMessageResult(
                context.Request.CreateResponse(customException.GetHttpStatusCode(), context.Exception.Message));
        }
        else
        {
            context.Result = new ResponseMessageResult(
                context.Request.CreateResponse(HttpStatusCode.InternalServerError, "An unexpected error occured"));
        }
    }
}

public abstract class ServerException: Exception {
//  standard constructors...
    public abstract HttpStatusCode GetHttpStatusCode();
}

public class UserAlreadyExistsExeption: ServerException {
//  again standard .ctors
    override public HttpStatusCode GetHttpStatusCode() => HttpStatusCode.Conflict;
}

public class UserAlreadyExistsExeption: Exception, IHasHttpErrorCode
{
    public UserAlreadyExistsExeption(): base("User already exists") {}
}

throw new UserAlreadyExistsExeption();

throw new UserAlreadyExistsExeption("User already exists");