How to avoid duplicate names?

Question

C++ / C#

There is a product, each new product must have a different name. How to write a function that will check if the name of a new product is the same as any other product?

public async Task<ProductDto> AddAsync(ProductDto item)
        {
            Product product = _mapper.Map<Product>(item);

            if (!await _unitOfWork.ProductRepository.IsDublicated(product.Label))
            {
                throw new DataDublication("Label already exists.");
            }

            await _unitOfWork.ProductRepository.AddAsync(product);

            return _mapper.Map<ProductDto>(product);
        }

public async Task<bool> IsDublicated(string label)
        {
            bool flag = false;

            var check = await _context.Set<Product>()
                .FirstOrDefaultAsync(property => property.Label.ToLower() == label.ToLower());

            return flag;
        }

I know that the problem is in the IsDublicated function, which returns false all the time. But, I don’t understand how I can throw the appropriate status after execution ...

Answer 1

IsDublicated always returns false because it's written incorrectly. You initialize a flag, then return it unchanged.
Try like this

public async Task<bool> IsDublicated(string label)
{
     var existedProduct = await _context.Set<Product>()
                .FirstOrDefaultAsync(property => property.Label.ToLower() == label.ToLower());
     return existedProduct != null;
}

And in the AddAsync function you have the wrong condition. Should be:

if (await _unitOfWork.ProductRepository.IsDublicated(product.Label))
{
     throw new DataDublication("Label already exists.");
}