基于泛型 T 的返回列表

Question

I want to convert list of TEntity to list of TDto .

    private List<TDto> ConvertUEntityToDto<TDto, TEntity>(IEnumerable<TEntity> entities)
    {
        if (entities is IEnumerable<Entity1>)
        {
            var result = new List<EntityDto1>
            foreach (var entity in entities)
            {
                result.Add(_mapper.Map<EntityDto1>(entity ));
            }

            return result;
        }

        else if (entities is IEnumerable<Entity2>)
        {
            var result = new List<EntityDto2>
            foreach (var entity in entities)
            {
                result.Add(_mapper.Map<EntityDto2>(entity));
            }

            return result;
        }

        return null;
    }

From code above i get an error:

Error CS0029 Cannot implicitly convert type 'System.Collections.Generic.List< EntityDto>' to 'System.Collections.Generic.List< TDto>'

How to return dynamically List of T from generic method in C#?

Answer 1

The meaning of generics are to use them! The if-else should precede your generic function (also see edit below).

if (entities is IEnumerable<Entity1>)
    ConvertUEntityToDto<TDto1, Entity1>(entities);
else if (entities is IEnumerable<Entity2>)
    ConvertUEntityToDto<TDto2, Entity2>(entities);
else ; //do nothing

and inside ConvertUEntityToDto it should use the T (TDto) as follows:

private List<TDto> ConvertUEntityToDto<TDto, TEntity>(IEnumerable<TEntity> entities)
{
    var result = new List<TDto>
    foreach (var entity in entities)
    {
        result.Add(_mapper.Map<TDto>(entity));
    }
    return result;
}

The error was because the return type didn't match with the signature of your function.

But I want to mention

You could just pass in a list, iterate it with foreach and if the element within the list is of type T, then Add to your new List<T> which you return.

Also, you should have constraints for T.

Edit:

What I mentioned above, to return all element of the list, only which matches the type TDto (and to get rid of the preceding if-else block), we can use the following snippet (by referencing System.Linq in the project) (thanks @Janne Matikainen to mention):

private List<TDto> ConvertUEntityToDto<TDto, TEntity, Tmixed>(IEnumerable<Tmixed> entities)
{
    return entities.Where(e => e is TEntity).Select(e => _mapper.Map<TDto>(e));
}

Which is equivalent to:

private List<TDto> ConvertUEntityToDto<TDto, TEntity, Tmixed>(IEnumerable<Tmixed> entities)
    => entities.Where(e => e is TEntity).Select(e => _mapper.Map<TDto>(e));