在LINQ中的一列上使用distinct来获取整行

Question

I need to retrieve the entire rows collection using distinct on one column in LINQ .

i am getting the Distinct using GroupBy but it only selects the one column values instead i need entire row.

**this list returns the collection which is unique in ThumbnailAltText column BUT only returns that column rather than entire row values. so i had to use var instead of type

 var catlist = _db.ac_Categories
    .Where(c => c.VisibilityId == 0 && c.ThumbnailAltText != null
        && (!c.ThumbnailAltText.StartsWith("gifts/")
        && !c.ThumbnailAltText.StartsWith("email/")
        && !c.ThumbnailAltText.StartsWith("news/")
        && !c.ThumbnailAltText.StartsWith("promotions/")
        && !c.ThumbnailAltText.StartsWith("the-knowledge/"))).ToList()
        .GroupBy(c=> c.ThumbnailAltText.Trim().Distinct()).ToList();
                                .GroupBy(c=> c.ThumbnailAltText.Trim().Distinct()).ToList();

same thing does not work with type like this and i am getting error.

List<ac_Categories> catlist = _db.ac_Categories
        .Where(c => c.VisibilityId == 0 && c.ThumbnailAltText != null
            && (!c.ThumbnailAltText.StartsWith("gifts/")
            && !c.ThumbnailAltText.StartsWith("email/")
            && !c.ThumbnailAltText.StartsWith("news/")
            && !c.ThumbnailAltText.StartsWith("promotions/")
            && !c.ThumbnailAltText.StartsWith("the-knowledge/"))).ToList()
            .GroupBy(c=> c.ThumbnailAltText.Trim().Distinct()).ToList();
                                    .GroupBy(c=> c.ThumbnailAltText.Trim().Distinct()).ToList();

ERROR: The 'Distinct' operation cannot be applied to the collection ResultType of the specified argument.

EDIT: I need a collection and not the first record, ID and other columns contains diff values only ThumbnailAltText my contain duplicates

Answer 1

Distinct on a string returns unique characters. I assume you want to return all rows, but every row should be unique according to the ThumbnailAltText , is that correct?

Then this should work, it returns simply the first row of each group:

var catlist = _db.ac_Categories
    .Where(c => c.VisibilityId == 0 && c.ThumbnailAltText != null
        && (!c.ThumbnailAltText.StartsWith("gifts/")
        && !c.ThumbnailAltText.StartsWith("email/")
        && !c.ThumbnailAltText.StartsWith("news/")
        && !c.ThumbnailAltText.StartsWith("promotions/")
        && !c.ThumbnailAltText.StartsWith("the-knowledge/")))
     .GroupBy(c=> c.ThumbnailAltText.Trim())
     .ToList()
     .Select(g => g.First())
     .ToList();

Answer 2

also you may use Distinct(IEnumerable, IEqualityComparer) extension like this

public class CategotiesEqualityComparer : IEqualityComparer<ac_Categories>
{
    public bool Equals(ac_Categories x, ac_Categories y)
    {
        return x.ThumbnailAltText.Trim() == y.ThumbnailAltText.Trim();
    }

    public int GetHashCode(ac_Categories obj)
    {
        return obj.ThumbnailAltText.Trim().GetHashCode();
    }
}

List<ac_Categories> catlist = _db.ac_Categories
    .Where(c => c.VisibilityId == 0 && c.ThumbnailAltText != null
        && (!c.ThumbnailAltText.StartsWith("gifts/")
        && !c.ThumbnailAltText.StartsWith("email/")
        && !c.ThumbnailAltText.StartsWith("news/")
        && !c.ThumbnailAltText.StartsWith("promotions/")
        && !c.ThumbnailAltText.StartsWith("the-knowledge/")))
    .Distinct(new CategotiesEqualityComparer())
    .ToList()