具有明显列表的C＃Linq类

Question

I have a problem trying to get a Distinct List of my Class Objects.

For this particular query, I am only interested in grabbing the DLType, There are 3 types of 'DLType' that appear in my List multiple times. I just want to grab those three types from the data and use it for the datasource for a combo box.

Here is My Class :-

public class DistributionList
{
    public int DistributionID { get; set; }
    public string DistributionName { get; set; }
    public string  DLType { get; set; }
}

Here is where I grab the data:-

public List<DistributionList> GetDistributionLists()
{
    DataTable dt = new DataTable();

    OleDbCommand cmd = new OleDbCommand("Select * FROM [DistributionLists]",
                                                 Connection);
    Adapter.SelectCommand = cmd;

    Connection.Open();
    Adapter.SelectCommand.ExecuteNonQuery();
    Adapter.Fill(dt);
    Connection.Close();

    var DistributionLists = (from d in dt.AsEnumerable()
             select new DistributionList
             {
                 DistributionID = d.Field<int>("DistributionID"),
                 DistributionName = d.Field<string>("DistributionName"),
                 DLType = d.Field<string>("DLType")
             }).ToList();

    return DistributionLists;

}

Here I pull the data into my Form :-

var distributionData = dc.GetDistributionLists();

Now i need to get a distinct list from the distributionData Object:-

var query = (from d in distributionData
                         select new DistributionList
                         {
                             DLType = d.DLType

                         }).Distinct().ToList();

But this doesn't work, Is there a way of doing this?

Answer 1

The problem with your Linq is that it generates a new DistributionList for each value - and because they're all different instances, Distinct returns them all. Try reordering the way you process the query, something like this:

var query = (from d in distributionData select d.DLType)
                  .Distinct()
                  .Select(t => new DistributionList() { DLType = t })
                  .ToList();

Answer 2

You need to implement Equals on your DistributionList class. See this answer for details.

Answer 3

不要选择new DistributionList ，请选择DLType

distributionData.Select(d => d.DLType).Distinct().ToList();

Answer 4

you just need to group list by some unique key and after select the first element from each group, something like this

(from d in distributionData
                         select new DistributionList
                         {
                             DLType = d.DLType

                         }).GroupBy(a=>a.DLType).Select(a=>a.First()).ToList()

Answer 5

You could use a Lambda Comparer for the Distinct Method:

public class LambdaComparer<T> : IEqualityComparer<T>
{
    private readonly Func<T, T, bool> _lambdaComparer;
    private readonly Func<T, int> _lambdaHash;

    public LambdaComparer(Func<T, T, bool> lambdaComparer) :
        this(lambdaComparer, o => 0)
    {
    }

    public LambdaComparer(Func<T, T, bool> lambdaComparer, Func<T, int> lambdaHash)
    {
        if (lambdaComparer == null)
            throw new ArgumentNullException("lambdaComparer");
        if (lambdaHash == null)
            throw new ArgumentNullException("lambdaHash");

        _lambdaComparer = lambdaComparer;
        _lambdaHash = lambdaHash;
    }

    public bool Equals(T x, T y)
    {
        return _lambdaComparer(x, y);
    }

    public int GetHashCode(T obj)
    {
        return _lambdaHash(obj);
    }
}

and then use it like follow:

var query = (from d in distributionData
             select d)
             .Distinct<DistributionList>
             (new LambdaComparer<DistributionList>((x, y) 
                                 => x.DLType == y.DLType));

Greetings