Question

I’m trying to use LINQ to create a new dictionary from an existing one, removing duplicates in the process.

The existing dictionary is as follows:

Dictionary<LoaderConfig, List<ColumnInfo>> InvalidColumns = new Dictionary<LoaderConfig, List<ColumnInfo>>();

public struct LoaderConfig
{
    public string ObjectName { get; set; }
    public DateTime? LoadDate { get; set; }
    public string Load { get; set; }
    public string TableName { get; set; }
}

public struct ColumnInfo
{
    public string ColumnName { get; set; }
    public string DataType { get; set; }
    public int DataLength { get; set; }
}

What I want to end up with is a Dictionary<string, List<ColumnInfo>> where the key is the TableName attribute of the LoaderConfig objects and the list of ColumnInfo objects are unique for each TableName.

I started with this based on another post that I found:

var alterations = InvalidColumns
    .GroupBy(pair => pair.Key.TableName)
    .Select(group => group.First())
    .ToDictionary(pair => pair.Key.TableName, pair => pair.Value);

Which doesn’t work because of the First(). I imagine there is a way to acheive this using LINQ extensions, I just need some help finding it.

Thanks!

Answer 1

Personaly I’ll do something like this :

First create a IEqualityComparer for ColumnInfo (used by Distinct)

    public struct ColumnInfo
    {
        public string ColumnName { get; set; }
        public string DataType { get; set; }
        public int DataLength { get; set; }

        public class ColumnNameComparer : IEqualityComparer<ColumnInfo>
        {
            public bool Equals(ColumnInfo x, ColumnInfo y)
            {
                return x.ColumnName == y.ColumnName;
            }

            public int GetHashCode(ColumnInfo obj)
            {
                return obj.ColumnName.GetHashCode();
            }
        }
    }

Then the query :

        var colComparer = new ColumnInfo.ColumnNameComparer();
        Dictionary<string, List<ColumnInfo>> res = InvalidColumns
            .GroupBy(i => i.Key.TableName)
            .ToDictionary(i => i.Key, i => i.SelectMany(j => j.Value.Distinct(colComparer)).ToList());