ado.net linq小技巧

导入excel的列很多，只保留abcd这几个列的值，其他的列都清空

                    var row_Distinct = dt.Select().Select(d =>
                     {
                         foreach (DataColumn col in dt.Columns)
                         {
                             if (col.ColumnName != "a" &&
                             col.ColumnName != "b" &&
                             col.ColumnName != "c" &&
                             col.ColumnName != "d")
                             {
                                 d.SetField(col, "");
                             }
                         }
                         return d;
                     }).Distinct(new DataRowEqualityComparer());

 导入excel的列很多，如果abcd这几个列的值相同则算重复行，Distinct

var row_Distinct = dt.Select().Distinct(new DataRowEqualityComparer());

public class DataRowEqualityComparer : IEqualityComparer<DataRow>
    {
        public bool Equals(DataRow x, DataRow y)
        {
            if (x["a"].ToString() == y["a"].ToString() &&
                x["b"].ToString() == y["b"].ToString() &&
                x["c"].ToString() == y["c"].ToString() &&
                x["d"].ToString() == y["d"].ToString())
            {
                return true;
            }
            else
            {
                return false;
            }
        }

        public int GetHashCode(DataRow obj)
        {
            var HashCode = $"{obj["a"]}{obj["b"]}{obj["c"]}{obj["d"]}".GetHashCode();
            return HashCode;
        }

    }