C＃-如何从数据表中选择不同的随机记录

Question

I am trying to fetch records from DataTable randomly with some limit. I am able to fetch the data randomly as follows -

 var r = new Random();
 var _randomSelection = dt.Rows.OfType<DataRow>().OrderBy(rand => r.Next()).Take(limit);

But the values seems to be redundant. I want to get the unique values. So far I have tried Distinct() as -

var _randomSelection = dt.Rows.OfType<DataRow>().OrderBy(rand => r.Next()).Take(limit).Distinct();

But that doesn't seems to be worked out. Can anybody please guide where am going wrong with this. Thanks.

Answer 1

   DataTable returnVals = dt.DefaultView.ToTable(true, "ColumnNameOnWhichYouWantDistinctRecords");

In ToTable method the first parameter specifies if you want Distinct records, the second specify by which column name we will make distinct.

After that if you want to take some number of it you can do

var result = returnVals.Rows.Cast<DataRow>().OrderBy(rand => r.Next()).Take(number);

Answer 2

If you want unique records according to one column:

var distinctRows = from r in table.AsEnumerable()
                   let id = r.Field<int>("Id")
                   group r by id into idGroup
                   select idGroup.First();

If you want unique records according to multiple columns, use an anonymous type:

var distinctRows = from r in table.AsEnumerable()
                   let keyCols = new { id=  r.Field<int>("Id"), col2 = r.Field<string>("Col2")}
                   group r by keyCols into keyGroup
                   select keyGroup.First();

if you want a dynamic way to compare all columns even if you don't know them:

public static class DataTableExtensions
{
    public class DataRowValuesComparer : IEqualityComparer<DataRow>
    {
        public bool Equals(DataRow x, DataRow y)
        {
            if(ReferenceEquals(x, y))
                return true;
            if (x == null || y == null)
                return false;
            if (!ReferenceEquals(x.Table, y.Table))
                return false; // different tables
            foreach (DataColumn column in x.Table.Columns)
            {
                object v1 = x[column];
                object v2 = y[column];
                if (ReferenceEquals(v1, v2)) continue;
                if (v1 == null || v2 == null) return false;
                if (!v1.Equals(v2)) return false;
            }
            return true;
        }

        public int GetHashCode(DataRow obj)
        {
            if (obj == null) return int.MinValue;
            unchecked
            {
                int hash = 19;
                foreach (DataColumn column in obj.Table.Columns)
                {
                    object value = obj[column];
                    hash = hash * 31 + (value?.GetHashCode() ?? 0);
                }
                return hash;
            }
        }
    }

    public static IEnumerable<DataRow> DistinctValues(this IEnumerable<DataRow> rows)
    {
        return rows.Distinct(new DataRowValuesComparer());
    }
}

Usage:

var distinctRows = table.AsEnumerable().DistinctValues();