A column named 'Item' already belongs to this DataTable

Question

I am trying to convert a List to a datatable with an extension method. Implementation is:

Extension method

public static class list2Dt
    {
        public static DataTable ToDataTable<T>(List<T> items)
        {
            DataTable dataTable = new DataTable(typeof(T).Name);

            //Get all the properties
            PropertyInfo[] Props = typeof(T).GetProperties(BindingFlags.Public | BindingFlags.Instance);
            foreach (PropertyInfo prop in Props)
            {
                //Setting column names as Property names
                dataTable.Columns.Add(prop.Name);
            }
            foreach (T item in items)
            {
                var values = new object[Props.Length];
                for (int i = 0; i < Props.Length; i++)
                {
                    //inserting property values to datatable rows
                    values[i] = Props[i].GetValue(item, null);
                }
                dataTable.Rows.Add(values);
            }
            //put a breakpoint here and check datatable
            return dataTable;
        }
    }

Controller

var noDups = firstTable.AsEnumerable()
                            .GroupBy(d => new
                            {
                                name = d.Field<string>("name"),
                                date = d.Field<string>("date")
                            })
                            .Where(d => d.Count() > 1) 
                            .Select(d => d.First())
                             .ToList();

                        DataTable secondTable = new DataTable();
                        secondTable.Columns.Add("name", typeof(string));
                        secondTable.Columns.Add("date", typeof(string));
                        secondTable.Columns.Add("clockIn", typeof(string));
                        secondTable.Columns.Add("clockOut", typeof(string));

                        secondTable = list2Dt.ToDataTable(noDups);

I am getting this following error:

An exception of type 'System.Data.DuplicateNameException' occurred in System.Data.dll but was not handled in user code

Additional information: A column named 'Item' already belongs to this DataTable.

Above error is raised on line:

dataTable.Columns.Add(prop.Name);

Can someone find where problem lies.

Answer 1

Your ToDataTable method is expecting a list of objects - most likely a list of simple DTOs or similar.

You are passing it a list of DataRow instances, of which that class has multiple overloads of property Item which means when you're trying to build up the new DataTable it will try to add multiple columns with the name Item which is invalid in a DataTable.

On way around this is to project noDups to a new object, rather than retain the DataRow:

public class MyClass
{
    public string Name{get;set;}
    public string Date{get;set;}
}

var noDups = firstTable.AsEnumerable()
                        .GroupBy(d => new
                        {
                            name = d.Field<string>("name"),
                            date = d.Field<string>("date")
                        })
                        .Where(d => d.Count() > 1) 
                        .Select(d => {
                              var first = d.First();
                              return new MyClass()
                                 {
                                     Name = (string)first["name"],
                                     Date = (string)first["date"]
                                 }
                         })

                         .ToList();