Use SQLBulkLoad with linq-toSQL in LinqPad - now with Property support.

BulkInsertAll.cs

// Add this to LinqPad "C# Program"
//  Example use:
//  Insead of this: (create a List<CS_POSITION>)
//  CS_POSITIONs.InsertOnSubmit(position);  
//  This:
//  positions.Add(position);
//  
//  Then instead of this:
//  SubmitChanges()
//  This:
//  BulkInsertAll<CS_POSITION>(positions)
void BulkInsertAll<T>(List<T> entities)
{
	var conn = (SqlConnection)this.Connection;

	Type t = typeof(T);
	var tableAttribute = (TableAttribute)t.GetCustomAttributes(typeof(TableAttribute), false).Single();
	var bulkCopy = new SqlBulkCopy(conn) { DestinationTableName = tableAttribute.Name };
	var properties = t.GetMembers().Where(p =>
	{
		var columnAttribute = Attribute.GetCustomAttribute(p, typeof(ColumnAttribute)) as ColumnAttribute;
		if (columnAttribute != null) return true;
		return false;
	}).ToArray();
	var table = new DataTable();
	foreach (var property in properties)
	{
		Type memberType = null;
		
		switch (property)
		{
			case PropertyInfo pi:
				memberType = pi.PropertyType;
				break;
			case FieldInfo fi:
				memberType = fi.FieldType;
				break;
			default:
				throw new InvalidOperationException();
		}
		
		if (memberType.IsGenericType && memberType.GetGenericTypeDefinition() == typeof(Nullable<>))
		{
			memberType = Nullable.GetUnderlyingType(memberType);
		}
		table.Columns.Add(new DataColumn(property.Name, memberType));
	}
	// https://stackoverflow.com/a/50995201/89176
	table.Columns.Cast<DataColumn>().ToList().ForEach(x =>
		bulkCopy.ColumnMappings.Add(new SqlBulkCopyColumnMapping(x.ColumnName, x.ColumnName)));
		
	foreach (var entity in entities)
	{
		var cols = properties.Select(property =>
		{
			switch (property)
			{
				case FieldInfo fi:
					return fi.GetValue(entity) ?? DBNull.Value;
				case PropertyInfo pi:
					return pi.GetValue(entity) ?? DBNull.Value;
				default:
					throw new InvalidOperationException();
			}
		}).ToArray();
		table.Rows.Add(cols);
	}
	conn.Open();
	bulkCopy.BulkCopyTimeout = 0;
	bulkCopy.WriteToServer(table);
	conn.Close();
}

I'll note that using the Linqpad-generated class for this operation isn't possible because it doesn't have the Table and Column attributes. Also, I had to use the name parameter for my Column attributes.