Working with NpgsqlConnection
http://www.sqlines.com/postgresql/npgsql_cs_result_sets

Copy command (Bulk insert)
http://www.npgsql.org/doc/copy.html

	public static void BulkInsertNpgsql<T>(this IDbConnection dbConn, IEnumerable<T> list, IEnumerable<string> insertFields = null)
	{
	if (list == null) return;
	if (list.Count() < 1) return;
	var objWithAttributes = list.FirstOrDefault();
	var modelDef = OrmLiteConfig.GetModelDefinition(objWithAttributes.GetType());
	if (insertFields == null) insertFields = new List<string>();
	var sbColumnNames = new StringBuilder();
	var sbColumnValues = new StringBuilder();
	var targetTableName = "";
	bool isFirstRecord = true;
	var insertFieldsCount = insertFields.Count();
	foreach (var record in list)
	{
	if (isFirstRecord) targetTableName = OrmLiteConfig.DialectProvider.GetQuotedTableName(modelDef);
	var defsCount = modelDef.FieldDefinitions.Count;
	var counter = 0;
	foreach (var fieldDef in modelDef.FieldDefinitions)
	{
	counter += 1;
	if (fieldDef.IsComputed) continue;
	if (fieldDef.AutoIncrement) continue;
	//insertFields contains Attribute "Name" of fields to insert ( that's how expressions work )
	if (insertFieldsCount > 0 && !insertFields.Contains(fieldDef.Name)) continue;
	if (sbColumnNames.Length > 0 && isFirstRecord) sbColumnNames.Append(",");
	try
	{
	if (isFirstRecord) sbColumnNames.Append(OrmLiteConfig.DialectProvider.GetQuotedColumnName(fieldDef.FieldName));
	var stringValue = "";
	var value = fieldDef.GetValue(record);
	if (value == null) stringValue = "\\N";
	else if (fieldDef.FieldType == typeof(DateTime))
	{
	var dateValue = (DateTime)value;
	const string iso8601Format = "yyyy-MM-dd HH:mm:ss.fff";
	stringValue = dateValue.ToString(iso8601Format);
	}
	else if (fieldDef.FieldType == typeof(Guid))
	{
	var guidValue = (Guid)value;
	stringValue = guidValue.ToString();
	}
	else
	{
	stringValue = value.ToString();
	}
	sbColumnValues.Append(stringValue);
	if (counter < defsCount)
	{
	sbColumnValues.Append("\t");
	}
	}
	catch (Exception ex)
	{
	throw;
	}
	}
	sbColumnValues.Append("\r\n");
	isFirstRecord = false;
	}
	var strColumnValues = sbColumnValues.ToString();
	byte[] byteArrayColumnValues = System.Text.Encoding.UTF8.GetBytes(strColumnValues);
	MemoryStream msColumnValues = new MemoryStream(byteArrayColumnValues);
	//var connection = dbConn as NpgsqlConnection;
	var ormliteConn = dbConn as OrmLiteConnection;
	var connection = ormliteConn.DbConnection as NpgsqlConnection;
	//var connection = (NpgsqlConnection)dbConn.Database.ToDbConnection();
	//var connection = dbConn.DbConnection;
	using (var command = new NpgsqlCommand(string.Format("COPY {0} ({1}) FROM STDIN DELIMITER '\t' ", targetTableName, sbColumnNames), connection))
	{
	var cin = new NpgsqlCopyIn(command, connection, msColumnValues);
	try
	{
	cin.Start();
	}
	catch (Exception e)
	{
	try
	{
	cin.Cancel("Undo copy");
	}
	catch (NpgsqlException e2)
	{
	// we should get an error in response to our cancel request:
	if (!("" + e2).Contains("Undo copy"))
	{
	throw new Exception("Failed to cancel copy: " + e2 + " upon failure: " + e);
	}
	}
	throw e;
	}
	}
	}
	private static string ToMySqlBulkInsertValue<T>(this FieldDefinition fieldDef, T record)
	{
	var stringValue = "";
	var value = fieldDef.GetValue(record);
	if (value == null) stringValue = "";
	else if (fieldDef.FieldType == typeof(DateTime))
	{
	var dateValue = (DateTime)value;
	const string iso8601Format = "yyyy-MM-dd HH:mm:ss.fff";
	stringValue = dateValue.ToString(iso8601Format);
	}
	else if (fieldDef.FieldType == typeof(Guid))
	{
	var guidValue = (Guid)value;
	stringValue = guidValue.ToString();
	}
	else
	{
	stringValue = value.ToString();
	}
	return stringValue;
	}
	// http://theonetechnologies.com/outsourcing/post/mysql-bulk-data-import-using-net-connector-mysqlbulkloader-class.aspx
	public static void BulkInsertMySql<T>(this IDbConnection myConnection, IEnumerable<T> list/*, IEnumerable<string> insertFields = null*/)
	{
	if (list == null) return;
	if (list.Count() < 1) return;
	var objWithAttributes = list.FirstOrDefault();
	var modelDef = OrmLiteConfig.GetModelDefinition(objWithAttributes.GetType());
	var targetTableName = OrmLiteConfig.DialectProvider.GetQuotedTableName(modelDef);
	var Server = HttpContext.Current.Server;
	string strFile = targetTableName + "_" + DateTime.Now.Ticks.ToString() + ".csv";
	//Create directory if not exist... Make sure directory has required rights..
	if (!Directory.Exists(Server.MapPath("~/TempFolder/")))
	Directory.CreateDirectory(Server.MapPath("~/TempFolder/"));
	strFile = Server.MapPath("~/TempFolder/") + strFile;
	//If file does not exist then create it and right data into it..
	if (!File.Exists(strFile))
	{
	FileStream fs = new FileStream(strFile, FileMode.Create, FileAccess.Write);
	fs.Close();
	fs.Dispose();
	}
	bool isFirstRecord = true;
	StreamWriter sw = new StreamWriter(strFile, false);
	//var insertFieldsCount = insertFields.Count();
	var defsCount = modelDef.FieldDefinitions.Count;
	foreach (var record in list)
	{
	if (isFirstRecord)
	{
	sw.Write(string.Join("\t", modelDef.FieldDefinitions.Select(x => x.Name)));
	sw.Write(sw.NewLine);
	isFirstRecord = false;
	}
	sw.Write(string.Join("\t", modelDef.FieldDefinitions.Select(x => x.ToMySqlBulkInsertValue<T>(record))));
	sw.Write(sw.NewLine);
	}
	sw.Close();
	sw.Dispose();
	var connection = myConnection as MySqlConnection;
	var bl = new MySqlBulkLoader(connection);
	bl.TableName = targetTableName;
	bl.FieldTerminator = "\t";
	bl.LineTerminator = sw.NewLine;
	bl.FileName = strFile;
	bl.NumberOfLinesToSkip = 1;
	var inserted = bl.Load();
	}
	public static void BulkInsertMsSql<T>(this IDbConnection dbConn, IEnumerable<T> list)
	{
	if (list == null) return;
	if (list.Count() < 1) return;
	var objWithAttributes = list.FirstOrDefault();
	var modelDef = OrmLiteConfig.GetModelDefinition(objWithAttributes.GetType());
	var targetTableName = OrmLiteConfig.DialectProvider.GetQuotedTableName(modelDef);
	using (var bulkCopy = new SqlBulkCopy(dbConn as SqlConnection))
	{
	bulkCopy.BatchSize = list.Count();
	bulkCopy.DestinationTableName = targetTableName;
	var table = new DataTable();
	foreach (var fieldDef in modelDef.FieldDefinitions)
	{
	if (fieldDef.IsComputed) continue;
	if (fieldDef.AutoIncrement) continue;
	bulkCopy.ColumnMappings.Add(fieldDef.Name, fieldDef.Name);
	table.Columns.Add(fieldDef.Name, Nullable.GetUnderlyingType(fieldDef.FieldType) ?? fieldDef.FieldType);
	}
	foreach (var record in list)
	{
	//var values = new List<object>();
	//var dataRows = new List<DataRow>();
	DataRow row = table.NewRow();
	foreach (var fieldDef in modelDef.FieldDefinitions)
	{
	if (fieldDef.IsComputed) continue;
	if (fieldDef.AutoIncrement) continue;
	//values.Add(fieldDef.GetValue(record));
	//values.Add(SqlServerDialect.Provider.ConvertDbValue(fieldDef.GetValue(record), fieldDef.FieldType));
	//row.
	row[fieldDef.Name] = SqlServerDialect.Provider.ConvertDbValue(fieldDef.GetValue(record), fieldDef.FieldType); //Nullable.GetUnderlyingType(fieldDef.FieldType) ?? fieldDef.FieldType;
	}
	table.Rows.Add(row);
	//table.Rows.Add(values);
	}
	bulkCopy.WriteToServer(table);
	}
	}

view raw gistfile1.txt hosted with by GitHub

Copy command
https://stackoverflow.com/questions/42108766/cant-import-rows-in-pgsql-with-npgsql-in-net-core-app-with-begintextimport-t

Using Npgsql and Import (bulk)
https://stackoverflow.com/questions/48467377/npgsql-begintextimport