You can access the same column information about the results of a query that you can for table schemas. See Columns for the columns returned.

Retrieving Result Set Metadata

You can use the GetSchemaTable method of the PostgreSQLDataReader to retrieve result set metadata. Call GetSchemaTable after calling ExecuteReader.

Each row of the DataTable describes a column in the query's result.

C#

string connectionString = "User=postgres;Password=admin;Database=postgres;Server=127.0.0.1;Port=5432";

using (PostgreSQLConnection conn = new PostgreSQLConnection(connectionString)) {
  PostgreSQLCommand cmd = new PostgreSQLCommand("SELECT * FROM \"postgres\".\"schema01\".Orders WHERE ShipCountry = 'USA'", conn);
  PostgreSQLDataReader rdr = cmd.ExecuteReader();
  DataTable schemaTable = rdr.GetSchemaTable();
  foreach (DataRow row in schemaTable.Rows) {
    foreach (DataColumn col in schemaTable.Columns) {
      Console.WriteLine("{0}: {1}", col.ColumnName, row[col]);
    }
  }
}

VB.NET

Dim connectionString As String = "User=postgres;Password=admin;Database=postgres;Server=127.0.0.1;Port=5432"

Using conn As New PostgreSQLConnection(connectionString)
  Dim cmd As New PostgreSQLCommand("SELECT * FROM \"postgres\".\"schema01\".Orders WHERE ShipCountry = 'USA'", conn)
  Dim rdr As PostgreSQLDataReader = cmd.ExecuteReader()
  Dim schemaTable As DataTable = rdr.GetSchemaTable()
  For Each row As DataRow In schemaTable.Rows
    For Each col As DataColumn In schemaTable.Columns
      Console.WriteLine("{0}: {1}", col.ColumnName, row(col))
    Next
  Next
End Using