Rellenar C# DataTable de forma asíncrona

Si desea un método de extensión, puede escribir directamente en el comando

public static class extensions
    {
         public async static Task<DataTable> ExecuteAndCreateDataTableAsync(this SqlCommand cmd)
         {
             using (var reader = await cmd.ExecuteReaderAsync().ConfigureAwait(false))
             {
                 var dataTable = reader.CreateTableSchema();
                 while (await reader.ReadAsync().ConfigureAwait(false))
                 {
                     var dataRow = dataTable.NewRow();
                     for (int i = 0; i < dataTable.Columns.Count; i++)
                     {
                         dataRow[i] = reader[i];
                     }
                     dataTable.Rows.Add(dataRow);
                 }
                 return dataTable;

             }
         }
         public static void LoadParams(this SqlCommand cmd, params SqlParameter[] parameters)
         {
             if (parameters != null)
             {
                 foreach (var parameter in parameters)
                 {
                     if (parameter != null)
                     {
                         if (parameter.Value == null)
                             parameter.Value = DBNull.Value;

                         cmd.Parameters.Add(parameter);
                     }
                 }
             }
         }


         private static DataTable CreateTableSchema(this SqlDataReader reader)
         {
             DataTable schema = reader.GetSchemaTable();
             DataTable dataTable = new DataTable();
             if (schema != null)
             {
                 foreach (DataRow drow in schema.Rows)
                 {
                     string columnName = System.Convert.ToString(drow["ColumnName"]);
                     DataColumn column = new DataColumn(columnName, (Type)(drow["DataType"]));
                     dataTable.Columns.Add(column);
                 }
             }
             return dataTable;
         }
    }

y tu método:

private async Task<DataTable> ExecuteQueryInternalAsync(string commandText, CommandType commandType, SqlConnection sqlConnection, SqlTransaction transaction, params SqlParameter[] parameters)
        {
            using (SqlCommand cmd = new SqlCommand(commandText, sqlConnection) { CommandType = commandType, CommandTimeout = this.config.MainConnectionTimeoutInSeconds })
            {
                if (transaction != null)
                    cmd.Transaction = transaction;

                cmd.LoadParams(parameters);

                if (sqlConnection.State == ConnectionState.Closed)
                    await sqlConnection.OpenAsync();

                var datatable =  await cmd.ExecuteAndCreateDataTableAsync();
                return datatable;
            }
        }