Tweaks and micro optimizations.

Author: electricessence

Extensions.cs

@@ -233,17 +233,12 @@ public static IEnumerable<object[]> AsEnumerable(this IDataReader reader)
             }
         }
 
-        /// <summary>
-        /// Enumerates all the remaining values of the current result set of a data reader.
-        /// </summary>
-        /// <param name="reader">The reader to enumerate.</param>
-        /// <param name="ordinals">The limited set of ordinals to include.  If none are specified, the returned objects will be empty.</param>
-        /// <returns>An enumeration of the values returned from a data reader.</returns>
-        public static IEnumerable<object[]> AsEnumerable(this IDataReader reader, IEnumerable<int> ordinals)
+
+        static IEnumerable<object[]> AsEnumerableInternal(this IDataReader reader, IEnumerable<int> ordinals, bool readStarted)
         {
-            if (reader.Read())
+            if (readStarted || reader.Read())
             {
-                var o = ordinals.ToArray();
+                var o = ordinals as int[] ?? ordinals.ToArray();
                 var fieldCount = o.Length;
                 if (fieldCount == 0)
                 {
@@ -266,6 +261,15 @@ public static IEnumerable<object[]> AsEnumerable(this IDataReader reader, IEnume
             }
         }
 
+        /// <summary>
+        /// Enumerates all the remaining values of the current result set of a data reader.
+        /// </summary>
+        /// <param name="reader">The reader to enumerate.</param>
+        /// <param name="ordinals">The limited set of ordinals to include.  If none are specified, the returned objects will be empty.</param>
+        /// <returns>An enumeration of the values returned from a data reader.</returns>
+        public static IEnumerable<object[]> AsEnumerable(this IDataReader reader, IEnumerable<int> ordinals)
+            => AsEnumerableInternal(reader, ordinals, false);
+
         /// <summary>
         /// Enumerates all the remaining values of the current result set of a data reader.
         /// </summary>
@@ -769,30 +773,18 @@ public static Dictionary<string, object> ToDictionary(this IDataRecord record, p
         public static Dictionary<string, object> ToDictionary(this IDataRecord record, IEnumerable<string> columnNames)
             => ToDictionary(record, new HashSet<string>(columnNames));
 
-        static QueryResult<Queue<object[]>> RetrieveBlanksInternal(IDataReader reader)
-            => new QueryResult<Queue<object[]>>(
-                Array.Empty<int>(),
-                Array.Empty<string>(),
-                new Queue<object[]>(AsEnumerable(reader, Enumerable.Empty<int>())));
 
-        static QueryResult<Queue<object[]>> RetrieveInternal(this IDataReader reader, int[] ordinals, string[] columnNames = null)
+
+        static QueryResult<Queue<object[]>> RetrieveInternal(this IDataReader reader, int[] ordinals, string[] columnNames = null, bool readStarted = false)
         {
             var fieldCount = ordinals.Length;
             if (columnNames == null) columnNames = ordinals.Select(n => reader.GetName(n)).ToArray();
             else if (columnNames.Length != fieldCount) throw new ArgumentException("Mismatched array lengths of ordinals and names.");
 
-            if (fieldCount == 0)
-            {
-                // No column names specified?  Then return results, but empty ones.  Simplify the results for counting.  
-                return RetrieveBlanksInternal(reader);
-            }
-            else
-            {
-                return new QueryResult<Queue<object[]>>(
-                    ordinals,
-                    columnNames,
-                    new Queue<object[]>(AsEnumerable(reader, ordinals)));
-            }
+            return new QueryResult<Queue<object[]>>(
+                ordinals,
+                columnNames,
+                new Queue<object[]>(AsEnumerableInternal(reader, ordinals, readStarted)));
         }
 
         /// <summary>
@@ -908,13 +900,15 @@ public static QueryResult<Queue<object[]>> Retrieve(this IDbCommand command, str
         public static IEnumerable<T> Results<T>(this IDataReader reader, IEnumerable<(string Field, string Column)> fieldMappingOverrides)
             where T : new()
         {
+            if (!reader.Read()) return Enumerable.Empty<T>();
+
             var x = new Transformer<T>(fieldMappingOverrides);
             // Ignore missing columns.
             var columns = reader.GetMatchingOrdinals(x.ColumnNames, true);
 
             return x.AsDequeueingEnumerable(RetrieveInternal(reader,
                 columns.Select(c => c.Ordinal).ToArray(),
-                columns.Select(c => c.Name).ToArray()));
+                columns.Select(c => c.Name).ToArray(), readStarted: true));
         }
 
         /// <summary>

QueryResult.cs

@@ -172,14 +172,24 @@ public static ISourceBlock<T> To<T>(this QueryResult<ISourceBlock<object[]>> sou
 		/// <returns>An block that dequeues the results and returns a column mapped dictionary for each entry</returns>
 		public static ISourceBlock<T> To<T>(this QueryResult<ISourceBlock<object[]>> source, IEnumerable<KeyValuePair<string, string>> fieldMappingOverrides)
 			where T : new()
-			=> source.To<T>(fieldMappingOverrides?.Select(kvp => (kvp.Key, kvp.Value)));
+			=> To<T>(source, fieldMappingOverrides?.Select(kvp => (kvp.Key, kvp.Value)));
 
-		/// <summary>
-		/// Returns an enumerable that dequeues the results and returns a column mapped dictionary for each entry.
-		/// </summary>
-		/// <param name="source">The query result.  Typically produced by a .Retrieve method.</param>
-		/// <returns>An enumerable that dequeues the results and returns a column mapped dictionary for each entry</returns>
-		public static IEnumerable<Dictionary<string, object>> AsMappedDictionaries(this QueryResult<IEnumerable<object[]>> source)
+        /// <summary>
+        /// Returns a block that attempts to map the fields to type T.
+        /// </summary>
+        /// <param name="source">The query result.  Typically produced by a .Retrieve method.</param>
+        /// <param name="fieldMappingOverrides">An optional override map of field names to column names where the keys are the property names, and values are the column names.</param>
+        /// <returns>An block that dequeues the results and returns a column mapped dictionary for each entry</returns>
+        public static ISourceBlock<T> To<T>(this QueryResult<ISourceBlock<object[]>> source, params (string Field, string Column)[] fieldMappingOverrides)
+            where T : new()
+            => To<T>(source, fieldMappingOverrides as IEnumerable<(string Field, string Column)>);
+
+        /// <summary>
+        /// Returns an enumerable that dequeues the results and returns a column mapped dictionary for each entry.
+        /// </summary>
+        /// <param name="source">The query result.  Typically produced by a .Retrieve method.</param>
+        /// <returns>An enumerable that dequeues the results and returns a column mapped dictionary for each entry</returns>
+        public static IEnumerable<Dictionary<string, object>> AsMappedDictionaries(this QueryResult<IEnumerable<object[]>> source)
 		{
 			var q = source.Result;
 			var names = source.Names;

SqlClient/Extensions.cs

@@ -207,7 +207,7 @@ public static async Task IterateReaderAsyncWhile(this SqlCommand command, Func<I
         public static async Task<QueryResult<Queue<object[]>>> RetrieveAsync(this SqlDataReader reader)
         {
             var fieldCount = reader.FieldCount;
-            var names = reader.GetNames();
+            var names = reader.GetNames(); // pull before first read.
             var buffer = new Queue<object[]>();
 
             while (await reader.ReadAsync())
@@ -223,7 +223,7 @@ public static async Task<QueryResult<Queue<object[]>>> RetrieveAsync(this SqlDat
                 buffer);
         }
 
-        static async Task<QueryResult<Queue<object[]>>> RetrieveAsyncInternal(SqlDataReader reader, int[] ordinals, string[] columnNames = null)
+        static async Task<QueryResult<Queue<object[]>>> RetrieveAsyncInternal(SqlDataReader reader, int[] ordinals, string[] columnNames = null, bool readStarted = false)
         {
             var fieldCount = ordinals.Length;
             if (columnNames == null) columnNames = ordinals.Select(n => reader.GetName(n)).ToArray();
@@ -240,8 +240,14 @@ static async Task<QueryResult<Queue<object[]>>> RetrieveAsyncInternal(SqlDataRea
             };
 
             var buffer = new Queue<object[]>();
-            while (await reader.ReadAsync())
-                buffer.Enqueue(handler(reader));
+            if (readStarted || await reader.ReadAsync())
+            {
+                do
+                {
+                    buffer.Enqueue(handler(reader));
+                }
+                while (await reader.ReadAsync());
+            }
 
             return new QueryResult<Queue<object[]>>(
                 ordinals,
@@ -305,13 +311,15 @@ public static Task<QueryResult<Queue<object[]>>> RetrieveAsync(this SqlDataReade
         /// <returns>A task containing the list of results.</returns>
         public static async Task<IEnumerable<T>> ResultsAsync<T>(this SqlDataReader reader, IEnumerable<(string Field, string Column)> fieldMappingOverrides) where T : new()
         {
+            if (!await reader.ReadAsync()) return Enumerable.Empty<T>();
+
             var x = new Transformer<T>(fieldMappingOverrides);
             // Ignore missing columns.
             var columns = reader.GetMatchingOrdinals(x.ColumnNames, true);
 
             return x.AsDequeueingEnumerable(await RetrieveAsyncInternal(reader,
                 columns.Select(c => c.Ordinal).ToArray(),
-                columns.Select(c => c.Name).ToArray()));
+                columns.Select(c => c.Name).ToArray(), readStarted: true));
         }
 
         /// <summary>