Add ExecuteReaderAsync overloads that return Task<DbDataReader> (#1295)

Dapper/Extensions.cs

+using System;
+using System.Threading.Tasks;
+
+namespace Dapper
+{
+    internal static class Extensions
+    {
+        /// <summary>
+        /// Creates a <see cref="Task{TResult}"/> with a less specific generic parameter that perfectly mirrors the
+        /// state of the specified <paramref name="task"/>.
+        /// </summary>
+        internal static Task<TTo> CastResult<TFrom, TTo>(this Task<TFrom> task)
+            where TFrom : TTo
+        {
+            if (task is null) throw new ArgumentNullException(nameof(task));
+
+            if (task.Status == TaskStatus.RanToCompletion)
+                return Task.FromResult((TTo)task.Result);
+
+            var source = new TaskCompletionSource<TTo>();
+            task.ContinueWith(OnTaskCompleted<TFrom, TTo>, state: source, TaskContinuationOptions.ExecuteSynchronously);
+            return source.Task;
+        }
+
+        private static void OnTaskCompleted<TFrom, TTo>(Task<TFrom> completedTask, object state)
+            where TFrom : TTo
+        {
+            var source = (TaskCompletionSource<TTo>)state;
+
+            switch (completedTask.Status)
+            {
+                case TaskStatus.RanToCompletion:
+                    source.SetResult(completedTask.Result);
+                    break;
+                case TaskStatus.Canceled:
+                    source.SetCanceled();
+                    break;
+                case TaskStatus.Faulted:
+                    source.SetException(completedTask.Exception.InnerExceptions);
+                    break;
+            }
+        }
+    }
+}

Dapper/WrappedReader.cs

@@ -93,6 +93,14 @@ public static IDataReader Create(IDbCommand cmd, IDataReader reader)
             cmd.Dispose();
             return null; // GIGO
         }
+        public static DbDataReader Create(IDbCommand cmd, DbDataReader reader)
+        {
+            if (cmd == null) return reader; // no need to wrap if no command
+
+            if (reader != null) return new DbWrappedReader(cmd, reader);
+            cmd.Dispose();
+            return null; // GIGO
+        }
     }
     internal sealed class DbWrappedReader : DbDataReader, IWrappedDataReader
     {
@@ -143,7 +151,7 @@ protected override void Dispose(bool disposing)
                 _cmd = null;
             }
         }
-        
+
         public override int FieldCount => _reader.FieldCount;
 
         public override bool GetBoolean(int i) => _reader.GetBoolean(i);