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/SqlMapper.Async.cs

@@ -1101,6 +1101,18 @@ public static async Task<GridReader> QueryMultipleAsync(this IDbConnection cnn,
         /// </code>
         /// </example>
         public static Task<IDataReader> ExecuteReaderAsync(this IDbConnection cnn, string sql, object param = null, IDbTransaction transaction = null, int? commandTimeout = null, CommandType? commandType = null) =>
+            ExecuteWrappedReaderImplAsync(cnn, new CommandDefinition(sql, param, transaction, commandTimeout, commandType, CommandFlags.Buffered), CommandBehavior.Default).CastResult<DbDataReader, IDataReader>();
+
+        /// <summary>
+        /// Execute parameterized SQL and return a <see cref="DbDataReader"/>.
+        /// </summary>
+        /// <param name="cnn">The connection to execute on.</param>
+        /// <param name="sql">The SQL to execute.</param>
+        /// <param name="param">The parameters to use for this command.</param>
+        /// <param name="transaction">The transaction to use for this command.</param>
+        /// <param name="commandTimeout">Number of seconds before command execution timeout.</param>
+        /// <param name="commandType">Is it a stored proc or a batch?</param>
+        public static Task<DbDataReader> ExecuteReaderAsync(this DbConnection cnn, string sql, object param = null, IDbTransaction transaction = null, int? commandTimeout = null, CommandType? commandType = null) =>
             ExecuteWrappedReaderImplAsync(cnn, new CommandDefinition(sql, param, transaction, commandTimeout, commandType, CommandFlags.Buffered), CommandBehavior.Default);
 
         /// <summary>
@@ -1114,6 +1126,14 @@ public static async Task<GridReader> QueryMultipleAsync(this IDbConnection cnn,
         /// or <see cref="T:DataSet"/>.
         /// </remarks>
         public static Task<IDataReader> ExecuteReaderAsync(this IDbConnection cnn, CommandDefinition command) =>
+            ExecuteWrappedReaderImplAsync(cnn, command, CommandBehavior.Default).CastResult<DbDataReader, IDataReader>();
+
+        /// <summary>
+        /// Execute parameterized SQL and return a <see cref="DbDataReader"/>.
+        /// </summary>
+        /// <param name="cnn">The connection to execute on.</param>
+        /// <param name="command">The command to execute.</param>
+        public static Task<DbDataReader> ExecuteReaderAsync(this DbConnection cnn, CommandDefinition command) =>
             ExecuteWrappedReaderImplAsync(cnn, command, CommandBehavior.Default);
 
         /// <summary>
@@ -1128,9 +1148,18 @@ public static async Task<GridReader> QueryMultipleAsync(this IDbConnection cnn,
         /// or <see cref="T:DataSet"/>.
         /// </remarks>
         public static Task<IDataReader> ExecuteReaderAsync(this IDbConnection cnn, CommandDefinition command, CommandBehavior commandBehavior) =>
+            ExecuteWrappedReaderImplAsync(cnn, command, commandBehavior).CastResult<DbDataReader, IDataReader>();
+
+        /// <summary>
+        /// Execute parameterized SQL and return a <see cref="DbDataReader"/>.
+        /// </summary>
+        /// <param name="cnn">The connection to execute on.</param>
+        /// <param name="command">The command to execute.</param>
+        /// <param name="commandBehavior">The <see cref="CommandBehavior"/> flags for this reader.</param>
+        public static Task<DbDataReader> ExecuteReaderAsync(this DbConnection cnn, CommandDefinition command, CommandBehavior commandBehavior) =>
             ExecuteWrappedReaderImplAsync(cnn, command, commandBehavior);
 
-        private static async Task<IDataReader> ExecuteWrappedReaderImplAsync(IDbConnection cnn, CommandDefinition command, CommandBehavior commandBehavior)
+        private static async Task<DbDataReader> ExecuteWrappedReaderImplAsync(IDbConnection cnn, CommandDefinition command, CommandBehavior commandBehavior)
         {
             Action<IDbCommand, object> paramReader = GetParameterReader(cnn, ref command);
 

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
     {