Imported Upstream version 4.0.0~alpha1

Former-commit-id: 806294f5ded97629b74c85c09952f2a74fe182d9

external/referencesource/System.Core/System/Linq/ParallelMergeOptions.cs

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+// ==++==
+//
+//   Copyright (c) Microsoft Corporation.  All rights reserved.
+// 
+// ==--==
+// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
+//
+// ParallelMergeOptions.cs
+//
+// <OWNER>[....]</OWNER>
+//
+// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
+
+namespace System.Linq
+{
+    /// <summary>
+    /// Specifies the preferred type of output merge to use in a query. This is a hint only, and may not be
+    /// respected by the system when parallelizing all queries.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Use <b>NotBuffered</b> for queries that will be consumed and output as streams, this has the lowest latency
+    /// between beginning query execution and elements being yielded. For some queries, such as those involving a 
+    /// sort (OrderBy, OrderByDescending), buffering is essential and a hint of NotBuffered or AutoBuffered will 
+    /// be ignored.
+    /// </para>
+    /// <para>
+    /// Use <b>AutoBuffered</b> for most cases; this is the default.  It strikes a balance between latency and
+    /// overall performance.
+    /// </para>
+    /// <para>
+    /// Use <b>FullyBuffered</b> for queries when the entire output can be processed before the information is 
+    /// needed. This option offers the best performance when all of the output can be accumulated before yielding
+    /// any information, though it is not suitable for stream processing or showing partial results mid-query.
+    /// </para>
+    /// </remarks>
+    public enum ParallelMergeOptions
+    {
+        /// <summary>
+        /// Use the default merge type, which is AutoBuffered.
+        /// </summary>
+        Default = 0,
+
+        /// <summary>
+        /// Use a merge without output buffers. As soon as result elements have been computed, 
+        /// make that element available to the consumer of the query.
+        /// </summary>
+        NotBuffered = 1,
+
+        /// <summary>
+        /// Use a merge with output buffers of a size chosen by the system. Results
+        /// will accumulate into an output buffer before they are available to the consumer of
+        /// the query.
+        /// </summary>
+        AutoBuffered = 2,
+
+        /// <summary>
+        /// Use a merge with full output buffers. The system will accumulate all of the
+        /// results before making any of them available to the consumer of the query.
+        /// </summary>
+        FullyBuffered = 3
+    }
+}