[Select] Introduce Identity for Select extension.

Author: reegeek

src/StructLinq.Benchmark/Select.cs

@@ -22,7 +22,7 @@ public Select()
             delegateRange = StructEnumerable.Range(0, Count).Select(x=> x * 2.0);
             convertRange = Enumerable.Range(0, Count).ToTypedEnumerable().Select(x=> x * 2.0);
             var multFunction = new MultFunction();
-            structRange = StructEnumerable.Range(0, Count).Select(ref multFunction, default(double));
+            structRange = StructEnumerable.Range(0, Count).Select(ref multFunction, Identity<double>.Instance);
         }
 
         [Benchmark(Baseline = true)]

src/StructLinq.Tests/SelectTests.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Collections.Generic;
 using System.Linq;
 using Xunit;
 
@@ -33,7 +32,7 @@ public void StructTest()
             var fct = new MultFunction();
             var structEnum = StructEnumerable
                 .Range(-50, 100)
-                .Select(ref fct, default(double))
+                .Select(ref fct, Identity<double>.Instance)
                 .ToArray();
             Assert.Equal(sys, structEnum);
 

src/StructLinq/Identity.cs

@@ -0,0 +1,10 @@
+namespace StructLinq
+{
+    public class Identity<T>
+    {
+        private Identity()
+        {
+        }
+        public static readonly Identity<T> Instance = new Identity<T>();
+    }
+}

src/StructLinq/Select/SelectStructEnumerable.cs

@@ -8,7 +8,7 @@ namespace StructLinq
     public static partial class StructEnumerable
     {
         public static ITypedEnumerable<TOut, SelectEnumerator<TIn, TOut, TEnumerator, TFunction>>
-            Select<TIn, TOut, TEnumerator, TFunction>(this ITypedEnumerable<TIn, TEnumerator> enumerable, ref TFunction function, TOut output = default(TOut))
+            Select<TIn, TOut, TEnumerator, TFunction>(this ITypedEnumerable<TIn, TEnumerator> enumerable, ref TFunction function, Identity<TOut> output)
             where TEnumerator : IEnumerator<TIn> 
             where TFunction : struct, IFunction<TIn, TOut>
         {
@@ -19,7 +19,7 @@ public static ITypedEnumerable<TOut, SelectEnumerator<TIn, TOut, TEnumerator, St
             where TEnumerator : IEnumerator<TIn>
         {
             var fct = function.ToStruct();
-            return enumerable.Select(ref fct, default(TOut));
+            return enumerable.Select(ref fct, Identity<TOut>.Instance);
         }
     }