Remove byte* property in ByteBufferAllocator (#5191)

* Remove byte* property in ByteBufferAllocator.

This allows consumers to read/write into native memory, but without
having to always pin the managed `byte[]` when working with managed
memory. This allows for users to not need to Dispose() ByteBuffers
when they are using the default ByteArrayAllocator class.

Instead, we use `Span<byte> GetSpan()` methods to get access to the
underlying memory buffer.

Fix #5181

* Add a set of benchmark tests.

* Add ReadOnly spans.

This allows consumers to use ReadOnlyMemory<byte> as the backing storage
for ByteBuffers, which is useful in read-only scenarios.

* Run tests using ENABLE_SPAN_T in appveyor.

* Fix FlatBuffers.Test.csproj to work on older MSBuild versions.

* Change the test script to test UNSAFE_BYTEBUFFER

* Address PR feedback.

Remove IDisposable from ByteBuffer.

* Respond to PR feedback.

tests/FlatBuffers.Benchmarks/FlatBufferBuilderBenchmark.cs

@@ -0,0 +1,101 @@
+/*
+ * Copyright 2014 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+using BenchmarkDotNet.Attributes;
+using MyGame.Example;
+
+namespace FlatBuffers.Benchmarks
+{
+    //[EtwProfiler] - needs elevated privileges
+    [MemoryDiagnoser]
+    public class FlatBufferBuilderBenchmark
+    {
+        private const int NumberOfRows = 10_000;
+
+        [Benchmark]
+        public void BuildNestedMonster()
+        {
+            const string nestedMonsterName = "NestedMonsterName";
+            const short nestedMonsterHp = 600;
+            const short nestedMonsterMana = 1024;
+
+            for (int i = 0; i < NumberOfRows; i++)
+            {
+                // Create nested buffer as a Monster type
+                var fbb1 = new FlatBufferBuilder(16);
+                var str1 = fbb1.CreateString(nestedMonsterName);
+                Monster.StartMonster(fbb1);
+                Monster.AddName(fbb1, str1);
+                Monster.AddHp(fbb1, nestedMonsterHp);
+                Monster.AddMana(fbb1, nestedMonsterMana);
+                var monster1 = Monster.EndMonster(fbb1);
+                Monster.FinishMonsterBuffer(fbb1, monster1);
+                var fbb1Bytes = fbb1.SizedByteArray();
+                fbb1 = null;
+
+                // Create a Monster which has the first buffer as a nested buffer
+                var fbb2 = new FlatBufferBuilder(16);
+                var str2 = fbb2.CreateString("My Monster");
+                var nestedBuffer = Monster.CreateTestnestedflatbufferVector(fbb2, fbb1Bytes);
+                Monster.StartMonster(fbb2);
+                Monster.AddName(fbb2, str2);
+                Monster.AddHp(fbb2, 50);
+                Monster.AddMana(fbb2, 32);
+                Monster.AddTestnestedflatbuffer(fbb2, nestedBuffer);
+                var monster = Monster.EndMonster(fbb2);
+                Monster.FinishMonsterBuffer(fbb2, monster);
+            }
+        }
+
+        [Benchmark]
+        public void BuildMonster()
+        {
+            for (int i = 0; i < NumberOfRows; i++)
+            {
+                var builder = new FlatBufferBuilder(16);
+                var str1 = builder.CreateString("MonsterName");
+                Monster.StartMonster(builder);
+                Monster.AddName(builder, str1);
+                Monster.AddHp(builder, 600);
+                Monster.AddMana(builder, 1024);
+                Monster.AddColor(builder, Color.Blue);
+                Monster.AddTestbool(builder, true);
+                Monster.AddTestf(builder, 0.3f);
+                Monster.AddTestf2(builder, 0.2f);
+                Monster.AddTestf3(builder, 0.1f);
+
+                var monster1 = Monster.EndMonster(builder);
+                Monster.FinishMonsterBuffer(builder, monster1);
+            }
+        }
+
+        [Benchmark]
+        public void TestTables()
+        {
+            FlatBufferBuilder builder = new FlatBufferBuilder(1024 * 1024 * 32);
+            for (int x = 0; x < 500000; ++x)
+            {
+                var offset = builder.CreateString("T");
+                builder.StartObject(4);
+                builder.AddDouble(3.2);
+                builder.AddDouble(4.2);
+                builder.AddDouble(5.2);
+                builder.AddOffset(offset.Value);
+                builder.EndObject();
+            }
+        }
+    }
+}

tests/FlatBuffers.Benchmarks/FlatBuffers.Benchmarks.csproj

@@ -0,0 +1,21 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp2.1</TargetFramework>
+    <LangVersion>latest</LangVersion>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <DefineConstants>$(DefineConstants);UNSAFE_BYTEBUFFER;BYTEBUFFER_NO_BOUNDS_CHECK;ENABLE_SPAN_T</DefineConstants>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="BenchmarkDotNet" Version="0.11.3" />
+    <PackageReference Include="BenchmarkDotNet.Diagnostics.Windows" Version="0.11.3" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Compile Include="..\..\net\FlatBuffers\*.cs" Link="FlatBuffers\%(FileName).cs" />
+    <Compile Include="..\MyGame\**\*.cs" Link="MyGame\Example\%(FileName).cs" />
+  </ItemGroup>
+
+</Project>

tests/FlatBuffers.Benchmarks/Program.cs

@@ -0,0 +1,30 @@
+/*
+ * Copyright 2014 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+using BenchmarkDotNet.Running;
+
+namespace FlatBuffers.Benchmarks
+{
+    public static class Program
+    {
+        public static void Main(string[] args)
+        {
+            BenchmarkSwitcher
+                .FromAssembly(typeof(Program).Assembly)
+                .Run(args);
+        }
+    }
+}

tests/FlatBuffers.Test/FlatBuffers.Test.csproj

@@ -31,6 +31,10 @@
   <PropertyGroup>
     <StartupObject />
   </PropertyGroup>
+  <PropertyGroup Condition="'$(UnsafeByteBuffer)' == 'true'">
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <DefineConstants>$(DefineConstants);UNSAFE_BYTEBUFFER</DefineConstants>
+  </PropertyGroup>
   <ItemGroup>
     <Reference Include="System" />
     <Reference Include="System.Core">