[C#] support Object API (#5710)

* [C#] support Object API

* fix sign-compare

* fix indent

* add new line before for loop.

* using auto whenever possible

* reduce the amout of blank lines.

* wip: support vectors of union

* done: support unions of vectors

* set C# version to 4.0
* remove null propagation operator
* remove auto property initializer
* remove expression-bodied method
* remove pattern matching

* add Example2 to NetTest.sh

* separate JavaUsage.md and CsharpUsage.md from JavaCsharpUsage.md

* add C# Object based API notes.

* support vs2010.

* remove range based for loop.

* remove System.Linq

* fix indent

* CreateSharedString to CreateString

* check shared attribute

* snake case

docs/source/JavaUsage.md

@@ -0,0 +1,114 @@
+Use in Java    {#flatbuffers_guide_use_java}
+==============
+
+## Before you get started
+
+Before diving into the FlatBuffers usage in Java, it should be noted that
+the [Tutorial](@ref flatbuffers_guide_tutorial) page has a complete guide to
+general FlatBuffers usage in all of the supported languages (including Java).
+This page is designed to cover the nuances of FlatBuffers usage,
+specific to Java.
+
+You should also have read the [Building](@ref flatbuffers_guide_building)
+documentation to build `flatc` and should be familiar with
+[Using the schema compiler](@ref flatbuffers_guide_using_schema_compiler) and
+[Writing a schema](@ref flatbuffers_guide_writing_schema).
+
+## FlatBuffers Java code location
+
+The code for the FlatBuffers Java library can be found at
+`flatbuffers/java/com/google/flatbuffers`. You can browse the library on the
+[FlatBuffers GitHub page](https://github.com/google/flatbuffers/tree/master/
+java/com/google/flatbuffers).
+
+## Testing the FlatBuffers Java libraries
+
+The code to test the libraries can be found at `flatbuffers/tests`.
+
+The test code for Java is located in [JavaTest.java](https://github.com/google
+/flatbuffers/blob/master/tests/JavaTest.java).
+
+To run the tests, use either [JavaTest.sh](https://github.com/google/
+flatbuffers/blob/master/tests/JavaTest.sh) or [JavaTest.bat](https://github.com/
+google/flatbuffers/blob/master/tests/JavaTest.bat), depending on your operating
+system.
+
+*Note: These scripts require that [Java](https://www.oracle.com/java/index.html)
+is installed.*
+
+## Using the FlatBuffers Java library
+
+*Note: See [Tutorial](@ref flatbuffers_guide_tutorial) for a more in-depth
+example of how to use FlatBuffers in Java.*
+
+FlatBuffers supports reading and writing binary FlatBuffers in Java.
+
+To use FlatBuffers in your own code, first generate Java classes from your
+schema with the `--java` option to `flatc`.
+Then you can include both FlatBuffers and the generated code to read
+or write a FlatBuffer.
+
+For example, here is how you would read a FlatBuffer binary file in Java:
+First, import the library and generated code. Then, you read a FlatBuffer binary
+file into a `byte[]`.  You then turn the `byte[]` into a `ByteBuffer`, which you
+pass to the `getRootAsMyRootType` function:
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.java}
+    import MyGame.Example.*;
+    import com.google.flatbuffers.FlatBufferBuilder;
+
+    // This snippet ignores exceptions for brevity.
+    File file = new File("monsterdata_test.mon");
+    RandomAccessFile f = new RandomAccessFile(file, "r");
+    byte[] data = new byte[(int)f.length()];
+    f.readFully(data);
+    f.close();
+
+    ByteBuffer bb = ByteBuffer.wrap(data);
+    Monster monster = Monster.getRootAsMonster(bb);
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Now you can access the data from the `Monster monster`:
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.java}
+    short hp = monster.hp();
+    Vec3 pos = monster.pos();
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+## Storing dictionaries in a FlatBuffer
+
+FlatBuffers doesn't support dictionaries natively, but there is support to
+emulate their behavior with vectors and binary search, which means you
+can have fast lookups directly from a FlatBuffer without having to unpack
+your data into a `Dictionary` or similar.
+
+To use it:
+-   Designate one of the fields in a table as the "key" field. You do this
+    by setting the `key` attribute on this field, e.g.
+    `name:string (key)`.
+    You may only have one key field, and it must be of string or scalar type.
+-   Write out tables of this type as usual, collect their offsets in an
+    array.
+-   Instead of calling standard generated method,
+    e.g.: `Monster.createTestarrayoftablesVector`,
+    call `createSortedVectorOfTables` (from the `FlatBufferBuilder` object).
+    which will first sort all offsets such that the tables they refer to
+    are sorted by the key field, then serialize it.
+-   Now when you're accessing the FlatBuffer, you can use
+    the `ByKey` accessor to access elements of the vector, e.g.:
+    `monster.testarrayoftablesByKey("Frodo")`.
+    which returns an object of the corresponding table type,
+    or `null` if not found.
+    `ByKey` performs a binary search, so should have a similar
+    speed to `Dictionary`, though may be faster because of better caching.
+    `ByKey` only works if the vector has been sorted, it will
+    likely not find elements if it hasn't been sorted.
+
+## Text parsing
+
+There currently is no support for parsing text (Schema's and JSON) directly
+from Java, though you could use the C++ parser through native call
+interfaces available to each language. Please see the
+C++ documentation for more on text parsing.
+
+<br>