Adds FixedSizeBuilder type

The existing ImmutableArray<T> builder pattern through the Builder type
uses a minimum of two array allocations to create a single
ImmutableArray<T> instance.  The ImmutableArray<T>.Builder type has an
array field which is manipulated in much the same way as the array
inside of List<T>.  The call to ToImmutable on the builder creates a
copy of this array at the current length and wraps the result in an
ImmutableArray<T>

This pattern is great for a number of sceanrios.  For example when the
length of the array is variable or the application has low memory
pressure.

There are many situations though where a no copy / single allocation
solution would be advantageous.  In particular where there are a known
fixed number of elements to be converted to an array.  There is no need
for a double copy solution here because the final size is known in
advance.  All that is needed is for the array to be created, populated
and then frozen in an ImmutableArray<T> instance.

A good example of this need is Roslyn within Visual Studio.  Roslyn is
very careful to avoid unnecessary allocations, especially on hot paths.
Many of the hot paths include building instances of ImmutableArray<T>.
To avoid the double allocation Roslyn pools ImmutableArray<T>.Builder
instances.  This successfully avoids the double allocation but increases
the overall Gen2 pressure in the system.

This PR proposes to fix this by adding a no copy builder pattern:
FixedSizeBuilder.  This is a very simple wrapper around T[] which allows
for the creation and population of the array without the ability to
store a long term reference to the underlying array.  Hence it can be
safely converted to an ImmutableArray<T> without the need to copy the
array.

The builder can be reused for subsequent ImmutableArray<T> builds via
the Reset method
The message tells u whatever the developer wrote if they wrote anything. It
may be related to the error or it may be a general message.  IME the text
of the exception has very rarely been helpful in tracking down the error
(if its present at all)

The important information is the stack trace which both provide equally
well.

The difficulty with tracking down the exact condition that lead to the
error is the same between an NRE and IVO.  Both are triggered by expression
and it's just as possible for compex checks to trigger an IVO as complex
expression to trigger an NRE.


On Tuesday, November 25, 2014, Andrew Arnott <notifications@github.com>
wrote:

> In
> src/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableArray`1+FixedSizeBuilder.cs:
>
> > +            /// Gets or sets the element at the specified index.
> > +            /// </summary>
> > +            /// <param name="index">The index.</param>
> > +            /// <exception cref="System.IndexOutOfRangeException" />
> > +            /// <exception cref="System.InvalidOperationException" />
> > +            public T this[int index]
> > +            {
> > +                get
> > +                {
> > +                    CheckIsInitialized();
> > +                    return _elements[index];
> > +                }
> > +
> > +                set
> > +                {
> > +                    CheckIsInitialized();
>
> I'll also add that NullReferenceExceptions are often very difficult to
> figure out because unless the method is absolutely trivial (which in this
> case it is but often isn't) there just isn't enough data in a callstack to
> know which dereferenced value was null.
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/dotnet/corefx/pull/154/files#r20918315>.
>


-- 
Jared Parsons
http://blog.paranoidcoding.com/
http://twitter.com/jaredpar