ROSequence should throw a more helpful exception on a bad slice.

[Kuinox]

Description

This code:

ReadOnlySequence<byte> bufferA = new ReadOnlySequence<byte>( new byte[50] );
ReadOnlySequence<byte> bufferB = new ReadOnlySequence<byte>( new byte[45] );
ReadOnlySequence<byte> slice = bufferA.Slice( bufferB.End );
long test = slice.Length;

Throws on the last line:

System.InvalidCastException: 'Unable to cast object of type 'System.Byte[]' to type 'System.Buffers.ReadOnlySequenceSegment`1[System.Byte]'.'

As discussed in #766, this behavior is expected, but:

• The exception could be more helpful here.
• The docs don't mention the late throwing behavior: https://docs.microsoft.com/en-us/dotnet/api/system.buffers.readonlysequence-1?view=net-5.0

Discussed this with @tannergooding on discord.

[tannergooding]

As from #766, we can't really check if a given position is part of a sequence due to perf.

That being said, all the Slice methods that take a SequencePosition do currently call into BoundsCheck
It might be reasonable to do the simplest sanity check that position might be valid as part of this so a better error message can be produced.

Namely something like:

 if (startObject == endObject)
 {
     if (!InRange(sliceStartIndex, startIndex, endIndex))
     {
         ThrowHelper.ThrowArgumentOutOfRangeException_PositionOutOfRange();
     }
 
+    if (position.GetObject() != startObject)
+    {
+        // throw, position can't be part of this sequence
+    }
 }
 else
 {
     // Multi-Segment Sequence
     // Storing this in a local since it is used twice within InRange()
     ulong startRange = (ulong)(((ReadOnlySequenceSegment<T>)startObject!).RunningIndex + startIndex);
     long runningIndex = 0;
     if (positionIsNotNull)
     {
         Debug.Assert(position.GetObject() != null);
         runningIndex = ((ReadOnlySequenceSegment<T>)position.GetObject()!).RunningIndex;
     }
 
     if (!InRange(
         (ulong)(runningIndex + sliceStartIndex),
         startRange,
         (ulong)(((ReadOnlySequenceSegment<T>)endObject!).RunningIndex + endIndex)))
     {
         ThrowHelper.ThrowArgumentOutOfRangeException_PositionOutOfRange();
     }
 
+    if (position.GetObject() is not ReadOnlySequenceSegment<T>)
+    {
+        // throw, position can't be part of this sequence
+    }
 }

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