Some quick shots  

- ValidateBufferArgs()  is not needed, because the [`Buffer.BlockCopy()`][1] method will handle these cases in the same manner. In addition, why do you call the `ValidtaeBufferArgs()` method before you check for  `if (!bIsOpen_m)` inside the `Write()` method ?  

- the naming of your member variables is well strange. First you are using hungarian notation which is an anti pattern if it is used this way. There are ways how this could be acceptable but not like this  

 `private bool bIsOpen_m;` indicating by the `b` prefix that it is a bool.  

 Next, you postfix the member variables with a `_m` **and** you always use `this.` with them. If you pre- or postfix the variables with either `m_` or `_m` than there is no need to use `this.`.  

- you have many places where it is obvious what type you are assigning by checking the right side of the assignment. In such cases you should use the [`var` type][2] and let the compiler determine the correct type.  

 >     byte[] newBuffer = new byte[count];  

 would e.g become  

        var newBuffer = new byte[count];


- **IDisposable**  

 Please read: [proper-use-of-the-idisposable-interface][3]  

 > If the user calls Dispose() on your object, then everything has been cleaned up. Later on, when the garbage collector comes along and calls Finalize, it will then call Dispose again.

 > Not only is this wasteful, but if your object has junk references to objects you already disposed of from the last call to Dispose(), you'll try to dispose them again! 

 
- **Regions** 

 Please read [are-regions-an-antipattern-or-code-smell][4]  

 > Is there a good use for regions?
> 
> No. There was a legacy use: generated code. Still, code generation
> tools just have to use partial classes instead. If C# has regions
> support, it's mostly because this legacy use, and because now that too
> many people used regions in their code, it would be impossible to
> remove them without breaking existent codebases.
> 
> Think about it as about goto. The fact that the language or the IDE
> supports a feature doesn't mean that it should be used daily. StyleCop
> SA1124 rule is clear: you should not use regions. Never.


  [1]: https://msdn.microsoft.com/en-us/library/system.buffer.blockcopy%28v=vs.110%29.aspx
  [2]: https://msdn.microsoft.com/en-us/library/bb383973.aspx
  [3]: http://stackoverflow.com/a/538238/2655508
  [4]: https://msdn.microsoft.com/en-us/library/system.buffer.blockcopy%28v=vs.110%29.aspx