Create dynamic code by using Reflection

  • Thread starter Thread starter Tony Johansson
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Tony Johansson

Hi!

I have hard to see a situation when I would use the reflection feature to
create the code dynamically ?
Can somebody give me a hint when this situation would arise ?
It's much much easier to create the code in the normal way.

//Tony
 
Tony Johansson said:
I have hard to see a situation when I would use the reflection feature to
create the code dynamically ?
Can somebody give me a hint when this situation would arise ?
It's much much easier to create the code in the normal way.
Click to expand...

Okay, I'll give you an example. Imagine the following challenge: You have
to write a program where the user types-in a function such as
"y=2*Cos(x)-1", and you have to paint an x-y graph showing that curve. Note
that the expression is entered by the user into a textbox. You do not know
it at compile time, so you cannot "create the code in the normal way", to
quote your own words.
To paint the graph, you would assign multiple values to x, and perform
the operations to get the valye of y. You would then plot the (x,y) points
into your output graphic.
So, you need to produce code that is able to evaluate quickly, hundreds
of times, a function that is entered by the user. How do you accomplish
this? The first way that comes to mind is to use the CSharpComiler class to
compile the expression, producing an assembly, and then invoke the method
inside the assembly by means of reflection. This works, but the compilation
process is not very fast (it uses a temporary file on disk) and it has the
drawback that the assembly remains loaded in memory after you are done using
it, so your program wil "leak" memory if you repeat the proccess many times
as the user types new expressions. Of course, you could load the assembly
into a different AppDomain and then unload it, but this would introduce
additional complexity, and you would need to use cross-domain calls to
evaluate the function.
So, what is another alternative? Write a syntax analyzer to analyze the
expression that the user typed, and then use Emit to produce some MSIL code
that is equivalent to the expression. You then execute this code. The code
is produced in memory (no temporary file), and the result is a DynamicMethod
that evaluates the function as needed. The DynamicMethod is subject to
garbage colection, so you will not leak memory if the process is repeated
multiple times.
If you want an example, you can take a look at an article the I wrote on
this subject. The text of the article is written in Spanish, but you can
copy and paste the C# code and it will work fine regardless of your language
:-). This is the URL:
http://www.scribd.com/doc/24018633/Como-crear-un-compilador-de-expresiones-en-NET
 
I have hard to see a situation when I would use the reflection feature to
create the code dynamically ?
Can somebody give me a hint when this situation would arise ?
It's much much easier to create the code in the normal way.
Click to expand...

Yes. But that is only possible if the code is known at
compile time.

If you allow users to enter text at runtime and want
to generate code based on that, then you need to
dynamically generate code.

Arne
 
Hi!

I have hard to see a situation when I would use the reflection feature to
create the code dynamically ?
Can somebody give me a hint when this situation would arise ?
It's much much easier to create the code in the normal way.

//Tony
Click to expand...

Dynamically generated code is used for things like mock frameworks,
for example. Recently, I created my own mock framework called Mockina.
http://mockina.codeplex.com.

Also, I have used it in the past to create lightweight decorator
classes, such that they implement an interface simply by redirecting
all calls to an underlying instance. The benefit here is that the
decorator can track things like the number of times a method is
called, how many references to the object are in memory and
maintaining referential integrity.

In the past, I wrote a really awesome tool that could read an Oracle
database schema and generated an assembly of Data Transfer Objects
matching the schema.

Dynamically generated code also gives a very fine-grained control over
code generation, even beyond what C# can allow syntactically. For
instance, did you know that the accessors "protected internal" means,
"Visible to subclasses OR classes in the same assembly." Some times
you want "Visible to subclasses IN the same assembly." You can't
achieve that with pure C# syntax.

A lot of folks use the CodeDom namespace to generate text files with
source code in them. Another option is to dynamically generate code
and then use a decompiler to generate the files. The CodeDom namespace
is actually limited in functionality; for instance, bitwise operations
and using statements are not available. The best decompiler on the
market can automatically convert try/finally/dispose patterns into
using blocks.

Generated code is also used in proxies, such as Web Services and .NET
Remoting. In fact, under the hood of most of the major development
tool enhancements in the past 5 years, reflection and dynamic code
generation are performing a large majority of the work. Pure coding is
great and all, but dynamic code generation is at the core of the
modern development practice. It is a wonder why it is not taught more
in schools. I would say that there is a great deal of benefit in
teaching assembler in MSIL or ByteCode at this point, as well as at
the machine level.
 
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