What does the Sample do
It compiles C# lambda expressions into 16-bit executable files.
Why is it Interesting?
C# expression trees allow you to analyze C# code and reinterpret it in a different way. E.g. LINQ-to-SQL takes expressions written in C# and turns them into SQL statements.
This sample turns (a small subset of) C# into 8086 machine code.
I have read many blogs about LINQ expression trees and how "cool" they are. However, very few posts explained what actually expression trees are for. This sample demonstrates one possible usage. Some day, I may expand this article to further discuss the place of expression trees in life.
Why .Com Files
I chose .com files because they are as simple as executable format gets. Just an array of 8086 machine codes with absolutely no headers or metadata. A compiler writer heaven. And it is still executable on all versions of Windows.
NB: "com" here is merely a file extension, as in xy.com. It has nothing to do with Component Object Model which appeared 10 or so years after .com files.
An expression may contain variables and three arithmetic operations: +, -, and *. All arithmetic is done in unsigned 16-bit integers.
When the generated COM file executes, it prints a banner, prompts the user for parameter values (if any), calculates the result and displays it. COM files shall be executed in a console window, e.g. from cmd.exe.
(x,y) => 2*x + 3*y when compiled and executed produces the following output (user input in bold):
C# for MS-DOS, version 1.0
((2 * x) + (3 * y)) = 80
Calling Expression Compiler
The sample provides class
ExpressionCompiler86 with the method
Compile(Expression expr). Here is how main class
Program compiles three expressions:
static void Compile<T>(string file, Expression<T> expr)
using (ExpressionCompiler86 compiler = new ExpressionCompiler86(file))
Compile<Func<int>>( "constant.com", () => 42);
Compile<Func<int>>( "noparams.com&", () => 1+2+3*4);
Compile<Func<int,int,int>>( "xy.com", (x,y) => 2*x + 3*y );
Please be lenient to the quality of code. It is not production code. It's a sample, or, rather, a proof of concept. It was written in one day. It contains absolutely no automated tests. There are absolutely no overflow checks or anything like that. Class names may be questionable. Bugs are likely.
- 4th September, 2008: Initial post