constants:
SymbolSize:

SymbolRectAuto = -2
SymbolSquareAuto=-1
Symbol10x10=0
Symbol12x12=1
Symbol14x14=2
Symbol16x16=3
Symbol18x18=4
Symbol20x20=5
Symbol22x22=6
Symbol24x24=7
Symbol26x26=8
Symbol32x32=9
Symbol36x36=10
Symbol40x40=11
Symbol44x44=12
Symbol48x48=13
Symbol52x52=14
Symbol64x64=15
Symbol72x72=16
Symbol80x80=17
Symbol88x88=18
Symbol96x96=19
Symbol104x104=20
Symbol120x120=21
Symbol132x132=22
Symbol144x144=23
Symbol8x18=24
Symbol8x32=25
Symbol12x26=26
Symbol12x36=27
Symbol16x36=28
Symbol16x48=29


Mode:

Ascii=0
C40=1
Text=2
X12=3
Edifact=4
Base256=5
AsciiGS1=6


Programming samples:

.........
using aipsys.datamatrix.encoder;
..........

Encode to Bitmap and show it:

ImageEncoder iee = new ImageEncoder();
ie.Margin = 4;
ie.PixelSize = 3;
ie.Scheme = 0;
ie.SymbolSize = -2;
string s = "http://www.aipsys.com";
pictureBox1.Image = iee.Encode2Image(s);

Encode and save the barcode image to file:

ImageEncoder ie = new ImageEncoder();
ie.Margin = 4;
ie.PixelSize = 3;
ie.Scheme = 0;
ie.SymbolSize = -2;
ie.Encode2ImageFile("http://www.aipsys.com", "c:\\test.gif");


the SDK can be downloaded from http://www.aipsys.com

The symbol size of Data Matrix ECC200 consists of even rows and even columns. Some symbols are square with sizes from 10*10 to 144*144; some symbols are rectangular with sizes from 8*18 to 16*48
Here is a sample code of create Data Matrix in .net.
using BusinessRefinery.Barcode;

DataMatrix barcode = new DataMatrix();
barcode.Code = "Data Matrix";
barcode.Format = ImageFormat.Gif;
barcode.drawBarcode2ImageFile("c:/data-matrix-csharp.gif");

I have ever used the following codes to generate DataMatrix barcode image sucessfully. It's also very simple.
BarcodeLib.Barcode.DataMatrix datamatrix = new BarcodeLib.Barcode.DataMatrix();
datamatrix.Data = "1dfefg%^7fdsg56566";

datamatrix.BackgroundColor = System.Drawing.Color.White;
datamatrix.ModuleSize =6;
datamatrix.RightMargin = 6;
datamatrix.ImageFormat = System.Drawing.Imaging.ImageFormat.Jpeg;


// save barcode image into your system
datamatrix.drawBarcode("c:/datamatrix.jpeg");

// Encode datamatrix barcode & output to byte array
byte[] barcodeInBytes = datamatrix.drawBarcodeAsBytes();

// Encode datamatrix barcode to Graphics object
Graphics graphics = ...;
datamatrix.drawBarcode(graphics);

// Encode datamatrix barcode and output to HttpResponse object
HttpResponse response = ...;
datamatrix drawBarcode(response);

// Encode datamatrix barcode and output to Stream object
Stream stream = ...;
datamatrix drawBarcode(stream);

I have ever used the following codes to generate DataMatrix barcode image sucessfully. It's also very simple.
BarcodeLib.Barcode.DataMatrix datamatrix = new BarcodeLib.Barcode.DataMatrix();
datamatrix.Data = "1dfefg%^7fdsg56566";

datamatrix.BackgroundColor = System.Drawing.Color.White;
datamatrix.ModuleSize =6;
datamatrix.RightMargin = 6;
datamatrix.ImageFormat = System.Drawing.Imaging.ImageFormat.Jpeg;


// save barcode image into your system
datamatrix.drawBarcode("c:/datamatrix.jpeg");

// Encode datamatrix barcode & output to byte array
byte[] barcodeInBytes = datamatrix.drawBarcodeAsBytes();

// Encode datamatrix barcode to Graphics object
Graphics graphics = ...;
datamatrix.drawBarcode(graphics);

// Encode datamatrix barcode and output to HttpResponse object
HttpResponse response = ...;
datamatrix drawBarcode(response);

// Encode datamatrix barcode and output to Stream object
Stream stream = ...;
datamatrix drawBarcode(stream);

Here is the sample Code I have used to create Data Matrix barcode in C#:
using System;
using System.Collections.Generic;
using System.Text;
using OnBarcode.Barcode;
using System.Drawing.Imaging;
using System.Drawing;


DataMatrix datamatrix = new DataMatrix();

// Barcode data to encode
datamatrix.Data = "OnBarcode";
// Data Matrix data mode
datamatrix.DataMode = DataMatrixDataMode.ASCII;
// Data Matrix format mode
datamatrix.FormatMode = DataMatrixFormatMode.Format_16X16;

/*
* Barcode Image Related Settings
*/
// Unit of meature for all size related setting in the library.
datamatrix.UOM = UnitOfMeasure.PIXEL;
// Bar module size (X), default is 3 pixel;
datamatrix.X = 3;
// Barcode image left, right, top, bottom margins. Defaults are 0.
datamatrix.LeftMargin = 0;
datamatrix.RightMargin = 0;
datamatrix.TopMargin = 0;
datamatrix.BottomMargin = 0;
// Image resolution in dpi, default is 72 dpi.
datamatrix.Resolution = 72;
// Created barcode orientation.
//4 options are: facing left, facing right, facing bottom, and facing top
datamatrix.Rotate = Rotate.Rotate0;

// Generate data matrix and encode barcode to gif format
datamatrix.ImageFormat = System.Drawing.Imaging.ImageFormat.Gif;
datamatrix.drawBarcode("C:\\datamatrix.gif");

/*
You can also call other drawing methods to generate barcodes

public void drawBarcode(Graphics graphics);

public void drawBarcode(string filename);

public Bitmap drawBarcode();

public void drawBarcode(Stream stream);

*/
The C# barcode data encoding guide is very helpful.