asp.net the compiler failed with error code 128 I OBJECT-ORIENTED PROGRAMMING in Font

Printing Data Matrix ECC200 in Font I OBJECT-ORIENTED PROGRAMMING

CHAPTER 5 I OBJECT-ORIENTED PROGRAMMING
Drawing Data Matrix In None
Using Barcode maker for Font Control to generate, create ECC200 image in Font applications.
www.OnBarcode.com
Code 3 Of 9 Encoder In None
Using Barcode creator for Font Control to generate, create Code 3/9 image in Font applications.
www.OnBarcode.com
the class, you must explicitly add a constructor. To do this, you need to add a member, which is always named new and is followed the constructor within parentheses. After this comes an equals sign followed by a block (delimited by braces), which contains expressions to initialize every field in the class. The following example defines a simple constructor for an empty class: #light type JustConstruct = class new() = {} end let constructed = new JustConstruct() This may come as a surprise to experienced object-oriented programmers since most object-oriented languages provide a default constructor for any class that doesn t define one. F# s philosophy is to provide a more powerful construct called implicit class construction that often subsumes the need for explicit constructors altogether. You ll return to implicit class construction in the next section. Furthermore, default constructors can easily leave some fields uninitialized and therefore null and can leave some at risk of causing a NullReferenceException. This is why a constructor in F# must initialize all fields defined by a class. Fields are defined using the keyword val, followed the name of the field, and then the name of the type separated from the property name by a colon. The next example shows a simple class, file1, that has two fields, path and innerFile, that are initialized in the constructor, which has one parameter, path: #light open System.IO type File1 = class val path: string val innerFile: FileInfo new(path) = { path = path ; innerFile = new FileInfo(path) } end let myFile1 = new File1("whatever.txt") It s possible to overload constructors; one simply adds a second constructor with a different number of parameters. If you want to overload with parameters of different types, then you must provide type annotations. The following example shows a class, File2, with two constructors, one with no parameters and one with one parameter: #light open System.IO type File2 = class val path: string val innerFile: FileInfo
Painting Barcode In None
Using Barcode encoder for Font Control to generate, create Barcode image in Font applications.
www.OnBarcode.com
Print Universal Product Code Version A In None
Using Barcode encoder for Font Control to generate, create UPCA image in Font applications.
www.OnBarcode.com
CHAPTER 5 I OBJECT-ORIENTED PROGRAMMING
Create Data Matrix ECC200 In None
Using Barcode maker for Font Control to generate, create Data Matrix 2d barcode image in Font applications.
www.OnBarcode.com
Drawing Code 128 Code Set A In None
Using Barcode maker for Font Control to generate, create Code-128 image in Font applications.
www.OnBarcode.com
new() = new File2("default.txt") new(path) = { path = path ; innerFile = new FileInfo(path) } end let myFile2 = new File2("whatever2.txt") Note that the only thing you can do in the initialization block of a constructor is to initialize the fields of a class or call another constructor to do that for you. If you want to do other things in a constructor, you must you use the keyword then after the block and follow it by the extra expressions you want in the constructor. This separates the initialization of fields from other code to ensure that nothing can happen to the fields of a class before they are properly initialized. If you want to access the fields in a class outside the initialization block, you must give a name to the instance you re creating by qualifying the constructor; you do this using the keyword as followed by the alias for the instance. The next example shows a constructor with some code following the initialization block. The alias, x, is defined for the instance. This is later used to test whether the file associated with the FileInfo object bound to the field innerFile exists. #light open System.IO type File3 = class val path: string val innerFile: FileInfo new(path) as x = { path = path ; innerFile = new FileInfo(path) } then if not x.innerFile.Exists then let textFile = x.innerFile.CreateText() textFile.Dispose() end let myFile3 = new File3("whatever.txt") By default, fields in a class are immutable, which means once they have been bound to a value, the value can be rebound to another value. For F# records with mutable fields and .NET objects, this does not mean their internal state cannot change; it simply means you cannot replace the whole value to which the field is bound. You can see this in the previous example; if the file you are creating doesn t exist, the file will be created, changing the value of the Exists flag to true. However, you cannot set the field innerFile to be another instance of the FileInfo object. From time to time, it can be useful to rebind a field to another value. To allow this to happen, F# provides the keyword mutable; when a field is defined as mutable, it can be rebound whenever the programmer chooses. The following example illustrates its usage. In this example, you see that the mutable keyword is applied to the FileInfo field so that you can change
EAN / UCC - 13 Creator In None
Using Barcode maker for Font Control to generate, create EAN / UCC - 13 image in Font applications.
www.OnBarcode.com
Generate EAN / UCC - 8 In None
Using Barcode generator for Font Control to generate, create GTIN - 8 image in Font applications.
www.OnBarcode.com
Reading Data Matrix ECC200 In VB.NET
Using Barcode reader for Visual Studio .NET Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
ECC200 Scanner In .NET
Using Barcode scanner for VS .NET Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
Recognizing UPC - 13 In C#.NET
Using Barcode reader for .NET framework Control to read, scan read, scan image in .NET applications.
www.OnBarcode.com
PDF-417 2d Barcode Creation In None
Using Barcode maker for Software Control to generate, create PDF417 image in Software applications.
www.OnBarcode.com
QR Code 2d Barcode Maker In Java
Using Barcode printer for Android Control to generate, create QR Code ISO/IEC18004 image in Android applications.
www.OnBarcode.com
DataBar Creator In Java
Using Barcode generation for Java Control to generate, create DataBar image in Java applications.
www.OnBarcode.com
Code 128 Code Set A Decoder In None
Using Barcode scanner for Software Control to read, scan read, scan image in Software applications.
www.OnBarcode.com
Barcode Creation In VS .NET
Using Barcode drawer for VS .NET Control to generate, create Barcode image in .NET applications.
www.OnBarcode.com
PDF-417 2d Barcode Reader In Visual C#.NET
Using Barcode recognizer for Visual Studio .NET Control to read, scan read, scan image in Visual Studio .NET applications.
www.OnBarcode.com
Painting Barcode In .NET Framework
Using Barcode printer for ASP.NET Control to generate, create Barcode image in ASP.NET applications.
www.OnBarcode.com
Barcode Scanner In Java
Using Barcode decoder for Java Control to read, scan read, scan image in Java applications.
www.OnBarcode.com
Read Universal Product Code Version A In VB.NET
Using Barcode scanner for Visual Studio .NET Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
Copyright © OnBarcode.com . All rights reserved.