Private Const
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Private Const
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c# .net c#-3.0 constants readonly
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I got three class files where in constant is declared as
Is it good to move this to new file to hold all common constants declared as
What are the pros and cons for this?
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There are some important differences between a const and a readonly field:
The const is evaluated at compile time. If you declare the const in a separate assembly that you reference from you application a change to the const will only affect the application if it is recompiled using the updated assembly. To quote from the .NET Design Guidelines for Developing Class Libraries :
Do use constant fields for constants that will never change.
For example, the Math class defines E and PI as static constants.
The compiler inserts the values of const fields directly into the calling code, which means that const values can never be changed without the risk of introducing a compatibility issue.
A readonly field can be initialized at run-time enabling you to perform run-time calculations to compute the value and use. A const can only be declared by a constant expression that can be fully evaluated at compile time. The only reference type that can be const is String .
About your specific question it really depends on how these constants are used. Obviously you shouldn't have multiple definitions of the same constant. Otherwise it is probably easier to understand if the constant is declared "near" where it is used, e.g. in the class or even the method where it is used.
In favour: your constants are all centralised in one place.
Against: your constants are no longer close to the point at which they're used.
For constants that are shared between classes it makes sense to break them out to a common single class so they're only specified once. However this implies "inappropriate coupling", so it might be that all the logic that uses this constant needs to be in the same class.
The two may have similar practical effects, but they are useful for signalling your intentions.
So, a const value is something that is available to all instances of your class and will never change. A readonly signals that you have a data value that could be different for each instance of you class, but will be immutable once the class is created. Immutability can be a really useful guarantee when you are sharing the instance of the class between different consumers. In passing, in CLR Via C#, Richter prefers readonly public members to properties with only public setters, I'll have to dig it out and remind myself why.
It might be that you're optimizing too early.
This might mean losing flexibility by making the constants shared and public -- hard to say without knowing what the existing classes are.
What if PageSize actually needs to vary between them in the future? Does it actually need to be synchronised across all three classes, or is this just a small tweak that seems like a good idea at the moment?
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Declares and defines one or more constants.
attributelist
Optional. List of attributes that apply to all the constants declared in this statement. See Attribute List in angle brackets (" < " and " > ").
accessmodifier
Optional. Use this to specify what code can access these constants. Can be Public , Protected , Friend , Protected Friend , Private , or Private Protected .
Shadows
Optional. Use this to redeclare and hide a programming element in a base class. See Shadows .
constantlist
Required. List of constants being declared in this statement.
Each constant has the following syntax and parts:
constantname [ As datatype ] = initializer
If you have a value that never changes in your application, you can define a named constant and use it in place of a literal value. A name is easier to remember than a value. You can define the constant just once and use it in many places in your code. If in a later version you need to redefine the value, the Const statement is the only place you need to make a change.
You can use Const only at module or procedure level. This means the declaration context for a variable must be a class, structure, module, procedure, or block, and cannot be a source file, namespace, or interface. For more information, see Declaration Contexts and Default Access Levels .
Local constants (inside a procedure) default to public access, and you cannot use any access modifiers on them. Class and module member constants (outside any procedure) default to private access, and structure member constants default to public access. You can adjust their access levels with the access modifiers.
Declaration Context. A constant declared at module level, outside any procedure, is a member constant ; it is a member of the class, structure, or module that declares it.
A constant declared at procedure level is a local constant ; it is local to the procedure or block that declares it.
Attributes. You can apply attributes only to member constants, not to local constants. An attribute contributes information to the assembly's metadata, which is not meaningful for temporary storage such as local constants.
Modifiers. By default, all constants are Shared , Static , and ReadOnly . You cannot use any of these keywords when declaring a constant.
At procedure level, you cannot use Shadows or any access modifiers to declare local constants.
Multiple Constants. You can declare several constants in the same declaration statement, specifying the constantname part for each one. Multiple constants are separated by commas.
Data Types. The Const statement can declare the data type of a variable. You can specify any data type or the name of an enumeration.
Default Type. If you do not specify datatype , the constant takes the data type of initializer . If you specify both datatype and initializer , the data type of initializer must be convertible to datatype . If neither datatype nor initializer is present, the data type defaults to Object .
Different Types. You can specify different data types for different constants by using a separate As clause for each variable you declare. However, you cannot declare several constants to be of the same type by using a common As clause.
Initialization. You must initialize the value of every constant in constantlist . You use initializer to supply an expression to be assigned to the constant. The expression can be any combination of literals, other constants that are already defined, and enumeration members that are already defined. You can use arithmetic and logical operators to combine such elements.
You cannot use variables or functions in initializer . However, you can use conversion keywords such as CByte and CShort . You can also use AscW if you call it with a constant String or Char argument, since that can be evaluated at compile time.
Scope. Local constants are accessible only from within their procedure or block. Member constants are accessible from anywhere within their class, structure, or module.
Qualification. Code outside a class, structure, or module must qualify a member constant's name with the name of that class, structure, or module. Code outside a procedure or block cannot refer to any local constants within that procedure or block.
The following example uses the Const statement to declare constants for use in place of literal values.
If you define a constant with data type Object , the Visual Basic compiler gives it the type of initializer , instead of Object . In the following example, the constant naturalLogBase has the run-time type Decimal .
The preceding example uses the ToString method on the Type object returned by the GetType Operator , because Type cannot be converted to String using CStr .
Required. Name of the constant. See Declared Element Names .
Required if Option Strict is On . Data type of the constant.
Required. Expression that is evaluated at compile time and assigned to the constant.
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You use the Const statement to declare a constant and set its value. By declaring a constant, you assign a meaningful name to a value. Once a constant is declared, it cannot be modified or assigned a new value.
You declare a constant within a procedure or in the declarations section of a module, class, or structure. Class or structure-level constants are Private by default, but may also be declared as Public , Friend , Protected , or Protected Friend for the appropriate level of code access.
The constant must have a valid symbolic name (the rules are the same as those for creating variable names) and an expression composed of numeric or string constants and operators (but no function calls).
Your computer might show different names or locations for some of the Visual Studio user interface elements in the following instructions. The Visual Studio edition that you have and the settings that you use determine these elements. For more information, see Personalizing the IDE .
Write a declaration that includes an access specifier, the Const keyword, and an expression, as in the following examples:
When Option Infer is Off and Option Strict is On , you must declare a constant explicitly by specifying a data type ( Boolean , Byte , Char , DateTime , Decimal , Double , Integer , Long , Short , Single , or String ).
When Option Infer is On or Option Strict is Off , you can declare a constant without specifying a data type with an As clause. The compiler determines the type of the constant from the type of the expression. For more information, see Constant and Literal Data Types .
Write a declaration that includes the As keyword and an explicit data type, as in the following examples:
You can declare multiple constants on a single line, although your code is more readable if you declare only a single constant per line. If you declare multiple constants on a single line, they must all have the same access level ( Public , Private , Friend , Protected , or Protected Friend ).
Separate the declarations with a comma and a space, as in the following example:
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You use the const keyword to declare a constant field or a constant local. Constant fields and locals aren't variables and may not be modified. Constants can be numbers, Boolean values, strings, or a null reference. Don’t create a constant to represent information that yo
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