Type parameters and constraints

If a generic type is declared in multiple parts, each part must state the type parameters. Each part must have the same number of type parameters, and the same name for each type parameter, in order.

When a partial generic type declaration includes constraints (where clauses), the constraints must agree with all other parts that include constraints. Specifically, each part that includes constraints must have constraints for the same set of type parameters, and for each type parameter the sets of primary, secondary, and constructor constraints must be equivalent. Two sets of constraints are equivalent if they contain the same members. If no part of a partial generic type specifies type parameter constraints, the type parameters are considered unconstrained.

The example

partial class Dictionary<K,V>
where K: IComparable<K>
where V: IKeyProvider<K>, IPersistable
{
...
}

partial class Dictionary<K,V>
where V: IPersistable, IKeyProvider<K>
where K: IComparable<K>
{
...
}

partial class Dictionary<K,V>
{
...
}

is correct because those parts that include constraints (the first two) effectively specify the same set of primary, secondary, and constructor constraints for the same set of type parameters, respectively.

Base class

When a partial class declaration includes a base class specification it must agree with all other parts that include a base class specification. If no part of a partial class includes a base class specification, the base class becomes System.Object (§10.1.4.1).

Base interfaces

The set of base interfaces for a type declared in multiple parts is the union of the base interfaces specified on each part. A particular base interface may only be named once on each part, but it is permitted for multiple parts to name the same base interface(s). There must only be one implementation of the members of any given base interface.

In the example

partial class C: IA, IB {...}

partial class C: IC {...}

partial class C: IA, IB {...}

the set of base interfaces for class C is IA, IB, and IC.

Typically, each part provides an implementation of the interface(s) declared on that part; however, this is not a requirement. A part may provide the implementation for an interface declared on a different part:

partial class X
{
int IComparable.CompareTo(object o) {...}
}

partial class X: IComparable
{
...
}

Members

With the exception of partial methods (§10.2.7), the set of members of a type declared in multiple parts is simply the union of the set of members declared in each part. The bodies of all parts of the type declaration share the same declaration space (§3.3), and the scope of each member (§3.7) extends to the bodies of all the parts. The accessibility domain of any member always includes all the parts of the enclosing type; a private member declared in one part is freely accessible from another part. It is a compile-time error to declare the same member in more than one part of the type, unless that member is a type with the partial modifier.

partial class A
{
int x; // Error, cannot declare x more than once

partial class Inner // Ok, Inner is a partial type
{
int y;
}
}

partial class A
{
int x; // Error, cannot declare x more than once

partial class Inner // Ok, Inner is a partial type
{
int z;
}
}

The ordering of members within a type is rarely significant to C# code, but may be significant when interfacing with other languages and environments. In these cases, the ordering of members within a type declared in multiple parts is undefined.

Partial methods

Partial methods can be defined in one part of a type declaration and implemented in another. The implementation is optional; if no part implements the partial method, the partial method declaration and all calls to it are removed from the type declaration resulting from the combination of the parts.

Partial methods cannot define access modifiers, but are implicitly private. Their return type must be void, and their parameters cannot have the out modifier. The identifier partial is recognized as a special keyword in a method declaration only if it appears right before the void type; otherwise it can be used as a normal identifier. A partial method cannot explicitly implement interface methods.

There are two kinds of partial method declarations: If the body of the method declaration is a semicolon, the declaration is said to be a defining partial method declaration. If the body is given as a block, the declaration is said to be an implementing partial method declaration. Across the parts of a type declaration there can be only one defining partial method declaration with a given signature, and there can be only one implementing partial method declaration with a given signature. If an implementing partial method declaration is given, a corresponding defining partial method declaration must exist, and the declarations must match as specified in the following:

• The declarations must have the same modifiers (although not necessarily in the same order), method name, number of type parameters and number of parameters.
• Corresponding parameters in the declarations must have the same modifiers (although not necessarily in the same order) and the same types (modulo differences in type parameter names).
• Corresponding type parameters in the declarations must have the same constraints (modulo differences in type parameter names).

An implementing partial method declaration can appear in the same part as the corresponding defining partial method declaration.

Only a defining partial method participates in overload resolution. Thus, whether or not an implementing declaration is given, invocation expressions may resolve to invocations of the partial method. Because a partial method always returns void, such invocation expressions will always be expression statements. Furthermore, because a partial method is implicitly private, such statements will always occur within one of the parts of the type declaration within which the partial method is declared.

If no part of a partial type declaration contains an implementing declaration for a given partial method, any expression statement invoking it is simply removed from the combined type declaration. Thus the invocation expression, including any constituent expressions, has no effect at run-time. The partial method itself is also removed and will not be a member of the combined type declaration.

If an implementing declaration exist for a given partial method, the invocations of the partial methods are retained. The partial method gives rise to a method declaration similar to the implementing partial method declaration except for the following:

• The partial modifier is not included
• The attributes in the resulting method declaration are the combined attributes of the defining and the implementing partial method declaration in unspecified order. Duplicates are not removed.
• The attributes on the parameters of the resulting method declaration are the combined attributes of the corresponding parameters of the defining and the implementing partial method declaration in unspecified order. Duplicates are not removed.

If a defining declaration but not an implementing declaration is given for a partial method M, the following restrictions apply:

• It is a compile-time error to create a delegate to method (§7.6.10.5).
• It is a compile-time error to refer to M inside an anonymous function that is converted to an expression tree type (§6.5.2).
• Expressions occurring as part of an invocation of M do not affect the definite assignment state (§5.3), which can potentially lead to compile-time errors.
• M cannot be the entry point for an application (§3.1).

Partial methods are useful for allowing one part of a type declaration to customize the behavior of another part, e.g., one that is generated by a tool. Consider the following partial class declaration:

partial class Customer
{
string name;

public string Name {

get { return name; }

set {
OnNameChanging(value);
name = value;
OnNameChanged();
}

}

partial void OnNameChanging(string newName);

partial void OnNameChanged();
}

If this class is compiled without any other parts, the defining partial method declarations and their invocations will be removed, and the resulting combined class declaration will be equivalent to the following:

class Customer
{
string name;

public string Name {

get { return name; }

set { name = value; }
}
}

Assume that another part is given, however, which provides implementing declarations of the partial methods:

partial class Customer
{
partial void OnNameChanging(string newName)
{
Console.WriteLine(“Changing “ + name + “ to “ + newName);
}

partial void OnNameChanged()
{
Console.WriteLine(“Changed to “ + name);
}
}

Then the resulting combined class declaration will be equivalent to the following:

class Customer
{
string name;

public string Name {

get { return name; }

set {
OnNameChanging(value);
name = value;
OnNameChanged();
}

}

void OnNameChanging(string newName)
{
Console.WriteLine(“Changing “ + name + “ to “ + newName);
}

void OnNameChanged()
{
Console.WriteLine(“Changed to “ + name);
}
}

Name binding

Although each part of an extensible type must be declared within the same namespace, the parts are typically written within different namespace declarations. Thus, different using directives (§9.4) may be present for each part. When interpreting simple names (§7.5.2) within one part, only the using directives of the namespace declaration(s) enclosing that part are considered. This may result in the same identifier having different meanings in different parts:

namespace N
{
using List = System.Collections.ArrayList;

partial class A
{
List x; // x has type System.Collections.ArrayList
}
}

namespace N
{
using List = Widgets.LinkedList;

partial class A
{
List y; // y has type Widgets.LinkedList
}
}

Class members

The members of a class consist of the members introduced by its class-member-declarations and the members inherited from the direct base class.

class-member-declarations:
class-member-declaration
class-member-declarations class-member-declaration

class-member-declaration:
constant-declaration
field-declaration
method-declaration
property-declaration
event-declaration
indexer-declaration
operator-declaration
constructor-declaration
destructor-declaration
static-constructor-declaration
type-declaration

The members of a class type are divided into the following categories:

· Constants, which represent constant values associated with the class (§10.4).

· Fields, which are the variables of the class (§10.5).

· Methods, which implement the computations and actions that can be performed by the class (§10.6).

· Properties, which define named characteristics and the actions associated with reading and writing those characteristics (§10.7).

· Events, which define notifications that can be generated by the class (§10.8).

· Indexers, which permit instances of the class to be indexed in the same way (syntactically) as arrays (§10.9).

· Operators, which define the expression operators that can be applied to instances of the class (§10.10).

· Instance constructors, which implement the actions required to initialize instances of the class (§10.11)

· Destructors, which implement the actions to be performed before instances of the class are permanently discarded (§10.13).

· Static constructors, which implement the actions required to initialize the class itself (§10.12).

· Types, which represent the types that are local to the class (§10.3.8).

Members that can contain executable code are collectively known as the function members of the class type. The function members of a class type are the methods, properties, events, indexers, operators, instance constructors, destructors, and static constructors of that class type.

A class-declaration creates a new declaration space (§3.3), and the class-member-declarations immediately contained by the class-declaration introduce new members into this declaration space. The following rules apply to class-member-declarations:

· Instance constructors, destructors and static constructors must have the same name as the immediately enclosing class. All other members must have names that differ from the name of the immediately enclosing class.

· The name of a constant, field, property, event, or type must differ from the names of all other members declared in the same class.

· The name of a method must differ from the names of all other non-methods declared in the same class. In addition, the signature (§3.6) of a method must differ from the signatures of all other methods declared in the same class, and two methods declared in the same class may not have signatures that differ solely by ref and out.

· The signature of an instance constructor must differ from the signatures of all other instance constructors declared in the same class, and two constructors declared in the same class may not have signatures that differ solely by ref and out.

· The signature of an indexer must differ from the signatures of all other indexers declared in the same class.

· The signature of an operator must differ from the signatures of all other operators declared in the same class.

The inherited members of a class type (§10.3.3) are not part of the declaration space of a class. Thus, a derived class is allowed to declare a member with the same name or signature as an inherited member (which in effect hides the inherited member).

The instance type

Each class declaration has an associated bound type (§4.4.3), the instance type. For a generic class declaration, the instance type is formed by creating a constructed type (§4.4) from the type declaration, with each of the supplied type arguments being the corresponding type parameter. Since the instance type uses the type parameters, it can only be used where the type parameters are in scope; that is, inside the class declaration. The instance type is the type of this for code written inside the class declaration. For non-generic classes, the instance type is simply the declared class. The following shows several class declarations along with their instance types:

class A<T> // instance type: A<T>
{
class B {} // instance type: A<T>.B

class C<U> {} // instance type: A<T>.C<U>
}

class D {} // instance type: D