20 Delegates

Article
02/07/2024

20.1 General

A delegate declaration defines a class that is derived from the class System.Delegate. A delegate instance encapsulates an invocation list, which is a list of one or more methods, each of which is referred to as a callable entity. For instance methods, a callable entity consists of an instance and a method on that instance. For static methods, a callable entity consists of just a method. Invoking a delegate instance with an appropriate set of arguments causes each of the delegate’s callable entities to be invoked with the given set of arguments.

Note: An interesting and useful property of a delegate instance is that it does not know or care about the classes of the methods it encapsulates; all that matters is that those methods be compatible (§20.4) with the delegate’s type. This makes delegates perfectly suited for “anonymous” invocation. end note

20.2 Delegate declarations

A delegate_declaration is a type_declaration (§14.7) that declares a new delegate type.

delegate_declaration
    : attributes? delegate_modifier* 'delegate' return_type delegate_header
    | attributes? delegate_modifier* 'delegate' ref_kind ref_return_type
      delegate_header
    ;

delegate_header
    : identifier '(' parameter_list? ')' ';'
    | identifier variant_type_parameter_list '(' parameter_list? ')'
      type_parameter_constraints_clause* ';'
    ;
    
delegate_modifier
    : 'new'
    | 'public'
    | 'protected'
    | 'internal'
    | 'private'
    | unsafe_modifier   // unsafe code support
    ;

unsafe_modifier is defined in §23.2.

It is a compile-time error for the same modifier to appear multiple times in a delegate declaration.

A delegate declaration that supplies a variant_type_parameter_list is a generic delegate declaration. Additionally, any delegate nested inside a generic class declaration or a generic struct declaration is itself a generic delegate declaration, since type arguments for the containing type shall be supplied to create a constructed type (§8.4).

The new modifier is only permitted on delegates declared within another type, in which case it specifies that such a delegate hides an inherited member by the same name, as described in §15.3.5.

The public, protected, internal, and private modifiers control the accessibility of the delegate type. Depending on the context in which the delegate declaration occurs, some of these modifiers might not be permitted (§7.5.2).

The delegate’s type name is identifier.

As with methods (§15.6.1), if ref is present, the delegate returns-by-ref; otherwise, if return_type is void, the delegate returns-no-value; otherwise, the delegate returns-by-value.

The optional parameter_list specifies the parameters of the delegate.

The return_type of a returns-by-value or returns-no-value delegate declaration specifies the type of the result, if any, returned by the delegate.

The ref_return_type of a returns-by-ref delegate declaration specifies the type of the variable referenced by the variable_reference (§9.5) returned by the delegate.

The optional variant_type_parameter_list (§18.2.3) specifies the type parameters to the delegate itself.

The return type of a delegate type shall be either void, or output-safe (§18.2.3.2).

All the parameter types of a delegate type shall be input-safe (§18.2.3.2). In addition, any output or reference parameter types shall also be output-safe.

Note: Output parameters are required to be input-safe due to common implementation restrictions. end note

Furthermore, each class type constraint, interface type constraint and type parameter constraint on any type parameters of the delegate shall be input-safe.

Delegate types in C# are name equivalent, not structurally equivalent.

Example:
delegate int D1(int i, double d);
delegate int D2(int c, double d);
The delegate types D1 and D2 are two different types, so they are not interchangeable, despite their identical signatures.

end example

Like other generic type declarations, type arguments shall be given to create a constructed delegate type. The parameter types and return type of a constructed delegate type are created by substituting, for each type parameter in the delegate declaration, the corresponding type argument of the constructed delegate type.

The only way to declare a delegate type is via a delegate_declaration. Every delegate type is a reference type that is derived from System.Delegate. The members required for every delegate type are detailed in §20.3. Delegate types are implicitly sealed, so it is not permissible to derive any type from a delegate type. It is also not permissible to declare a non-delegate class type deriving from System.Delegate. System.Delegate is not itself a delegate type; it is a class type from which all delegate types are derived.

20.3 Delegate members

Every delegate type inherits members from the Delegate class as described in §15.3.4. In addition, every delegate type shall provide a non-generic Invoke method whose parameter list matches the parameter_list in the delegate declaration, whose return type matches the return_type or ref_return_type in the delegate declaration, and for returns-by-ref delegates whose ref_kind matches that in the delegate declaration. The Invoke method shall be at least as accessible as the containing delegate type. Calling the Invoke method on a delegate type is semantically equivalent to using the delegate invocation syntax (§20.6) .

Implementations may define additional members in the delegate type.

Except for instantiation, any operation that can be applied to a class or class instance can also be applied to a delegate class or instance, respectively. In particular, it is possible to access members of the System.Delegate type via the usual member access syntax.

20.4 Delegate compatibility

A method or delegate type M is compatible with a delegate type D if all of the following are true:

D and M have the same number of parameters, and each parameter in D has the same by-reference parameter modifier as the corresponding parameter in M.
For each value parameter, an identity conversion (§10.2.2) or implicit reference conversion (§10.2.8) exists from the parameter type in D to the corresponding parameter type in M.
For each by-reference parameter, the parameter type in D is the same as the parameter type in M.
One of the following is true:
- D and M are both returns-no-value
- D and M are returns-by-value (§15.6.1, §20.2), and an identity or implicit reference conversion exists from the return type of M to the return type of D.
- D and M are both returns-by-ref, an identity conversion exists between the return type of M and the return type of D, and both have the same ref_kind.

This definition of compatibility allows covariance in return type and contravariance in parameter types.

Example:
delegate int D1(int i, double d);
delegate int D2(int c, double d);
delegate object D3(string s);

class A
{
    public static int M1(int a, double b) {...}
}

class B
{
    public static int M1(int f, double g) {...}
    public static void M2(int k, double l) {...}
    public static int M3(int g) {...}
    public static void M4(int g) {...}
    public static object M5(string s) {...}
    public static int[] M6(object o) {...}
}
The methods A.M1 and B.M1 are compatible with both the delegate types D1 and D2, since they have the same return type and parameter list. The methods B.M2, B.M3, and B.M4 are incompatible with the delegate types D1 and D2, since they have different return types or parameter lists. The methods B.M5 and B.M6 are both compatible with delegate type D3.

end example

Example:
delegate bool Predicate<T>(T value);

class X
{
    static bool F(int i) {...}
    static bool G(string s) {...}
}
The method X.F is compatible with the delegate type Predicate<int> and the method X.G is compatible with the delegate type Predicate<string>.

end example

Note: The intuitive meaning of delegate compatibility is that a method is compatible with a delegate type if every invocation of the delegate could be replaced with an invocation of the method without violating type safety, treating optional parameters and parameter arrays as explicit parameters. For example, in the following code:
delegate void Action<T>(T arg);

class Test
{
    static void Print(object value) => Console.WriteLine(value);

    static void Main()
    {
        Action<string> log = Print;
        log("text");
    }
}
The Print method is compatible with the Action<string> delegate type because any invocation of an Action<string> delegate would also be a valid invocation of the Print method.

If the signature of the Print method above were changed to Print(object value, bool prependTimestamp = false) for example, the Print method would no longer be compatible with Action<string> by the rules of this clause.

end note

20.5 Delegate instantiation

An instance of a delegate is created by a delegate_creation_expression (§12.8.17.6), a conversion to a delegate type, delegate combination or delegate removal. The newly created delegate instance then refers to one or more of:

The static method referenced in the delegate_creation_expression, or
The target object (which cannot be null) and instance method referenced in the delegate_creation_expression, or
Another delegate (§12.8.17.6).

Example:

delegate void D(int x);

class C
{
    public static void M1(int i) {...}
    public void M2(int i) {...}
}

class Test
{
    static void Main()
    {
        D cd1 = new D(C.M1); // Static method
        C t = new C();
        D cd2 = new D(t.M2); // Instance method
        D cd3 = new D(cd2);  // Another delegate
    }
}

end example

The set of methods encapsulated by a delegate instance is called an invocation list. When a delegate instance is created from a single method, it encapsulates that method, and its invocation list contains only one entry. However, when two non-null delegate instances are combined, their invocation lists are concatenated—in the order left operand then right operand—to form a new invocation list, which contains two or more entries.

When a new delegate is created from a single delegate the resultant invocation list has just one entry, which is the source delegate (§12.8.17.6).

Delegates are combined using the binary + (§12.10.5) and += operators (§12.21.4). A delegate can be removed from a combination of delegates, using the binary - (§12.10.6) and -= operators (§12.21.4). Delegates can be compared for equality (§12.12.9).

Example: The following example shows the instantiation of a number of delegates, and their corresponding invocation lists:
delegate void D(int x);

class C
{
    public static void M1(int i) {...}
    public static void M2(int i) {...}
}

class Test
{
    static void Main() 
    {
        D cd1 = new D(C.M1); // M1 - one entry in invocation list
        D cd2 = new D(C.M2); // M2 - one entry
        D cd3 = cd1 + cd2;   // M1 + M2 - two entries
        D cd4 = cd3 + cd1;   // M1 + M2 + M1 - three entries
        D cd5 = cd4 + cd3;   // M1 + M2 + M1 + M1 + M2 - five entries
        D td3 = new D(cd3);  // [M1 + M2] - ONE entry in invocation
                             // list, which is itself a list of two methods.
        D td4 = td3 + cd1;   // [M1 + M2] + M1 - two entries
        D cd6 = cd4 - cd2;   // M1 + M1 - two entries in invocation list
        D td6 = td4 - cd2;   // [M1 + M2] + M1 - two entries in invocation list,
                             // but still three methods called, M2 not removed.
   }
}
When cd1 and cd2 are instantiated, they each encapsulate one method. When cd3 is instantiated, it has an invocation list of two methods, M1 and M2, in that order. cd4’s invocation list contains M1, M2, and M1, in that order. For cd5, the invocation list contains M1, M2, M1, M1, and M2, in that order.

When creating a delegate from another delegate with a delegate_creation_expression the result has an invocation list with a different structure from the original, but which results in the same methods being invoked in the same order. When td3 is created from cd3 its invocation list has just one member, but that member is a list of the methods M1 and M2 and those methods are invoked by td3 in the same order as they are invoked by cd3. Similarly when td4 is instantiated its invocation list has just two entries but it invokes the three methods M1, M2, and M1, in that order just as cd4 does.

The structure of the invocation list affects delegate subtraction. Delegate cd6, created by subtracting cd2 (which invokes M2) from cd4 (which invokes M1, M2, and M1) invokes M1 and M1. However delegate td6, created by subtracting cd2 (which invokes M2) from td4 (which invokes M1, M2, and M1) still invokes M1, M2 and M1, in that order, as M2 is not a single entry in the list but a member of a nested list. For more examples of combining (as well as removing) delegates, see §20.6.

end example

Once instantiated, a delegate instance always refers to the same invocation list.

Note: Remember, when two delegates are combined, or one is removed from another, a new delegate results with its own invocation list; the invocation lists of the delegates combined or removed remain unchanged. end note

20.6 Delegate invocation

C# provides special syntax for invoking a delegate. When a non-null delegate instance whose invocation list contains one entry, is invoked, it invokes the one method with the same arguments it was given, and returns the same value as the referred to method. (See §12.8.10.4 for detailed information on delegate invocation.) If an exception occurs during the invocation of such a delegate, and that exception is not caught within the method that was invoked, the search for an exception catch clause continues in the method that called the delegate, as if that method had directly called the method to which that delegate referred.

Invocation of a delegate instance whose invocation list contains multiple entries, proceeds by invoking each of the methods in the invocation list, synchronously, in order. Each method so called is passed the same set of arguments as was given to the delegate instance. If such a delegate invocation includes reference parameters (§15.6.2.3.3), each method invocation will occur with a reference to the same variable; changes to that variable by one method in the invocation list will be visible to methods further down the invocation list. If the delegate invocation includes output parameters or a return value, their final value will come from the invocation of the last delegate in the list. If an exception occurs during processing of the invocation of such a delegate, and that exception is not caught within the method that was invoked, the search for an exception catch clause continues in the method that called the delegate, and any methods further down the invocation list are not invoked.

Attempting to invoke a delegate instance whose value is null results in an exception of type System.NullReferenceException.

Example: The following example shows how to instantiate, combine, remove, and invoke delegates:
delegate void D(int x);

class C
{
    public static void M1(int i) => Console.WriteLine("C.M1: " + i);

    public static void M2(int i) => Console.WriteLine("C.M2: " + i);

    public void M3(int i) => Console.WriteLine("C.M3: " + i);
}

class Test
{
    static void Main()
    {
        D cd1 = new D(C.M1);
        cd1(-1);             // call M1
        D cd2 = new D(C.M2);
        cd2(-2);             // call M2
        D cd3 = cd1 + cd2;
        cd3(10);             // call M1 then M2
        cd3 += cd1;
        cd3(20);             // call M1, M2, then M1
        C c = new C();
        D cd4 = new D(c.M3);
        cd3 += cd4;
        cd3(30);             // call M1, M2, M1, then M3
        cd3 -= cd1;          // remove last M1
        cd3(40);             // call M1, M2, then M3
        cd3 -= cd4;
        cd3(50);             // call M1 then M2
        cd3 -= cd2;
        cd3(60);             // call M1
        cd3 -= cd2;          // impossible removal is benign
        cd3(60);             // call M1
        cd3 -= cd1;          // invocation list is empty so cd3 is null
        // cd3(70);          // System.NullReferenceException thrown
        cd3 -= cd1;          // impossible removal is benign
    }
}
As shown in the statement cd3 += cd1;, a delegate can be present in an invocation list multiple times. In this case, it is simply invoked once per occurrence. In an invocation list such as this, when that delegate is removed, the last occurrence in the invocation list is the one actually removed.

Immediately prior to the execution of the final statement, cd3 -= cd1;, the delegate cd3 refers to an empty invocation list. Attempting to remove a delegate from an empty list (or to remove a non-existent delegate from a non-empty list) is not an error.

The output produced is:
C.M1: -1
C.M2: -2
C.M1: 10
C.M2: 10
C.M1: 20
C.M2: 20
C.M1: 20
C.M1: 30
C.M2: 30
C.M1: 30
C.M3: 30
C.M1: 40
C.M2: 40
C.M3: 40
C.M1: 50
C.M2: 50
C.M1: 60
C.M1: 60
end example

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