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Delegates are instantiated using a single parameter: the name of a method that matches the template specified in the delegates declaration. Existing delegate instances can also be assigned to another instance of the same delegate type, in which case a copy of the delegate invocation list will be assigned to the target delegate. Delegates can be added together using the + and += operators. This results in the target delegate having the combined invocation list of the two operand delegates. Method references
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5. Data Types
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are maintained in the order in which they are added. Adding the same method twice results in two references to the same method being maintained on the invocation list. Removing method references from a delegate instance is achieved using the - or -= operator. Where there are multiple references to the method, only the last instance will be removed. Attempting to remove a method that is not on the invocation list is not an error. Continuing from the foregoing example:
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//Instantiating and modifying delegates MyDelegate d1 = new MyDelegate(Method1); MyDelegate d2 = new MyDelegate(Method2); MyDelegate d3 = d1 + d2; d3 += new MyDelegate(Method3); d3 -= d1; d3 -= new MyDelegate(Method3);
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A delegate is concerned only with the signature and return type of a method, not the type that implements the method; any instance or static method that matches the template can be used with a delegate.
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Invoking a delegate
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Invoking a delegate is achieved using the delegate instance as if it were a method with the return type and parameters specified in the delegate declaration. For example:
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//Invoking a delegate int i = d3(6, "Test");
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This will cause each method on the invocation list to be called in sequence using the parameters provided. If passing an object reference or using a ref parameter, subsequent methods will see any changes made to the arguments by previous methods. The return value will be the value returned from the last method in the invocation list.
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Modifiers
The applicability of modifiers to a delegate declaration depends on the context in which the delegate is declared. Delegates can be declared as a top-level type, being direct members of an enclosing namespace, or they can be nested within the definition of a class or struct. Table 5-9 summarizes modifier availability.
Table 5-9. Delegate Declaration Modifier Availability
Delegate Declaration Context Member of Namespace Accessibility public protected private internal
Member of Class Member of Struct
N/A N/A (default)
(default)
N/A (default)
5. Data Types
protected internal Inheritance new abstract sealed virtual override Other readonly volatile static extern
Events
N/A N/A N/A (implicit) N/A N/A N/A N/A N/A N/A
N/A N/A N/A (implicit) N/A N/A N/A N/A N/A N/A
N/A (implicit) N/A N/A N/A N/A N/A N/A
In the .NET Framework, delegates are most frequently used in conjunction with event members. For a complete description of events, see the "Events" section later in this chapter.
Conversion
Converting instances of one type or value to another can be implicit or explicit. The compiler handles implicit conversions automatically; the programmer need take no action. Implicit conversions can occur when conversion of one type to another will not cause loss of information. When no implicit conversion exists between two types, explicit conversion is used. The programmer forces explicit conversion using casts. If a cast is not specified when an explicit cast is required, a compiler error will occur. The .NET class library includes the System.Convert utility class to convert between different types. This includes the conversion between string, Boolean, date, and value types.
Implicit Conversion
Different data types support different implicit conversions. We discuss these in the following sections.
Implicit numeric conversion
For the simple numeric types, the implicit conversions described in Table 5-10 are possible.
Table 5-10. Supported Implicit Numeric Conversions
From Type sbyte byte short
To Types short, int, long, float, double, or decimal short, ushort, int, uint, long, ulong, float, double, or decimal int, long, float, double, or decimal
5. Data Types
ushort int uint long or ulong char float
int, uint, long, ulong, float, double, or decimal long, float, double, or decimal long, ulong, float, double, or decimal float, double, or decimal ushort, int, uint, long, ulong, float, double, or decimal double
Implicit enumeration conversion
The integer literal 0 (zero) implicitly converts to any enumeration type. Function members should accommodate the fact that 0 is valid where an enum type is expected.
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