Strongly Typed Delegates


In the previous article, you saw that you create specific delegate types using the delegate keyword.

The abstract Delegate class provides the infrastructure for loose coupling and invocation. Concrete Delegate types become much more useful by embracing and enforcing type safety for the methods that are added to the invocation list for a delegate object. When you use the delegate keyword and define a concrete delegate type, the compiler generates those methods.

In practice, this would lead to creating new delegate types whenever you need a different method signature. This work could get tedious after a time. Every new feature requires new delegate types.

Thankfully, this isn't necessary. The .NET Core framework contains several types that you can reuse whenever you need delegate types. These are generic definitions so you can declare customizations when you need new method declarations.

The first of these types is the Action type, and several variations:

public delegate void Action();
public delegate void Action<in T>(T arg);
public delegate void Action<in T1, in T2>(T1 arg1, T2 arg2);
// Other variations removed for brevity.

The in modifier on the generic type argument is covered in the article on covariance.

There are variations of the Action delegate that contain up to 16 arguments such as Action<T1,T2,T3,T4,T5,T6,T7,T8,T9,T10,T11,T12,T13,T14,T15,T16>. It's important that these definitions use different generic arguments for each of the delegate arguments: That gives you maximum flexibility. The method arguments need not be, but may be, the same type.

Use one of the Action types for any delegate type that has a void return type.

The framework also includes several generic delegate types that you can use for delegate types that return values:

public delegate TResult Func<out TResult>();
public delegate TResult Func<in T1, out TResult>(T1 arg);
public delegate TResult Func<in T1, in T2, out TResult>(T1 arg1, T2 arg2);
// Other variations removed for brevity

The out modifier on the result generic type argument is covered in the article on covariance.

There are variations of the Func delegate with up to 16 input arguments such as Func<T1,T2,T3,T4,T5,T6,T7,T8,T9,T10,T11,T12,T13,T14,T15,T16,TResult>. The type of the result is always the last type parameter in all the Func declarations, by convention.

Use one of the Func types for any delegate type that returns a value.

There's also a specialized Predicate<T> type for a delegate that returns a test on a single value:

public delegate bool Predicate<in T>(T obj);

You may notice that for any Predicate type, a structurally equivalent Func type exists For example:

Func<string, bool> TestForString;
Predicate<string> AnotherTestForString;

You might think these two types are equivalent. They are not. These two variables cannot be used interchangeably. A variable of one type cannot be assigned the other type. The C# type system uses the names of the defined types, not the structure.

All these delegate type definitions in the .NET Core Library should mean that you do not need to define a new delegate type for any new feature you create that requires delegates. These generic definitions should provide all the delegate types you need under most situations. You can simply instantiate one of these types with the required type parameters. In the case of algorithms that can be made generic, these delegates can be used as generic types.

This should save time, and minimize the number of new types that you need to create in order to work with delegates.

In the next article, you'll see several common patterns for working with delegates in practice.