64 lines
2.3 KiB
Markdown
64 lines
2.3 KiB
Markdown
---
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title: Delegates
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---
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## Delegates
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A C# delegate represents a reference to a method that has a given set
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of parameters and a given return type. When you instantiate the delegate, you can associate it with
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any method that is compatible with the delegate type: has the same amount of parameters, each is of the
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same type and the type of the return value is also the same.
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You can either use an instance method or a static method when you assign it to a delegate.
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Delegate allow you to pass methods as parameters to other methods.
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Delegates are often used to implement callback functions. The most typical example are event handlers: you register
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a method that is to be called whenever a certain event happens (the mouse button is clicked, for example).
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### Short explanation for developers
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Delegates are like function pointers in C type languages like C or C++. However, they are type safe.
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Unlike simple function pointers they
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contain information about the object instance whose method will be called when invoking the delegate, and have
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strict type checks for the arguments and return value of the function.
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## Example
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You declare a delegate similar to how you declare a function, but add the `delegate` keyword. For example:
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```csharp
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public delegate string StringOperation ( string s1, string s2 );
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```
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Any method that takes two `string` arguments and returns `string` can be assigned to a variable of this delegate type.
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After you have created the delegate type, you can use it just like any other type. You can declare a local variable,
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other class members or pass them as parameters to other methods.
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```csharp
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StringOperation a;
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```
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Before invoking the delegate, you will need to assign a value to it. Let's assume we have a concatenation method
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that has the following implementation:
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```csharp
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private string Concatenate ( string one, string two ) {
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return one + " " + two;
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}
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```
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You can then assign this to the delegate variable and invoke it like a function.
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```csharp
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StringOperation op = Concatenate;
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string result = op("Hello", "World");
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Console.WriteLine ( result ); // print "Hello World" to the console
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```
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## More information
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Read more about delegates [here](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/delegates/).
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