对Func，Delegates和Anonymous类型感到困惑

Question

Reading this line of code example from a book:

Func<string,int> returnLength;
returnLength = delegate (string text) { return text.Length; };

Console.WriteLine(returnLength("Hello"));

It says

Func<string,double,int> is equivalent to a delegate type of the form public delegate int SomeDelegate(string arg1, double arg2)

So Func is a delegate? Then what is that delegate we have defined again in the code example? We define a variable from a Func that is like a delegate and then assign it again to another delegate ? I am super confused and can't understand this concept. :(

Can someone explain it?

Answer 1

So Func is a delegate?

Yes, it's defined as follows:

public delegate TResult Func<in T, out TResult>(T arg)

It's described as:

Encapsulates a method that has one parameter and returns a value of the type specified by the TResult parameter.

Read Func Delegate for more.

Then what is that delegate we have defined again in the code example?

That's the anonymous method we create to assign it to the Func -typed variable. You could use named method or lambda expression as well, as long as the input and return types match the Func generic parameters.

We define a variable from a Func that is like a delegate and then assign it again to another delegate?

No, we create a variable typed as Func<string, int> and then create a delegate matching that type and assign it to the variable.

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Additional sources from MSDN:

• Anonymous Functions (C# Programming Guide)
• Anonymous Methods (C# Programming Guide)
• Delegates with Named vs. Anonymous Methods (C# Programming Guide)
• Lambda Expressions (C# Programming Guide)

Answer 2

the same code you could just write

Func<string,int> returnLength = (s) => {return s.Length;}

assigning delegate to a Func is just like assigning a variable to another..

Answer 3

Your code can also be expressed like this:

Func<string,int> returnLength = delegate (string text) { return text.Length; };

Console.WriteLine(returnLength("Hello"));

A func is a delegate.

1. Func<string, int> is the declaration,
2. delegate (string text) { return text.Length; } delegate (string text) { return text.Length; } is the function that you are assigning.
   1. This delegate (function) accepts a string ( string text ) and returns an integer ( return text.Length; ).

This would return 5 as the text.Length of "Hello" is 5. It is not as you are creating 2 delegates in this example, rather you are just assigning one.

Answer 4

Yes a Func is equivalent to a delegate type with a return type. The delegate you have defined is the delegate that is executed when the method is called returning the int type.

This can be shortcutted down to

Func<string, int> returnLength = (text) => { return text.length; };

In this situation the delegate returns a value type of int (Int32)

Now Action is also a delegate type without a return value such as.

Action<string> action = delegate(string text) { text = ""; };

or

Action<string> action = (text) => { text = ""; };

Now anonymous types are much different than a delegate , Action or Func . Now Func & Action were implemented in .Net 3.5 after delegates were introduced.