Related
By just briefing a book (leading one) , one thing caught my eyes - the lambda expression definition :
A lambda expression is an unnamed method written in place of a
delegate instance.
in place of delegate instance ???
A delegate instance is an object that refers-to/encapsulate target method/s :
In the following sample The right side(where the lambda expression would be) is not a delegate instance. it is a method.
TransformerDelegate t = SquareMethod;
So the definition should have been corrected-to/mention :
lambda expression are unnamed method written in place of a method(!)
being referenced by delegate variable.
TransformerDelegate sqr = x => x * x;
^
|
---------------+
|
this is the location for method/anonymous methods.
do you see what I mean ? Am I right?
p.s. I did understand the msdn's one : ( but want to see if the book had made a mistake)
A lambda expression is an anonymous function that can contain
expressions and statements, and can be used to create delegates or
expression tree types.
The value of a lambda expression is a delegate instance.
So the book is probably referring to code like:
MySquareDelegate f1 = x => x * x;
MySquareDelegate f2 = new MySquareDelegate(MySquareMethod);
MySquareDelegate f3 = MySquareMethod; // just shorthand for the previous line
It is tricky stuff to explain in 1 sentence, your own version
lambda expression are unnamed method written in place of a method(!) being referenced by delegate variable.
is talking about "a method instead of a method", the original about "a method instead of a delegate instance" where the method is implicitly converted to a delegate instance. Both seem incomplete at least.
A definition of a lambda should also include that it is an inline method.
Other answers miss the fact that lambda expressions do not necessarily represent methods. Sometimes, they represent expression trees.
The compiler implicitly converts lambda expressions to one type of object or the other, depending on context.
To specify a method, you need to specify parameters and a body. In a lambda expression, these are separated by =>. Examples:
() => 4; //empty parameter list
x => x.ToString(); //one parameter
(a, b) => a.Equals(b); //two parameters
// ^^^^^^ ^^^^^^^^^^^^
// | |
// parameter list body
These lambda expressions can be converted to Func<int>, Func<object, string>, and Func<object, object, bool> respectively. The could also be converted to Expression<Func<int>>, Expression<Func<object, string>>, and Expression<Func<object, object, bool>> respectively.
An anonymous method:
delegate (object p, object q) { return string.Concat(p, q); }
// ^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
// parameter list body
Here are two examples of lambda conversion:
Func<object, object, bool> aDelegate = (o1, o2) => object.Equals(o1, o2);
Expression<Func<object, object, bool>> anExpressionTree = (o1, o2) => object.Equals(o1, o2);
In method group conversion, the parameters and method body are specified by overload resolution. To simplify a bit, the compiler looks at the methods with the indicated name and chooses the correct overload based on the context. For example, consider these overloads of SquareMethod:
int SquareMethod(int a) { return a * a; }
double SquareMethod(double a) { return a * a; }
These statements involve method group conversion and overload resolution:
Func<int, int> squareAnInt = SquareMethod;
Func<double, double> squareADouble = SquareMethod;
Finally, statement lambdas cannot be translated to expression trees:
Action<object> anAction = o => { Console.WriteLine(o); };
Func<object, int> aFunc = o =>
{
var s = (o ?? "").ToString();
Console.WriteLine(s);
return s.Length;
};
The C# language specification (somewhat confusingly) uses the term "anonymous function" to cover both lambda expressions and anonymous methods. Anonymous functions can be implicitly converted to a compatible delegate type, and so can method groups. Therefore, if we have a delegate type called DelegateType, and a declaration/assignment like this:
DelegateType d = [something];
Then [something] can be a method group or an anonymous function. In other words, it can be a method group, an anonymous method or a lambda expression.
So, your correction to the book's text would be better to say "in place of a method group", but I would say
A lambda expression is an unnamed method that, like a named method group, can be used to create a delegate instance.
I might also add
In some cases, a lambda expression can be used to create an expression tree rather than a delegate instance.
Basically a delegate is nothing but an object that knows how to call a specific method. So a delegate instance literally acts as a delegate for the caller: the caller invokes the delegate, and then the delegate calls the target method.
Delegate is clear to understand
delegate int del(int i);
but why can we use
del myDelegate = x => x * x;
Questions here:
How can I assure x is int type in the lambda expresison?
How can I know lambda expression return a int?
C# compiler is smart enough to implicitly figure out the type of x on the left side of => from the context. Since it knows that you are assigning the lambda to a variable of type del, the compiler knows that x is an int.
As far as the return type goes, the compiler knows that x is an int, therefore the type of the x * x expression must also be int. That's how the compiler knows the return type of the lambda.
Note that the same code would not have compiled without the exact type of myDelegate specified:
// This does not compile!
var myDelegate = x => x*x;
Because for you to have x, you had to give it a type. It will detect this type rather than making you tell it over and over what type it is.
Right now you're letting the compiler determine what the lamdba expression is doing. You can sublclass using Action and Func
Actions return type is void while Func's return type is whatever you specify as the last type parameter. You can specify additional inputs in both
Func<in, in2, in3, output>
For returning an int
var f = new Func<int, int>(x => x*x);
If I have two yield return methods with the same signature, the compiler does not seem to be recognizing them to be similar.
I have two yield return methods like this:
public static IEnumerable<int> OddNumbers(int N)
{
for (int i = 0; i < N; i++)
if (i % 2 == 1) yield return i;
}
public static IEnumerable<int> EvenNumbers(int N)
{
for (int i = 0; i < N; i++)
if (i % 2 == 0) yield return i;
}
With this, I would expect the following statement to compile fine:
Func<int, IEnumerable<int>> generator = 1 == 0 ? EvenNumbers : OddNumbers; // Does not compile
I get the error message
Type of conditional expression cannot be determined because there is
no implicit conversion between 'method group' and 'method group'
However, an explicit cast works:
Func<int, IEnumerable<int>> newGen = 1 == 0 ? (Func<int, IEnumerable<int>>)EvenNumbers : (Func<int, IEnumerable<int>>)OddNumbers; // Works fine
Am I missing anything or Is this a bug in the C# compiler (I'm using VS2010SP1)?
Note: I have read this and still believe that the first one should've compiled fine.
EDIT: Removed the usage of var in the code snippets as that wasn't what I intended to ask.
No. It is not a bug. It has nothing with yield. The thing is that expression type method group can be converted to delegate type only when it is assigned directly like: SomeDel d = SomeMeth.
C# 3.0 specification:
§6.6 Method group conversions
An implicit conversion (§6.1) exists from a method group (§7.1) to a
compatible delegate type.
This is the only implicit conversion possible with method groups.
How ternary operator is evaluated in terms of types inferences:
A ? B : C:
Make sure that either B or C can be implicitly cast to one another's type. For example A ? 5 : 6.0 will be double because 5 can be implicitly cast to double. Type of A and B in this case is method group and there is no conversion between method group. Only to delegate and it can be enforced as you did.
There are many possible delegate types that could match the signature of the EvenNumbers and OddNumbers methods. For example:
Func<int, IEnumerable<int>>
Func<int, IEnumerable>
Func<int, object>
any number of custom delegate types
The compiler won't try to guess which compatible delegate type you're expecting. You need to be explicit and tell it -- with a cast in your example -- exactly which delegate type you want to use.
Well even
var gen = OddNumbers;
does not work. So you can't expect ternary operator to work.
I guess var can't infer a delegate type.
The yield Return has nothing to do with this.
You are not setting generator to an IEnumerable<int>, you are setting it to a MethodGroup, i.e. a function without the brackets to make the call.
The second statement casts the MethodGroups to Delegates which can be compared.
Perhaps you mean to do somthing like but,
var generator = 1 == 0 ? EvenNumbers(1) : OddNumbers(1);
I couldn't say for sure.
it doesn't have anything to do with iterators, the same code fails to compile if the methods are simple functions. The compiler is reluctant to automatically convert a method to a delegate object, forgetting to use the () in a method call is too common a mistake. You have to do it explicitly.
Rollup of what works and does not:
Does not work:
var generator = 1 == 0 ? EvenNumbers : OddNumbers;
Func<int, IEnumerable<int>> generator = 1 == 0 ? EvenNumbers : OddNumbers;
Does work:
var generator = 1 == 0 ? (Func<int, IEnumerable<int>>)EvenNumbers : OddNumbers;
If it was anything to do with yield or var the latter should also fail.
My guess is a problem with the ternary operator.
The problem is that the statement
var gen = OddNumbers;
Can be interpreted both as
Func<int, IEnumerable<int>> gen = OddNumbers;
and
Expression<Func<int, IEnumerable<int>> gen = OddNumbers;
The compiler can't decide that, so you have to do this.
A method (method group) does not have an intrinsic type, only delegates do. Which is why the ternary operator cannot infer a type to return, and thus is why you have to cast one or the other return value as the type you want to return.
I have the following code:
Func<string, bool> comparer = delegate(string value) {
return value != "0";
};
However, the following does not compile:
var comparer = delegate(string value) {
return value != "0";
};
Why can't the compiler figure out it is a Func<string, bool>? It takes one string parameter, and returns a boolean. Instead, it gives me the error:
Cannot assign anonymous method to an
implicitly-typed local variable.
I have one guess and that is if the var version compiled, it would lack consistency if I had the following:
var comparer = delegate(string arg1, string arg2, string arg3, string arg4, string arg5) {
return false;
};
The above wouldn't make sense since Func<> allows only up to 4 arguments (in .NET 3.5, which is what I am using). Perhaps someone could clarify the problem. Thanks.
UPDATE: This answer was written over ten years ago and should be considered to be of historical interest; in C# 10 the compiler will infer some delegate types.
Others have already pointed out that there are infinitely many possible delegate types that you could have meant; what is so special about Func that it deserves to be the default instead of Predicate or Action or any other possibility? And, for lambdas, why is it obvious that the intention is to choose the delegate form, rather than the expression tree form?
But we could say that Func is special, and that the inferred type of a lambda or anonymous method is Func of something. We'd still have all kinds of problems. What types would you like to be inferred for the following cases?
var x1 = (ref int y)=>123;
There is no Func<T> type that takes a ref anything.
var x2 = y=>123;
We don't know the type of the formal parameter, though we do know the return. (Or do we? Is the return int? long? short? byte?)
var x3 = (int y)=>null;
We don't know the return type, but it can't be void. The return type could be any reference type or any nullable value type.
var x4 = (int y)=>{ throw new Exception(); }
Again, we don't know the return type, and this time it can be void.
var x5 = (int y)=> q += y;
Is that intended to be a void-returning statement lambda or something that returns the value that was assigned to q? Both are legal; which should we choose?
Now, you might say, well, just don't support any of those features. Just support "normal" cases where the types can be worked out. That doesn't help. How does that make my life easier? If the feature works sometimes and fails sometimes then I still have to write the code to detect all of those failure situations and give a meaningful error message for each. We still have to specify all that behaviour, document it, write tests for it, and so on. This is a very expensive feature that saves the user maybe half a dozen keystrokes. We have better ways to add value to the language than spending a lot of time writing test cases for a feature that doesn't work half the time and doesn't provide hardly any benefit in cases where it does work.
The situation where it is actually useful is:
var xAnon = (int y)=>new { Y = y };
because there is no "speakable" type for that thing. But we have this problem all the time, and we just use method type inference to deduce the type:
Func<A, R> WorkItOut<A, R>(Func<A, R> f) { return f; }
...
var xAnon = WorkItOut((int y)=>new { Y = y });
and now method type inference works out what the func type is.
Only Eric Lippert knows for sure, but I think it's because the signature of the delegate type doesn't uniquely determine the type.
Consider your example:
var comparer = delegate(string value) { return value != "0"; };
Here are two possible inferences for what the var should be:
Predicate<string> comparer = delegate(string value) { return value != "0"; }; // okay
Func<string, bool> comparer = delegate(string value) { return value != "0"; }; // also okay
Which one should the compiler infer? There's no good reason to choose one or the other. And although a Predicate<T> is functionally equivalent to a Func<T, bool>, they are still different types at the level of the .NET type system. The compiler therefore cannot unambiguously resolve the delegate type, and must fail the type inference.
Eric Lippert has an old post about it where he says
And in fact the C# 2.0 specification
calls this out. Method group
expressions and anonymous method
expressions are typeless expressions
in C# 2.0, and lambda expressions join
them in C# 3.0. Therefore it is
illegal for them to appear "naked" on
the right hand side of an implicit
declaration.
Different delegates are considered different types. e.g., Action is different than MethodInvoker, and an instance of Action can't be assigned to a variable of type MethodInvoker.
So, given an anonymous delegate (or lambda) like () => {}, is it an Action or a MethodInvoker? The compiler can't tell.
Similarly, if I declare a delegate type taking a string argument and returning a bool, how would the compiler know you really wanted a Func<string, bool> instead of my delegate type? It can't infer the delegate type.
The following points are from the MSDN regarding Implicitly Typed Local Variables:
var can only be used when a local variable is declared and initialized in the same statement; the variable cannot be initialized to null, or to a method group or an anonymous function.
The var keyword instructs the compiler to infer the type of the variable from the expression on the right side of the initialization statement.
It is important to understand that the var keyword does not mean "variant" and does not indicate that the variable is loosely typed, or late-bound. It just means that the compiler determines and assigns the most appropriate type.
MSDN Reference: Implicitly Typed Local Variables
Considering the following regarding Anonymous Methods:
Anonymous methods enable you to omit the parameter list.
MSDN Reference: Anonymous Methods
I would suspect that since the anonymous method may actually have different method signatures, the compiler is unable to properly infer what the most appropriate type to assign would be.
My post doesn't answer the actual question, but it does answer the underlying question of :
"How do I avoid having to type out some fugly type like Func<string, string, int, CustomInputType, bool, ReturnType>?" [1]
Being the lazy/hacky programmer that I am, I experimented with using Func<dynamic, object> - which takes a single input parameter and returns an object.
For multiple arguments, you can use it like so:
dynamic myParams = new ExpandoObject();
myParams.arg0 = "whatever";
myParams.arg1 = 3;
Func<dynamic, object> y = (dynObj) =>
{
return dynObj.arg0.ToUpper() + (dynObj.arg1 * 45); //screw type casting, amirite?
};
Console.WriteLine(y(myParams));
Tip: You can use Action<dynamic> if you don't need to return an object.
Yeah I know it probably goes against your programming principles, but this makes sense to me and probably some Python coders.
I'm pretty novice at delegates... just wanted to share what I learned.
[1] This assumes that you aren't calling a method that requires a predefined Func as a parameter, in which case, you'll have to type that fugly string :/
Other answers were correct at the time they were written, but starting from C# 10.0 (from 2021), the compiler can infer a suitable delegate type (like some Func<...>, Action<...> or generated delegate type) in such cases.
See C# 10 Features - Lambda improvements.
var comparer = delegate(string value) {
return value != "0";
}; // OK in C# 10.0, picks 'Func<string, bool>' in this case
Of course the more usual syntax is to us =>, so:
var comparer = (string value) => {
return value != "0";
}; // OK in C# 10.0, picks 'Func<string, bool>' in this case
How is about that?
var item = new
{
toolisn = 100,
LangId = "ENG",
toolPath = (Func<int, string, string>) delegate(int toolisn, string LangId)
{
var path = "/Content/Tool_" + toolisn + "_" + LangId + "/story.html";
return File.Exists(Server.MapPath(path)) ? "<a style=\"vertical-align:super\" href=\"" + path + "\" target=\"_blank\">execute example</a> " : "";
}
};
string result = item.toolPath(item.toolisn, item.LangId);
I get asked this question a lot and I thought I'd solicit some input on how to best describe the difference.
They are actually two very different things. "Delegate" is actually the name for a variable that holds a reference to a method or a lambda, and a lambda is a method without a permanent name.
Lambdas are very much like other methods, except for a couple subtle differences.
A normal method is defined in a "statement" and tied to a permanent name, whereas a lambda is defined "on the fly" in an "expression" and has no permanent name.
Some lambdas can be used with .NET expression trees, whereas methods cannot.
A delegate is defined like this:
delegate Int32 BinaryIntOp(Int32 x, Int32 y);
A variable of type BinaryIntOp can have either a method or a labmda assigned to it, as long as the signature is the same: two Int32 arguments, and an Int32 return.
A lambda might be defined like this:
BinaryIntOp sumOfSquares = (a, b) => a*a + b*b;
Another thing to note is that although the generic Func and Action types are often considered "lambda types", they are just like any other delegates. The nice thing about them is that they essentially define a name for any type of delegate you might need (up to 4 parameters, though you can certainly add more of your own). So if you are using a wide variety of delegate types, but none more than once, you can avoid cluttering your code with delegate declarations by using Func and Action.
Here is an illustration of how Func and Action are "not just for lambdas":
Int32 DiffOfSquares(Int32 x, Int32 y)
{
return x*x - y*y;
}
Func<Int32, Int32, Int32> funcPtr = DiffOfSquares;
Another useful thing to know is that delegate types (not methods themselves) with the same signature but different names will not be implicitly casted to each other. This includes the Func and Action delegates. However if the signature is identical, you can explicitly cast between them.
Going the extra mile.... In C# functions are flexible, with the use of lambdas and delegates. But C# does not have "first-class functions". You can use a function's name assigned to a delegate variable to essentially create an object representing that function. But it's really a compiler trick. If you start a statement by writing the function name followed by a dot (i.e. try to do member access on the function itself) you'll find there are no members there to reference. Not even the ones from Object. This prevents the programmer from doing useful (and potentially dangerous of course) things such as adding extension methods that can be called on any function. The best you can do is extend the Delegate class itself, which is surely also useful, but not quite as much.
Update: Also see Karg's answer illustrating the difference between anonymous delegates vs. methods & lambdas.
Update 2: James Hart makes an important, though very technical, note that lambdas and delegates are not .NET entities (i.e. the CLR has no concept of a delegate or lambda), but rather they are framework and language constructs.
The question is a little ambiguous, which explains the wide disparity in answers you're getting.
You actually asked what the difference is between lambdas and delegates in the .NET framework; that might be one of a number of things. Are you asking:
What is the difference between lambda expressions and anonymous delegates in the C# (or VB.NET) language?
What is the difference between System.Linq.Expressions.LambdaExpression objects and System.Delegate objects in .NET 3.5?
Or something somewhere between or around those extremes?
Some people seem to be trying to give you the answer to the question 'what is the difference between C# Lambda expressions and .NET System.Delegate?', which doesn't make a whole lot of sense.
The .NET framework does not in itself understand the concepts of anonymous delegates, lambda expressions, or closures - those are all things defined by language specifications. Think about how the C# compiler translates the definition of an anonymous method into a method on a generated class with member variables to hold closure state; to .NET, there's nothing anonymous about the delegate; it's just anonymous to the C# programmer writing it. That's equally true of a lambda expression assigned to a delegate type.
What .NET DOES understand is the idea of a delegate - a type that describes a method signature, instances of which represent either bound calls to specific methods on specific objects, or unbound calls to a particular method on a particular type that can be invoked against any object of that type, where said method adheres to the said signature. Such types all inherit from System.Delegate.
.NET 3.5 also introduces the System.Linq.Expressions namespace, which contains classes for describing code expressions - and which can also therefore represent bound or unbound calls to methods on particular types or objects. LambdaExpression instances can then be compiled into actual delegates (whereby a dynamic method based on the structure of the expression is codegenned, and a delegate pointer to it is returned).
In C# you can produce instances of System.Expressions.Expression types by assigning a lambda expression to a variable of said type, which will produce the appropriate code to construct the expression at runtime.
Of course, if you were asking what the difference is between lambda expressions and anonymous methods in C#, after all, then all this is pretty much irelevant, and in that case the primary difference is brevity, which leans towards anonymous delegates when you don't care about parameters and don't plan on returning a value, and towards lambdas when you want type inferenced parameters and return types.
And lambda expressions support expression generation.
One difference is that an anonymous delegate can omit parameters while a lambda must match the exact signature. Given:
public delegate string TestDelegate(int i);
public void Test(TestDelegate d)
{}
you can call it in the following four ways (note that the second line has an anonymous delegate that does not have any parameters):
Test(delegate(int i) { return String.Empty; });
Test(delegate { return String.Empty; });
Test(i => String.Empty);
Test(D);
private string D(int i)
{
return String.Empty;
}
You cannot pass in a lambda expression that has no parameters or a method that has no parameters. These are not allowed:
Test(() => String.Empty); //Not allowed, lambda must match signature
Test(D2); //Not allowed, method must match signature
private string D2()
{
return String.Empty;
}
Delegates are equivalent to function pointers/method pointers/callbacks (take your pick), and lambdas are pretty much simplified anonymous functions. At least that's what I tell people.
A delegate is a function signature; something like
delegate string MyDelegate(int param1);
The delegate does not implement a body.
The lambda is a function call that matches the signature of the delegate. For the above delegate, you might use any of;
(int i) => i.ToString();
(int i) => "ignored i";
(int i) => "Step " + i.ToString() + " of 10";
The Delegate type is badly named, though; creating an object of type Delegate actually creates a variable which can hold functions -- be they lambdas, static methods, or class methods.
I don't have a ton of experience with this, but the way I would describe it is that a delegate is a wrapper around any function, whereas a lambda expression is itself an anonymous function.
A delegate is always just basically a function pointer. A lambda can turn into a delegate, but it can also turn into a LINQ expression tree. For instance,
Func<int, int> f = x => x + 1;
Expression<Func<int, int>> exprTree = x => x + 1;
The first line produces a delegate, while the second produces an expression tree.
Short version:
A delegate is a type that represents references to methods. C# lambda expression is a syntax to create delegates or expression trees.
Kinda long version:
Delegate is not "the name for a variable" as it's said in the accepted answer.
A delegate is a type (literally a type, if you inspect IL, it's a class) that represents references to methods (learn.microsoft.com).
This type could be initiated to associate its instance with any method with a compatible signature and return type.
namespace System
{
// define a type
public delegate TResult Func<in T, out TResult>(T arg);
}
// method with the compatible signature
public static bool IsPositive(int int32)
{
return int32 > 0;
}
// initiated and associate
Func<int, bool> isPositive = new Func<int, bool>(IsPositive);
C# 2.0 introduced a syntactic sugar, anonymous method, enabling methods to be defined inline.
Func<int, bool> isPositive = delegate(int int32)
{
return int32 > 0;
};
In C# 3.0+, the above anonymous method’s inline definition can be further simplified with lambda expression
Func<int, bool> isPositive = (int int32) =>
{
return int32 > 0;
};
C# lambda expression is a syntax to create delegates or expression trees. I believe expression trees are not the topic of this question (Jamie King about expression trees).
More could be found here.
lambdas are simply syntactic sugar on a delegate. The compiler ends up converting lambdas into delegates.
These are the same, I believe:
Delegate delegate = x => "hi!";
Delegate delegate = delegate(object x) { return "hi";};
A delegate is a reference to a method with a particular parameter list and return type. It may or may not include an object.
A lambda-expression is a form of anonymous function.
A delegate is a Queue of function pointers, invoking a delegate may invoke multiple methods. A lambda is essentially an anonymous method declaration which may be interpreted by the compiler differently, depending on what context it is used as.
You can get a delegate that points to the lambda expression as a method by casting it into a delegate, or if passing it in as a parameter to a method that expects a specific delegate type the compiler will cast it for you. Using it inside of a LINQ statement, the lambda will be translated by the compiler into an expression tree instead of simply a delegate.
The difference really is that a lambda is a terse way to define a method inside of another expression, while a delegate is an actual object type.
It is pretty clear the question was meant to be "what's the difference between lambdas and anonymous delegates?" Out of all the answers here only one person got it right - the main difference is that lambdas can be used to create expression trees as well as delegates.
You can read more on MSDN: http://msdn.microsoft.com/en-us/library/bb397687.aspx
Delegates are really just structural typing for functions. You could do the same thing with nominal typing and implementing an anonymous class that implements an interface or abstract class, but that ends up being a lot of code when only one function is needed.
Lambda comes from the idea of lambda calculus of Alonzo Church in the 1930s. It is an anonymous way of creating functions. They become especially useful for composing functions
So while some might say lambda is syntactic sugar for delegates, I would says delegates are a bridge for easing people into lambdas in c#.
Some basic here.
"Delegate" is actually the name for a variable that holds a reference to a method or a lambda
This is a anonymous method -
(string testString) => { Console.WriteLine(testString); };
As anonymous method do not have any name we need a delegate in which we can assign both of these method or expression. For Ex.
delegate void PrintTestString(string testString); // declare a delegate
PrintTestString print = (string testString) => { Console.WriteLine(testString); };
print();
Same with the lambda expression. Usually we need delegate to use them
s => s.Age > someValue && s.Age < someValue // will return true/false
We can use a func delegate to use this expression.
Func< Student,bool> checkStudentAge = s => s.Age > someValue && s.Age < someValue ;
bool result = checkStudentAge ( Student Object);
Lambdas are simplified versions of delegates. They have some of the the properties of a closure like anonymous delegates, but also allow you to use implied typing. A lambda like this:
something.Sort((x, y) => return x.CompareTo(y));
is a lot more concise than what you can do with a delegate:
something.Sort(sortMethod);
...
private int sortMethod(SomeType one, SomeType two)
{
one.CompareTo(two)
}
Heres an example I put up awhile on my lame blog. Say you wanted to update a label from a worker thread. I've got 4 examples of how to update that label from 1 to 50 using delegates, anon delegates and 2 types of lambdas.
private void button2_Click(object sender, EventArgs e)
{
BackgroundWorker worker = new BackgroundWorker();
worker.DoWork += new DoWorkEventHandler(worker_DoWork);
worker.RunWorkerAsync();
}
private delegate void UpdateProgDelegate(int count);
private void UpdateText(int count)
{
if (this.lblTest.InvokeRequired)
{
UpdateProgDelegate updateCallBack = new UpdateProgDelegate(UpdateText);
this.Invoke(updateCallBack, new object[] { count });
}
else
{
lblTest.Text = count.ToString();
}
}
void worker_DoWork(object sender, DoWorkEventArgs e)
{
/* Old Skool delegate usage. See above for delegate and method definitions */
for (int i = 0; i < 50; i++)
{
UpdateText(i);
Thread.Sleep(50);
}
// Anonymous Method
for (int i = 0; i < 50; i++)
{
lblTest.Invoke((MethodInvoker)(delegate()
{
lblTest.Text = i.ToString();
}));
Thread.Sleep(50);
}
/* Lambda using the new Func delegate. This lets us take in an int and
* return a string. The last parameter is the return type. so
* So Func<int, string, double> would take in an int and a string
* and return a double. count is our int parameter.*/
Func<int, string> UpdateProgress = (count) => lblTest.Text = count.ToString();
for (int i = 0; i < 50; i++)
{
lblTest.Invoke(UpdateProgress, i);
Thread.Sleep(50);
}
/* Finally we have a totally inline Lambda using the Action delegate
* Action is more or less the same as Func but it returns void. We could
* use it with parameters if we wanted to like this:
* Action<string> UpdateProgress = (count) => lblT…*/
for (int i = 0; i < 50; i++)
{
lblTest.Invoke((Action)(() => lblTest.Text = i.ToString()));
Thread.Sleep(50);
}
}
I assume that your question concerns c# and not .NET, because of the ambiguity of your question, as .NET does not get alone - that is, without c# - comprehension of delegates and lambda expressions.
A (normal, in opposition to so called generic delegates, cf later) delegate should be seen as a kind of c++ typedef of a function pointer type, for instance in c++ :
R (*thefunctionpointer) ( T ) ;
typedef's the type thefunctionpointer which is the type of pointers to a function taking an object of type T and returning an object of type R. You would use it like this :
thefunctionpointer = &thefunction ;
R r = (*thefunctionpointer) ( t ) ; // where t is of type T
where thefunction would be a function taking a T and returning an R.
In c# you would go for
delegate R thedelegate( T t ) ; // and yes, here the identifier t is needed
and you would use it like this :
thedelegate thedel = thefunction ;
R r = thedel ( t ) ; // where t is of type T
where thefunction would be a function taking a T and returning an R. This is for delegates, so called normal delegates.
Now, you also have generic delegates in c#, which are delegates that are generic, i.e. that are "templated" so to speak, using thereby a c++ expression. They are defined like this :
public delegate TResult Func<in T, out TResult>(T arg);
And you can used them like this :
Func<double, double> thefunctor = thefunction2; // call it a functor because it is
// really as a functor that you should
// "see" it
double y = thefunctor(2.0);
where thefunction2 is a function taking as argument and returning a double.
Now imagine that instead of thefunction2 I would like to use a "function" that is nowhere defined for now, by a statement, and that I will never use later. Then c# allows us to use the expression of this function. By expression I mean the "mathematical" (or functional, to stick to programs) expression of it, for instance : to a double x I will associate the double x*x. In maths you write this using the "\mapsto" latex symbol. In c# the functional notation has been borrowed : =>. For instance :
Func<double, double> thefunctor = ( (double x) => x * x ); // outer brackets are not
// mandatory
(double x) => x * x is an expression. It is not a type, whereas delegates (generic or not) are.
Morality ? At end, what is a delegate (resp. generic delegate), if not a function pointer type (resp. wrapped+smart+generic function pointer type), huh ? Something else ! See this and that.
Well, the really oversimplified version is that a lambda is just shorthand for an anonymous function. A delegate can do a lot more than just anonymous functions: things like events, asynchronous calls, and multiple method chains.