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);
Related
Intro
I am working on some code that relies heavily on reflection. Some parts that are at the core are used repeatedly. Optimizing this in even the slightest way will give me a lot(to various degrees) of performance benefits.
Context
The core consists of getting data out of an object. Currently I use reflection to get a PropertyInfo object, and then use the following code to construct a Func<object,object>:
public static Func<object, object> BuildUntypedGetter(PropertyInfo propertyInfo)
{
var targetType = propertyInfo.DeclaringType;
var methodInfo = propertyInfo.GetGetMethod();
var exTarget = System.Linq.Expressions.Expression.Parameter(typeof(object), "t");
var exBody0 = System.Linq.Expressions.Expression.Convert(exTarget, targetType);
var exBody = System.Linq.Expressions.Expression.Call(exBody0, methodInfo);
var exBody2 = System.Linq.Expressions.Expression.Convert(exBody, typeof(object));
var lambda = System.Linq.Expressions.Expression.Lambda<Func<object, object>>(exBody2, exTarget);
var action = lambda.Compile();
return action;
}
This resulting Func<object,object> is what I then cache and use. The actual typing and security for not calling it on other objects types but the original is something I do not have to worry about as this is taken care of.
In the above code, the lambda that is created looks like the following:
.Lambda #Lambda1<System.Func`2[System.Object,System.Object]>(System.Object $t) {
(System.Object).Call ((MyObjectType)$t).get_Id()
}
where Id is the attribute that this was generated on.
As you can see, this is merely a redirection. All it does is call the getter.
Question
Is there a way to just return the getter (get_Id) as a Func<object,object> without the extra cast and call? OR are there maybe other more efficient ways to call an object.attribute getter with an object type as instance type?
Is there a way to just return the getter (get_Id) as a
Func without the extra cast and call?
C# will not let you call getter without the cast (of course with the exception of dynamic type, but that's another story), because that would violate type safety, that C# guarantees.
But you can emit IL code, that does not contain such cast and calls getter directly. Of course, when you outsmart compiler like this, it is then your responsibility to call this method only on objects of correct type, otherwise your application is likely to crash.
Following code shows how to emit such IL code. My measurements indicates, that in Release mode, emitted delegate is approx. 3x faster than your compiled lambda (in Debug mode, it seems to be actually a bit slower).
public static Func<object, object> EmitUntypedGetter(PropertyInfo pi)
{
DynamicMethod method = new DynamicMethod(
"PropertyGetter",
typeof(Object),
new[] { typeof(Object) },
Assembly.GetExecutingAssembly().ManifestModule);
ILGenerator il = method.GetILGenerator(100);
// Load object onto the stack.
il.Emit(OpCodes.Ldarg_0);
// Call property getter
il.EmitCall(OpCodes.Callvirt, pi.GetGetMethod(), null);
// If property returns value-type, value must be boxed
if(pi.PropertyType.IsValueType)
il.Emit(OpCodes.Box, pi.PropertyType);
// Exit method
il.Emit(OpCodes.Ret);
return (Func<Object, Object>)method.CreateDelegate(typeof(Func<Object, Object>));
}
EDIT:
On my computer, performance of emitted code is consistently much faster that lambda version.
Tested on Windows 10 Home, CPU Intel Core2 Q9400, compiled with Visual Studio 2017 CE v. 15.4.0, Console App targeting .NET Framework 4.7, Release mode (Optimize code option in project properties enabled), executed outside of Visual Studio (with VS attached, some optimizations are disabled)
My results:
Compiled lambda (value type) : 40827 ms
Compiled lambda (reference type) : 37558 ms
Emit (value type) : 16963 ms
Emit (reference type) : 11903 ms
Program used for testing:
public struct MyClass
{
public int I => 42;
public string S => "foo";
}
public static void Main()
{
var valueTypeProperty = typeof(MyClass).GetProperty("I");
var referenceTypeProperty = typeof(MyClass).GetProperty("S");
var lambdaValueTypeGetterDelegate = BuildUntypedGetter(valueTypeProperty);
var lambdaReferenceTypeGetterDelegate = BuildUntypedGetter(referenceTypeProperty);
var emitValueTypeGetterDelegate = EmitUntypedGetter(valueTypeProperty);
var emitReferenceTypeGetterDelegate = EmitUntypedGetter(referenceTypeProperty);
//warm-up - ensures that delegates are properly jitted
lambdaValueTypeGetterDelegate(new MyClass());
lambdaReferenceTypeGetterDelegate(new MyClass());
emitValueTypeGetterDelegate(new MyClass());
emitReferenceTypeGetterDelegate(new MyClass());
TestDelegate("Compiled lambda (value type) ", lambdaValueTypeGetterDelegate);
TestDelegate("Compiled lambda (reference type) ", lambdaReferenceTypeGetterDelegate);
TestDelegate("Emit (value type) ", emitValueTypeGetterDelegate);
TestDelegate("Emit (reference type) ", emitReferenceTypeGetterDelegate);
Console.ReadLine();
}
private static void TestDelegate(string description, Func<object, object> getterDelegate)
{
const long LOOPS_COUNT = 1_000_000_000;
var obj = new MyClass();
Stopwatch sw = new Stopwatch();
sw.Start();
for (long i = 0; i < LOOPS_COUNT; i++)
{
getterDelegate(obj);
}
sw.Stop();
Console.WriteLine($"{description}: {sw.ElapsedMilliseconds} ms");
}
Is there a way to just return the getter (get_Id) as a Func without the extra cast and call?
Not in a way that satisfies your requirements. While it is generally possible to dynamically bind a delegate directly to a method (or property accessor), the delegate type must be signature-compatible with the method you are binding to. That's not the case for you.
Consider this example:
class MyClass {
string Id { get; set; }
}
Here, your accessor expressed in terms of Func<,> would be Func<MyClass, String>. You, however, want a Func<object, object>. You're changing both the argument type and the return type. That is only permissible within certain constraints.
The type Func<-T, +TReturn> is contravariant on its input type T and covariant on its output type TReturn. Thus, a Func<,> instance is convertible to another instantiation when the target type is more specific for T or less specific for TReturn. For example:
Func<object, *> is convertible to Func<string, *> because a function that takes in an object can always take in a string. The reverse is not true.
Func<*, string> is convertible to Func<*, object> because a function that returns a string always returns an object. Again, the reverse is not true.
In C#, these rules are strictly enforced. A generic parameter can be covariant when it only appears as an output; or contravariant when it only appears as an input. Furthermore, this variance is only applicable to reference types, meaning Func<*, int> is not assignable to Func<*, object>.
I'm afraid there's no way around it: any method you bind to a Func<object, object> must accept an object argument, which means you need an intermediate method that casts down to your source type. It also needs to explicitly box primitive return values. While you could eliminate some of the conversions by emitting the IL directly, as #Ňuf has shown, I don't think you'll see a meaningful impact.
However, depending on how you're using the values you pull out, there may be a better way. Instead of using generated code to pull values into the C# universe where you can operate on them, it may be feasible to do the opposite: push that logic down into generated code, which you could specialize according to the types you're working with. Or, alternatively, make that logic generic, and use the runtime code generation to dispatch to the correct generic instantiation.
In C#, we have var data type but we can't use it as functions return type.
Why this is not possible?
public var myFunction()
{
var = some operations
}
I believe it's partly due to the design of the compiler. Eric Lippert blogged about why fields can't use implicit typing, and I suspect some of the same arguments hold for methods.
But you could easily end up with ambiguity anyway. For example:
var Method1(bool callMethod2)
{
return callMethod2 ? Method2() : null;
}
var Method2()
{
return Method1(false);
}
What should the type be here?
A simpler example:
var Method1(bool throwException)
{
if (!throwException)
{
return Method1(true);
}
throw new Exception("Bang!");
}
Admittedly this sort of ambiguity could simply be disallowed, but I suspect that the design team felt that the added complexity of both design and implementation wasn't worth the benefit. Don't forget that they're running with limited resources - given a choice between var for methods and async/await, I'd pick the latter in a heartbeat. (Admittedly there are other features I'd have picked instead of dynamic, but that's a different matter...)
Note that return type inference is performed for lambda expressions, so the very idea of it isn't crazy. For example:
IEnumerable<string> x = new[] { "x", "y", "z" };
var result = x.Select(s => { return s.Length; }); // Long form
There the compiler infers the complete type of the lambda expression when it performs overload resolution on Select, converting it to a Func<string, int>. It's not inconceivable to apply the same ideas to methods - just complicated.
var is NOT a datatype in C#. That's why you cannot use it as a return parameter. The compiler infers the type at compile time from the right handside of the assignment and bearing in mind that it is known at compile time you need to use the real type as return value. In C# 4.0 you could use the dynamic type:
public dynamic myFunction()
{
var = some operations
}
I'd like to say
int x = magic(), y = moremagic();
return i => i + (x/y);
and have the x be captured as a constant instead of a variable reference. The idea is that x will never change and so when the expression is later compiled, the compiler to can do constant folding and produce more efficient code -- i.e. calculating x/y once instead of on every call, via pointer dereferences into a closure record.
There is no way to mark x as readonly within a method, and the compiler is not clever enough to detect that it doesn't change after the creation of the expression.
I'd hate to have to build the expression by hand. Any brilliant ideas?
UPDATE: I ended up using the marvelous LinqKit to build a partial evaluator that will do the substitutions I want. The transform is only safe if you know that relevant references will not change, but it worked for my purposes. It is possible to restrict the partial evaluation only to direct members of your closure, which you control, by adding an extra check or two in there, which is fairly obvious on inspection of the sample code provided in the LinqKit.
/// <summary>Walks your expression and eagerly evaluates property/field members and substitutes them with constants.
/// You must be sure this is semantically correct, by ensuring those fields (e.g. references to captured variables in your closure)
/// will never change, but it allows the expression to be compiled more efficiently by turning constant numbers into true constants,
/// which the compiler can fold.</summary>
public class PartiallyEvaluateMemberExpressionsVisitor : ExpressionVisitor
{
protected override Expression VisitMemberAccess(MemberExpression m)
{
Expression exp = this.Visit(m.Expression);
if (exp == null || exp is ConstantExpression) // null=static member
{
object #object = exp == null ? null : ((ConstantExpression)exp).Value;
object value = null; Type type = null;
if (m.Member is FieldInfo)
{
FieldInfo fi = (FieldInfo)m.Member;
value = fi.GetValue(#object);
type = fi.FieldType;
}
else if (m.Member is PropertyInfo)
{
PropertyInfo pi = (PropertyInfo)m.Member;
if (pi.GetIndexParameters().Length != 0)
throw new ArgumentException("cannot eliminate closure references to indexed properties");
value = pi.GetValue(#object, null);
type = pi.PropertyType;
}
return Expression.Constant(value, type);
}
else // otherwise just pass it through
{
return Expression.MakeMemberAccess(exp, m.Member);
}
}
}
No there is no way to do this in C#. The compiler does not support capturing variables by value / const. Nor can you convert a non-const value into a const value at runtime in this manner.
Additionally the C# compiler only does constant folding during the initial compilation for known constant values. If it was possible to freeze a value at runtime into a constant it wouldn't participate in compiler constant folding because it happens at runtime.
The compiler doesn't do this type of "value caching". Constant folding is done at compile time for constants only, not for readonly fields and certainly not for local variables which do not have a known value at compile time.
You have to make this yourself, but it has to stay a closure reference (since the value is in fact not determinable at compile time, which is why it is likely to be put in the closure when the expression is built):
int x = magic(), y = moremagic();
int xy = x/y;
return i => i + xy;
x can't be a constant, because you're doing runtime magic to determine what it is. However, if you know that x and y don't change, try:
int x = magic(), y = moremagic();
int xOverY = x/y;
return i => i + xOverY;
I should also mention that even though the compiled IL code for i => i + (x/y) will show the division, the JIT compiler is almost certainly going to optimize this away.
One technique I used in vb2005 was to use a generic delegate factory to effect by-value closures. I only implemented it for subs rather than functions, but it could be done for functions as well. If extended in that way:
FunctionOf.NewInv()
would be a static function which would accept as parameters a function (described later), a T3, and a T4. The passed-in function should accept parameters of types T2, T3, and T4, and return a T1. The function returned by NewInv would accept one parameter of type T2 and call the passed-in function with that parameter and the ones given to NewInv.
The invocation would look something like:
return FunctionOf.NewInv((i,x,y) => i+x/y, x, y)
If you(like me) are creating some expression builder for SQL queries you may consirer the following: first create a class variable, make it a constant and then access it like this:
var constant= Expression.Constant(values);
var start = Expression.MakeMemberAccess(constant, values.GetMemberInfo(f => f.Start));
var end = Expression.MakeMemberAccess(constant, values.GetMemberInfo(f => f.End));
var more = Expression.GreaterThanOrEqual(memberBody, start);
var less = Expression.LessThanOrEqual(memberBody, end);
In C# I am trying to write code where I would be creating a Func delegate which is in itself generic. For example the following (non-Generic) delegate is returning an arbitrary string:
Func<string> getString = () => "Hello!";
I on the other hand want to create a generic which acts similarly to generic methods. For example if I want a generic Func to return default(T) for a type T. I would imagine that I write code as follows:
Func<T><T> getDefaultObject = <T>() => default(T);
Then I would use it as
getDefaultObject<string>() which would return null and if I were to write getDefaultObject<int>() would return 0.
This question is not merely an academic excercise. I have found numerous places where I could have used this but I cannot get the syntax right. Is this possible? Are there any libraries which provide this sort of functionality?
Well you can't overload anything based only on the return value, so this includes variables.
You can however get rid of that lambda expression and write a real function:
T getDefaultObject<T>() { return default(T); }
and then you call it exactly like you want:
int i=getDefaultObject<int>(); // i=0
string s=getDefaultObject<string>(); // s=null
Though one might find practical workarounds like Stephen Cleary's
Func<T> CreateGetDefaultObject<T>() { return () => default(T); }
where you can specify the generics directly, this is a quite interesting problem from a theoretical point that cannot be solved by C#'s current type system.
A type which, as you call it, is in itself generic, is referred to as a higher-rank type.
Consider the following example (pseudo-C#):
Tuple<int[], string[]> Test(Func<?> f) {
return (f(1), f("Hello"));
}
In your proposed system, a call could look like that:
Test(x => new[] { x }); // Returns ({ 1 }, { "Hello" })
But the question is: How do we type the function Test and it's argument f?
Apparently, f maps every type T to an array T[] of this type. So maybe?
Tuple<int[], string[]> Test<T>(Func<T, T[]> f) {
return (f(1), f("Hello"));
}
But this doesn't work. We can't parameterize Test with any particular T, since f should can be applied to all types T. At this point, C#'s type system can't go further.
What we needed was a notation like
Tuple<int[], string[]> Test(forall T : Func<T, T[]> f) {
return (f(1), f("Hello"));
}
In your case, you could type
forall T : Func<T> getDefaultValue = ...
The only language I know that supports this kind of generics is Haskell:
test :: (forall t . t -> [t]) -> ([Int], [String])
test f = (f 1, f "hello")
See this Haskellwiki entry on polymorphism about this forall notation.
This isn't possible, since a delegate instance in C# cannot have generic parameters. The closest you can get is to pass the type object as a regular parameter and use reflection. :(
In many cases, casting to dynamic helps remove the pain of reflection, but dynamic doesn't help when creating new instances, such as your example.
You can't do this, because generic type parameters have to be known at runtime. You have to use the activator class:
Object o = Activator.CreateInstance(typeof(StringBuilder));
which will do exactly what you want to. You can write it as the following:
public T Default<T>()
{
return (T)Activator.CreateInstance(typeof(T));
}
Edit
Blindy's solution is better.
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.