I have read about Func<>, which says it iss a delegate and you can use it like for example:
Func<class,bool>
means you send a class or anything and get a bool result, this is what I got!
but what does the following mean?
Func<Class, bool> predicate
I have no idea, can you make it clear for me?
The former will not compile since class is a registered keyword and can only be used for class definitions.
The latter is a Func<T, TResult> which is a function that takes a parameter of type T and returns an object of type TResult. So in your case, the function takes an object of type Class and returns a bool.
The naming of Class is unfortunate but it’s actually allowed to use that identifier for a type. The following would be an example that takes such an object of type Class and then checks a property of it, returning true or false depending on the success of the comparison.
Func<Class,bool> predicate = obj => obj.Prop == "Foo";
var x = new Class();
x.Prop = "Foo";
Console.WriteLine(predicate(x)); // true
// with the following class definition
public class Class
{
public string Prop { get; set; }
}
A Func<> object is callable, just like methods, so we can use parentheses to call it and pass the object here. This would be equivalent to a method like this:
public bool predicate(Class obj)
{
return obj.Prop == "Foo";
}
But the usual way to use Func<> is using lambda expressions to be able to create functions quickly inline. This is very commonly used in LINQ where you use lambda expressions, and as such Func<> objects, all the time:
var filtered = listOfOjects.Where(obj => obj.Prop == "Foo").ToList();
// ^^^^^^^^^^^^^^^^^^^^^^^^
// this is the predicate function from above
// so you could also pass the predicate Func from above:
var filtered = listOfObjects.Where(predicate).ToList()
Func<MyClass,bool> is a delegate type
In Func<MyClass, bool> predicate , predicate is a delegate variable.
You would normally see something like
Func<MyClass, bool> predicate = c => c.IsValid;
...
if (predicate(myClass1)) DoSomething();
That's just how you instantiate the Func. Compare to a string:
string str;
If you want to instantiate and assign it at the same time, you do something like this:
Func<string, bool> isLongerThanThree = input => input.Length > 3;
isLongerThanThree("string"); // Returns "true"
Are you referring to the word predicate?
That's just the name of a parameter. Notice the similarity:
MyMethod(int myAge)
MyMethod(List<bool> myBooleans)
MyMethod(Func<Class,bool> myPredicate)
Also notice the similarities between:
int myAge = 30;
myAge is a variable of type int who has been given the value of 30.
Func<Class,bool> myPredicate = (x => x.IsAlive);
myPredicate is a variable of type Func<Class,bool> who has been given the value of (x => x.IsAlive).
Func<> is a delegate which represents a method which return a result. C# provides signatures for up to 15 input arguments, which should be enough to represent all possible methods you will ever need :)
it is hardly event to imagine method which has 15 input arguments. Beside Func<> there are also some special version of the delegate like Predicate<T> which is nothing else that Func<in T, bool TResult> or Action<> which represent a function without return value, return value is void.
Delegates(C# Programming Guide)
You can assign any method, static or instance or even anonymous with the matching signature to the Func<>. For example:
Func<MyClass, bool> predicate = (myClass) => {return true;} //anonymoys method
public class MyClass
{
public bool MyPredicate(MyClass myClass)
{
return true;
}
public static bool MyStaticPredicate(MyClass myClass)
{
return true;
}
}
Func<MyClass, bool> predicate = new MyClass().MyPredicate;
Func<MyClass, bool> staticPredicate = MyClass.MyStaticPredicate;
Related
I can't understand how this is working.
Test expects to get a method with the signature of InterfaceObj as input, but we never pass it in Test2 (the Test itself holds a reference to Actual that implements InterfaceObj and acts as the input, but Test2 doesn't know it.. or is it?)
Actual implements InterfaceObj.
public bool Test(Func<InterfaceObj, bool> pointerToMethod)
{
Actual actual = new Actual();
return pointerToMethod(actual);
}
public bool Test2()
{
return Test(x => x.Test());
}
The type of the lambda parameters can be automatically inferred by the compiler. You could also specify the type explicitly:
public bool Test2()
{
return Test((InterfaceObj x) => x.Test());
}
x => y can be assigned to a Func<T, TResult> with x being type T and y being type TResult.
And since your method Test() only accepts Func<InterfaceObj, bool>, x can only be of type InterfaceObj.
It also works when assigning a variable:
Func<InterfaceObj, bool> myFunction = x => x != null; // x is of type InterfaceObj
The lambda itself is not called, you still need to invoke it:
InterfaceObj obj = ...;
bool result1 = myFunction.Invoke(obj);
// or, shorter:
bool result2 = myFunction(obj);
NB Instead of Func<T, bool>, it is usually better to use the more specific Predicate<T> which also provides more semantic value to callers of your method.
I have a class with a member Predicate which I would like to use in a Linq expression:
using System.Linq;
class MyClass
{
public bool DoAllHaveSomeProperty()
{
return m_instrumentList.All(m_filterExpression);
}
private IEnumerable<Instrument> m_instrumentList;
private Predicate<Instrument> m_filterExpression;
}
As I read that "Predicate<T> is [...] completely equivalent to Func<T, bool>" (see here), I would expect this to work, since All takes in as argument: Func<Instrument, bool> predicate.
However, I get the error:
Argument 2: cannot convert from 'System.Predicate<MyNamespace.Instrument>' to 'System.Type'
Is there a way to convert the predicate to an argument that this function will swallow?
The two types represent the same logical signature, but that doesn't mean they're just interchangable. A straight assignment won't work, for example - but you can create a new Func<T, bool> from the Predicate<T, bool>. Sample code:
Predicate<string> pred = x => x.Length > 10;
// Func<string, bool> func = pred; // Error
Func<string, bool> func = new Func<string, bool>(pred); // Okay
This is a bit like having two enum types with the same values - you can convert between them, but you have to do so explicitly. They're still separate types.
In your case, this means you could write:
public bool DoAllHaveSomeProperty()
{
return m_instrumentList.All(new Func<T, bool>(m_filterExpression));
}
The lambda expression approach suggested by other answers will work too, of course.
public bool DoAllHaveSomeProperty()
{
return m_instrumentList.All(i => m_filterExpression(i));
}
You can convert a predicate to a method by calling Invoke. All delegates have this member. Delegates don't have structural identity, but methods can be converted to matching delegates. This fix has a minor performance cost, as it adds an extra layer of indirection. However, most solutions to this problem have that problem. Eric Lippert discusses this in more detail at https://web.archive.org/web/20140625132124/http://blog.coverity.com/2014/06/18/delegates-structural-identity/ .
In your specific case, replace return m_instrumentList.All(m_filterExpression); with return m_instrumentList.All(m_filterExpression.Invoke);
Sample code demonstrating the actual problem.
void Main()
{
Predicate<int> t1 = Foo;
Func<int,bool> t2 = Foo;
Predicate<int> t3 = t2.Invoke; //Legal
Func<int,bool> t4 = t1.Invoke; //Legal
Predicate<int> t5 = t2; //Illegal
Func<int,bool> t6 = t1; //Illegal
}
bool Foo(int x)
{
return x > 20;
}
return m_instrumentList.All(i => m_filterExpression(i));
Since there are a lot of answers i will add one more just for fun.
If you want your code to compile you can use extention methods
//Original Code
class MyClass4
{
public bool DoAllHaveSomeProperty()
{
return m_instrumentList.All(m_filterExpression);
}
private IEnumerable<Instrument> m_instrumentList;
private Predicate<Instrument> m_filterExpression;
}
Add this class in the same namespace
public static class MyExtentions
{
public static bool All(this IEnumerable<Instrument> enumer, Predicate<Instrument> pred)
{
return enumer.All(e => pred(e));
}
}
As Brian said, you can convert a predicate to a method via Invoke:
public bool DoAllHaveSomeProperty()
{
return m_instrumentList.All(m_filterExpression.Invoke);
}
Let's suppose I have defined Func as follows:
Func<MyClass, object> f = o => o.StringProperty;
or
Func<MyClass, object> f = o => o.Property.SomeMethod();
Is there way to get the actual return type without specifically calling it?
You can get the return type like this:
f.Method.ReturnType
But this will return you the type object. If you want to get Method or String or something that derives from object, you won't be able to have the information unless you call the method.
Actually you could, but this would mean that you'd have to dissassemble the method core and then analyze it to see what the method can return. But even then, there might be many different return types.
So the answer is: if you want to know that it returns an object, then yes you can, otherwise it's not worth the trouble, and it's better to find another way of doing what you need.
Since you are retrieving these Func<MyClass, object> delegates at runtime from other sources, the type information is essentially lost.
Instead, where these functions are defined, you can have the callers essentially encode that type information in a wrapped delegate by taking advantage of the LINQ Expression API (EDIT: Silly me, far more simpler at this point; we already have the generic compile time information):
public class MyClassDelegate
{
private readonly Func<MyClass, object> Function;
public Type ReturnType { get; private set; }
private MyClassDelegate(Func<MyClass, object> function, Type returnType)
{
this.Function = function;
this.ReturnType = returnType;
}
public object Invoke(MyClass context)
{
return Function(context);
}
public static MyClassDelegate Create<TReturnType>(Func<MyClass, TReturnType> function)
{
Func<MyClass, object> nonTypedFunction = o => function(o);
return new MyClassDelegate(nonTypedFunction, typeof(TReturnType));
}
}
(A derived generic MyClassDelegate<TReturnType> : MyClassDelegate class could be made as well to get around some of the sillyness in the Create method, or avoid value-type boxing, or to have the return type information available at compile time or even by reflecting on whatever MyClassDelegate<TReturnType> is.)
Callers defining the delegates instead of working directly with a Func<MyClass, object> would instead work with this class and define their delegates as:
MyClassDelegate f1 = MyClassDelegate.Create(o => o.StringProperty);
MyClassDelegate f2 = MyClassDelegate.Create(o => o.Property.SomeMethod());
Your API would require a MyClassDelegate, with which you can easily access their types:
Console.WriteLine(f1.ReturnType.FullName); //string
Console.WriteLine(f2.ReturnType.FullName); //whatever `SomeMethod()` is declared to return
Finally, you can invoke the delegates or even create Func<MyClass, object> delegates still:
f1.Invoke(myClassInstance);
Func<MyClass, object> f3 = f1.Invoke;
You can do something close to this by using a generic method to make the compiler infer the type arguments:
static Func<T1, R> Infer<T1, R>(Func<T1, R> f) { return f; }
And then:
var func = Infer((string s) => s.Length);
This will encode the return type into the type of func at compilation time.
Of course for a more generally applicable solution you would need a bunch of overloads of Infer to cover Action and Func with one, two, three, etc arguments.
If you then want to get the return type at runtime, for any kind of Func it's as simple as func.Method.ReturnType as ppetrov has already pointed out.
I have a requirement to modify a method so that it has an extra parameter that will take a lambda expression that will be used on an internal object to return the value of the given property. Forgive my probable incorrect use of terminology as this is my first foray into LINQ expressions!
I have tried searching for an answer, but as I mentioned, my terminology seems to be off and the examples I can find are far too complex or deal with expressions for collection functions such as .Where(), which I am familiar with.
What I have so far (cut down version):
class MyClass
{
private MyObject _myObject = new MyObject() { Name = "Test", Code = "T" };
private string MyMethod(int testParameter, ??? selector)
{
//return _myObject.Name;
//return _myObject.Code;
return ???;
}
}
I would like to call it something like this:
string result = _myClassInstance.MyMethod(1, (x => x.Name));
or:
string result = _myClassInstance.MyMethod(1, (x => x.Code));
Obviously the parts which I am missing is the selector parameter in MyMethod, how to apply it to the local variable and how to pass the required property into the method when I am invoking it.
Any help would be appreciated, also extra bonus points for a VB.NET solutions as well as unfortunately the final implementation needs to be in our lone VB project!
private string MyMethod(int testParameter, Func<MyObject, string> selector)
{
return selector(_myObject);
}
When using Func delegates, the last parameter is the return type and the first N-1 are the argument types. In this case, there is a single MyObject argument to selector and it returns a string.
You can invoke it like:
string name = _myClassInstance.MyMethod(1, x => x.Name);
string result = _myClassInstance.MyMethod(1, x => x.Code);
Since the return type of MyMethod matches the return type of your selector delegate, you could make it generic:
private T MyMethod<T>(int testParameter, Func<MyObject, T> selector)
{
MyObject obj = //
return selector(obj);
}
I don't know VB.Net but it looks like it would be:
Public Function MyMethod(testParameter as Integer, selector as Func(Of MyObject, String))
Return selector(_myObject)
End Function
and the generic version would be:
Public Function MyMethod(Of T)(testParameter as Integer, selector Func(Of MyObject, T))
Return selector(_myObject)
End Function
I will show you a different approach that is very flexible (see DotNetFiddle at the bottom): You can easily write your own LINQ functions to extend existing functions or write your own functions and benefit from the power of LINQ queries.
In this example, I am improving Linq's Distinct function in a way so you can specify a field, which is used for grouping.
Usage (Example):
var myQuery=(from x in Customers select x).MyDistinct(d => d.CustomerID);
In this example the query is being grouped by CustomerID and the first element of each group is returned.
Declaration of MyDistinct:
public static class Extensions
{
public static IEnumerable<T> MyDistinct<T, V>(this IEnumerable<T> query,
Func<T, V> f)
{
return query.GroupBy(f).Select(x=>x.First());
}
}
You can see that f, the 2nd parameter, is declared as Func<T, V>, so it can be used by the .GroupBy statement.
Coming back to the code in your question, if you have declared
class MyObject
{
public string Name;
public string Code;
}
private MyObject[] _myObject = {
new MyObject() { Name = "Test1", Code = "T"},
new MyObject() { Name = "Test2", Code = "Q"},
new MyObject() { Name = "Test2", Code = "T"},
new MyObject() { Name = "Test5", Code = "Q"}
};
you could use that with the newly defined function MyDistinct as follows:
var myQuery = (from x in _myObject select x).MyDistinct(d => d.Code);
which will return
Name Code
Test1 T
Test2 Q
or you can use .MyDistinct(d => d.Name) in the query, which returns:
Name Code
Test1 T
Test2 Q
Test5 Q
Notice that because MyDistinct is declared with the generics T and V, it recognizes and uses the right object types automatically and returns MyObject elements.
Advanced usage
Notice that MyDistinct always takes the first element of each group. What if you need a condition defining which element you need?
Here's how you can do it:
public static class Extensions
{
public static IEnumerable<T> MyDistinct<T, V>(this IEnumerable<T> query,
Func<T, V> f,
Func<IGrouping<V,T>,T> h=null)
{
if (h==null) h=(x => x.First());
return query.GroupBy(f).Select(h);
}
}
This modification either allows you to use it exactly as before, i.e. by specifying one parameter like .MyDistinct(d => d.Name), but it also allows you to specify a having condition such as x => x.FirstOrDefault(y => y.Name.Contains("1")||y.Name.Contains("2")) as a second parameter like so:
var myQuery2 = (from x in _myObject select x).MyDistinct(d => d.Name,
x=>x.FirstOrDefault(y=>y.Name.Contains("1")||y.Name.Contains("2"))
);
If you run this query, the result is:
Name Code
Test1 T
Test2 Q
null
because Test5 does not meet the condition (it does not contain 1 or 2), you're getting null in the 3rd row.
Note: If you want to expose just the condition, you can have it even simpler by implementing it as:
public static IEnumerable<T> MyDistinct2<T, V>(this IEnumerable<T> query,
Func<T, V> f,
Func<T,bool> h=null
)
{
if (h == null) h = (y => true);
return query.GroupBy(f).Select(x=>x.FirstOrDefault(h));
}
In this case, the query would just look like:
var myQuery3 = (from x in _myObject select x).MyDistinct2(d => d.Name,
y => y.Name.Contains("1") || y.Name.Contains("2")
);
so you don't need to write x=>x.FirstOrDefault(... condition ...).
Try it in DotNetFiddle
in C#
The parameter type you are looking for Func
private string MyMethod(int testParameter, Func<MyClass,string> selector){
return selector(_myObject);
}
in VB you still want Func the syntax is a little different.
Function MyMethod(ByVal testParameter As Integer, ByVal selector as Func(Of MyClass,string) as string
return selector(_myObject)
End Function
class MyClass
{
private MyObject _myObject = new MyObject() { Name = "Test", Code = "T" };
private string MyMethod(int testParameter, Func<MyObject, string> selector)
{
return selector(_myObject );
}
}
You can do that with a delegate of your selector:
delegate string SampleDelegate(MyObject obj);
private string MyMethod(int testParameter, SampleDelegate selector)
{
return selector(_myObject);
}
You are probably looking for the Delegate class ("Delegate" in VB, "delegate" in C#), or one of its subtypes.
This page has some examples you will probably find useful, especially near the bottom of the page.
Here is a VB example of what you would want to do:
Public Class MyClass
Private Property _myObject As MyObject = New MyObject With {.Name = "Test", .Code = "T"}
Private Function MyMethod(testParameter As Integer, selector As Func(Of MyObject, String)) As String
Return selector(_myObject).ToString
End Function
End Class
I have a method which i want to convert to Extension Method
public static string GetMemberName<T>(Expression<Func<T>> item)
{
return ((MemberExpression)item.Body).Member.Name;
}
and calling it like
string str = myclass.GetMemberName(() => new Foo().Bar);
so it evaluates to str = "Bar"; // It gives the Member name and not its value
Now when i try to convert this to extension method by this
public static string GetMemberName<T>(this Expression<Func<T>> item)
{
return ((MemberExpression)item.Body).Member.Name;
}
and call it like
string str = (() => new Foo().Bar).GetMemberName();
Error says Operator '.' cannot be applied to operand of type 'lambda expression'
Where am I wrong?
There are really two things here, first, passing () => new Foo().Bar into the method that accepts Expression<Func<T>> treats the specified expression tree as a Expression<Func<T>>, but () => new Foo().Bar is not an Expression<Func<T>> on its own.
Second, in order to get your extension method to accept any lambda (such as you're supplying), you'd have to use the type that corresponds to any expression tree. But, as you may have already guessed based on the message ... to operand of type 'lambda expression' where you'd usually see the name of the type inside the quotes, that lambda expressions are treated specially by the language, making what you're trying to do, without casting first, impossible.
The way to invoke your extension method in extension method form would be (in the case that Bar is of type string)
((Expression<Func<string>>)(() => new Foo().Bar)).GetMemberName()`
which doesn't seem like it would be all that desirable.
Where am I wrong?
The compiler is telling you exactly what's wrong - you can't use . on a lambda expression.
The lambda expression doesn't have any particular type - it's just convertible to the expression tree.
A member-access expression (which is what you're trying to do) is only available in the forms
primary-expression . identifier type-argument-list(opt)
predefined-type . identifier type-argument-list(opt)
qualified-alias-member . identifier type-argument-list(opt)
... and a lambda expression isn't a primary expression.
Interestingly, this argument doesn't hold for an anonymous method expression, but for you still can't use a member access expression on that, either. Section 7.6.4 of the C# spec lists how a member access expression is bound, and the bulk of the options are either under "If E is a predefined-type or a primary-expression classified as a type" (which doesn't apply to anonymous methods) or "If E is a property access, variable, or value, the type of which is T" - but an anonymous method is an anonymous function, and as per section 7.15: "An anonymous function does not have a value or type in and of itself".
EDIT: You can still use extension methods on expression trees, you just can't use them directly on lambda expressions. So this will work:
Expression<Func<int>> expr = () => new Foo().Bar;
string name = expr.GetMemberName();
... but it's obviously not as useful. (Ditto with a cast as per mlorbetske's answer.)
To get typed expression, you will have to write it out. As others have said, there is no way compiler will automatically infer it from a lambda expression since a lambda expression can mean two things - either a delegate or an expression tree.
You can get the expression relatively simpler, by letting the compiler infer the type for you partially, like (from this answer):
public sealed class Lambda
{
public static Func<T> Func<T>(Func<T> func)
{
return func;
}
public static Expression<Func<T>> Expression<T>(Expression<Func<T>> expression)
{
return expression;
}
}
public sealed class Lambda<S>
{
public static Func<S, T> Func<T>(Func<S, T> func)
{
return func;
}
public static Expression<Func<S, T>> Expression<T>(Expression<Func<S, T>> expression)
{
return expression;
}
}
//etc, to cover more cases
Call it like:
var expr1 = Lambda.Expression(() => new Foo().Bar);
var expr2 = Lambda<string>.Expression(x => x.Length); //etc
Your options are:
Cast backward, to exact expression type and then call extension method on it
var name = ((Expression<Func<BarType>>)(() => new Foo().Bar)).GetMemberName();
looks ugly - cure worse than cause.
Get the expression first into a variable
Expression<Func<BarType>> expr = () => new Foo().Bar;
var name = expr.GetMemberName();
Slightly better, but still looks little off for a trivial thing.
Using the Lambda classes written above
var name = Lambda.Expression(() => new Foo().Bar).GetMemberName();
Even better. It's just a little less typing.
Your first pattern, which I think is the best you can get as far as readability goes
I dont think you can improve upon that considering C# rules related to lambda expressions. That said I think few improvements can be made.
First, extend the functionality to other lambda expression types. You would need more than Func<T> types to handle all cases. Decide what are the expression types you have to handle. For instance if you have a Func<S, T> type (as in your question - Bar on Foo), it looks better. Compare this
myclass.GetMemberName(() => new Foo().Bar);
with
myclass.GetMemberName<Foo>(x => x.Bar);
I would say these overloads would do:
//for static methods which return void
public static string GetMemberName(Expression<Action> expr);
//for static methods which return non-void and properties and fields
public static string GetMemberName<T>(Expression<Func<T>> expr);
//for instance methods which return void
public static string GetMemberName<T>(Expression<Action<T>> expr);
//for instance methods which return non-void and properties and fields
public static string GetMemberName<S, T>(Expression<Func<S, T>> expr);
Now these can be used not just in the cases mentioned in comments, surely there are overlapping scenarios. For instance, if you already have instance of Foo, then its easier to call the second overload (Func<T>) overload for name of property Bar, like myclass.GetMemberName(() => foo.Bar).
Secondly, implement a GetMemberName functionality common to all these overloads. An extension method on Expression<T> or LambdaExpression would do. I prefer the latter so that you get to call it even in non-strongly typed scenarios. I would write it like this, from this answer:
public static string GetMemberName(this LambdaExpression memberSelector)
{
Func<Expression, string> nameSelector = null;
nameSelector = e => //or move the entire thing to a separate recursive method
{
switch (e.NodeType)
{
case ExpressionType.Parameter:
return ((ParameterExpression)e).Name;
case ExpressionType.MemberAccess:
return ((MemberExpression)e).Member.Name;
case ExpressionType.Call:
return ((MethodCallExpression)e).Method.Name;
case ExpressionType.Convert:
case ExpressionType.ConvertChecked:
return nameSelector(((UnaryExpression)e).Operand);
case ExpressionType.Invoke:
return nameSelector(((InvocationExpression)e).Expression);
case ExpressionType.ArrayLength:
return "Length";
default:
throw new Exception("not a proper member selector");
}
};
return nameSelector(memberSelector.Body);
}
Lastly, GetMemberName is not a good name if you're to call it non-extension way. expression.GetMemberName() sounds more logical. Member.NameFrom<int>(x => x.ToString()) or MemberName.From<string>(x => x.Length) etc are more descriptive names for static calls.
So overall the class might look like:
public static class Member
{
public static string NameFrom(Expression<Action> expr)
{
return expr.GetMemberName();
}
public static string NameFrom<T>(Expression<Func<T>> expr)
{
return expr.GetMemberName();
}
public static string NameFrom<T>(Expression<Action<T>> expr)
{
return expr.GetMemberName();
}
public static string NameFrom<T>(Expression<Func<T, object>> expr)
{
return expr.GetMemberName();
}
}
And usage:
var name1 = Member.NameFrom(() => Console.WriteLine());
var name2 = Member.NameFrom(() => Environment.ExitCode);
var name3 = Member.NameFrom<Control>(x => x.Invoke(null));
var name4 = Member.NameFrom<string>(x => x.Length);
Most concise and clean.
For properties and fields, it can be transformed to an anonymous class and then using reflection the member name can be read, as shown here.
public static string GetMemberName<T>(T item) where T : class
{
if (item == null)
return null;
return typeof(T).GetProperties()[0].Name;
}
Call it like
var name = GetMemberName(new { new Foo().Bar });
It's faster, but has certain quirks, like not very refactor friendly, and doesnt help in case of methods as members. See the thread..
Of all I prefer 4.