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.
Related
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);
}
I have the following method where T is used inside a Func:
public void DoSomething<T>(string someString, Func<T, bool> someMethod)
{
if(someCondition)
{
string A;
bool resultA = someMethod(A);
}
else
{
string[] B;
bool resultB = someMethod(B);
}
// Some other stuff here ...
}
I am invoking the DoSomething method in the following manner:
DoSomething<string>("abc", someMethod);
DoSomething<string[]>("abc", someMethod);
And the someMethod exists with the following definitions:
bool someMethod(string simpleString);
bool someMethod(string[] stringArray);
Now the compilation fails with the following errors in method DoSomething:
cannot convert from 'string' to 'T'
cannot convert from 'string[]' to 'T'
I am unable to figure out if there is a solution to the problem, or what I am trying is not feasible. It looks similar to question How can I pass in a func with a generic type parameter?, though it was not helpful for my scenario.
Your example seems a little inconsistent, but if you were writing things generically, it should look more like this:
public void DoSomething<T>(string someString, Func<T, bool> someMethod)
{
T a;
someMethod(a);
}
Notice that instead of using if to choose between types, and then declaring the type as either a string or string[], we simply declare the type as T, which will get substituted when the code is compiled so that it will be appropriate for the function.
The moment you find yourself picking between types using if or switch case, you probably don't want a generic solution; the logic isn't, in fact, generic at all. It is specific. In that sort of case, just write two prototypes:
public void DoSomething(string someString, Func<string, bool> someMethod)
{
string A;
bool resultA = someMethod(A);
}
public void DoSomething(string someString, Func<string[], bool> someMethod)
{
string[] A;
bool resultA = someMethod(A);
}
This is known as method overloading. The compiler will automatically pick the right method with the right arguments by inferring the types from the supplied function.
You can achieve it via reflection:
public void DoSomething<T>(string someString, Func<T, bool> someMethod)
{
var args = new Dictionary<Type, object>
{
[typeof(string)] = "string", //string A;
[typeof(string[])] = new[] { "string" }, //string[] B;
};
var arg = args[typeof(T)];
var result = (bool)someMethod.Method.Invoke(someMethod.Target, new[] { arg });
}
Usage:
DoSomething<string>("abc", someMethod);
DoSomething<string[]>("abc", someMethod);
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;
I'm trying to write a generic method that supplies parameters and calls a function, like this:
class MyClass {
public int Method(float arg) => 0;
}
TResult Call<T1, TResult>(Func<T1, TResult> func) =>
func(default(T1));
void Main()
{
var m = new MyClass();
var r1 = Call<float, int>(m.Method);
var r2 = Call(m.Method); // CS0411
}
The last line fails to compile with CS0411. Is there any workaround to get type inference to work here?
Use case: using AutoFixture to generate function call parameters.
Unfortunately no, that's a limitation of type inference in C#. It doesn't really do much with return types, which is what's needed in your case to fully infer the generic arguments to Call<>.
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