How to call Expression<Func<T1, T2, TResult>> - c#

I need to call a function that combines two types into one.
This is the expression:
private static readonly Expression<Func<T1, T2, TResult>> T1T2ToTResultProjection = (T1, T2) => T1 == null ?
null :
new TResult(
This is how I have tried to call it:
TResult result = T1T2ToTResultProjection(T1, T2);
How can I call this?
Is it possible to call it with a lambda expression?


How to convert delegate `Func<T1, Func<T2, Task<TResult>>>` to `Func<T1, Task<Func<T2, TResult>>`?

For my delegate (with signature Func<T1, Func<T2, Task<TResult>>) to be applicable it has to confirm to the signature Func<T1, Task<TResult>>. I would like to have a function such as:
public static Func<T1, Task<Func<T2, TResult>> TaskToOuterScope<T1, T2, TResult>(Func<T1, Func<T2, Task<TResult>>> f)
// throw new NotImplementedException();
How to implement this TaskToOuterScope-function?
I tried:
public static Func<T1, Task<Func<T2, TResult>> TaskToOuterScope<T1, T2, TResult>(Func<T1, Func<T2, Task<TResult>>> f)
return async (T1 arg1) => await (async (T2 arg2) => await f(arg1)(arg2));
However, this gives CS4001 Cannot await 'lambda expression'.
Googling gave pages that are either about currying (the signature Func<T1, Func<T2, Task<TResult>> is the result from currying) or about async/await but non about the combination or more specifically about moving task to outer scope in higher order functions.
Is conversion from Func<T1, Func<T2, Task<TResult>> to Func<T1, Task<Func<T2, TResult>>> possible?
And if so, how can I do it?
You can't.
Task<TResult> is a task whose result depends on two parameters, t1 and t2. You can see that you cannot transform it to a task Task<Func<T2, TResult>> whose result only depends on one parameter t1.
Whatever asynchronous task is being executed requires (by the definition you have given) those two parameters upfront before it starts executing. It cannot take t1 then, given t2 at a later point, return its result synchronously.

Can a lambda expression be declared and invoked at the same time in C#?

In VB.NET, a lambda expression can be declared and invoked on the same line:
'Output 3
Console.WriteLine((Function(num As Integer) num + 1)(2))
Is this possible in C#?
You have to tell the compiler a specific delegate type. For example, you could cast the lambda expression:
Console.WriteLine(((Func<int, int>)(x => x + 1))(2));
EDIT: Or yes, you can use a delegate creation expression as per Servy's answer:
Console.WriteLine(new Func<int, int>(i => i + 1)(2));
Note that this isn't really a normal constructor call - it's special syntax for delegate creation which looks like a regular constructor call. Still clever though :)
You can make it slightly cleaner with a helper class:
public static class Functions
public static Func<T> Of<T>(Func<T> input)
return input;
public static Func<T1, TResult> Of<T1, TResult>
(Func<T1, TResult> input)
return input;
public static Func<T1, T2, TResult> Of<T1, T2, TResult>
(Func<T1, T2, TResult> input)
return input;
... then:
Console.WriteLine(Functions.Of<int, int>(x => x + 1)(2));
Console.WriteLine(Functions.Of((int x) => x + 1)(2));
Console.WriteLine(new Func<int, int>(i => i + 1)(2));
Uses a few less parentheses to use the Func's constructor than a cast.
Yes, though it's messy:
Console.WriteLine(((Func<int, int>) (num => num + 1))(2));
Kind or, you would have to use the Func object :
var square = new Func<double, double>(d => d*d)(2);

How do I describe an Action<T> delegate that returns a value (non-void)?

The Action<T> delegate return void. Is there any other built-in delegate which returns non void value?
Yes. Func<> returns the type specified as the final generic type parameter, such that Func<int> returns an int and Func<int, string> accepts an integer and returns a string. Examples:
Func<int> getOne = () => 1;
Func<int, string> convertIntToString = i => i.ToString();
Action<string> printToScreen = s => Console.WriteLine(s);
// use them
Sure, the Func Delegates return T.
Func<TResult> is "TResult method()"
Func<TInput, TResult> is "TResult method(TInput param)"
All the way down to
Func<T1, T2, T3, T4, TResult>
Also, for the sake of completeness, there is Predicate which returns bool.
Predicate<T> is "bool method(T param)"

How to declare a generic delegate with an out parameter [duplicate]

This question already has answers here:
Func<T> with out parameter
(4 answers)
Closed 9 years ago.
Func<a, out b, bool>, just don't compile, how to declare that i want the second parameter be an out one?
I want to use it like this:
public class Foo()
public Func<a, out b, bool> DetectMethod;
Actually, Func is just a simple delegate declared in the .NET Framework. Actually, there are several Func delegates declared there:
delegate TResult Func<TResult>()
delegate TResult Func<T, TResult>(T obj)
delegate TResult Func<T1, T2, TResult>(T1 obj1, T2 obj2)
delegate TResult Func<T1, T2, T3, TResult>(T1 obj1, T2 obj2, T3 obj3)
delegate TResult Func<T1, T2, T3, T4, TResult>(T1 obj1, T2 obj2, T3 obj3, T4 obj4)
delegate TResult Func<T1, T2, ... , T16, TResult>(T1 obj1, T2 obj2, ..., T16 obj16)
So the only thing you can do is declare your custom delegate:
delegate bool MyFunc<T1, T2>(T1 a, out T2 b)
You need to make your own delegate type, like this:
delegate bool MyFunc(Type1 a, out Type2 b);
You might want to rethink your design. Do you really need to complicate your code by adding an out parameter?
You can wrap the bool return type and the second out type in their own class (or .NET 4.0 Tuple) and use that as a return type:
public Func<Type1, Tuple<Type2, bool>> DetectMethod;
Of course when you want to use the delegates to reference try-parse methods, you are on the right track and you'll need to define a new delegate as others already described.

Func<T> with out parameter

Can I pass a method with an out parameter as a Func?
public IList<Foo> FindForBar(string bar, out int count) { }
// somewhere else
public IList<T> Find(Func<string, int, List<T>> listFunction) { }
Func needs a type so out won't compile there, and calling listFunction requires an int and won't allow an out in.
Is there a way to do this?
ref and out are not part of the type parameter definition so you can't use the built-in Func delegate to pass ref and out arguments. Of course, you can declare your own delegate if you want:
delegate V MyDelegate<T,U,V>(T input, out U output);
Why not create a class to encapsulate the results?
public class Result
public IList<Foo> List { get; set; }
public Int32 Count { get; set; }
The Func family of delegates (or Action for that matter) are nothing but simple delegate types declared like
//.NET 4 and above
public delegate TResult Func<out TResult>()
public delegate TResult Func<in T, out TResult>(T obj)
//.NET 3.5
public delegate TResult Func<T1, T2, TResult>(T1 obj1, T2 obj2)
public delegate TResult Func<T1, T2, T3, TResult>(T1 obj1, T2 obj2, T3 obj3)
etc. Delegates as such can have out/ref parameters, so in your case its only a matter of custom implementation by yourself as other answers have pointed out. As to why Microsoft did not pack this by default, think of the sheer number of combinations it would require.
delegate TResult Func<T1, T2, TResult>(T1 obj1, T2 obj2)
delegate TResult Func<T1, T2, TResult>(out T1 obj1, T2 obj2)
delegate TResult Func<T1, T2, TResult>(T1 obj1, out T2 obj2)
delegate TResult Func<T1, T2, TResult>(out T1 obj1, out T2 obj2)
for just two parameters. We have not even touched ref. It would actually be cumbersome and confusing for developers.
You could wrap it in a lambda/delegate/function/method that exposed the right interface and called FindForBar, but I suspect that FindForBar has count as an out parameter as a reason, so you'd need to be sure throwing that information away was ok/safe/desirable/had the right results (you'd need to be sure of this even if you could just directly pass in FindForBar).