How to create type of func with specified generic types?

Question

I want to create Func type with specified generic type arguments, something like that like, but only with one statement

Type create_type(Type[] types) { return typeof(Func<>).MakeGenericType(types); // error if types.Length != 1 return typeof(Func<,>).MakeGenericType(types); // error if types.Length != 2 return typeof(Func<,,>).MakeGenericType(types); // error if types.Length != 3 return typeof(Func<,,,>).MakeGenericType(types); // error if types.Length != 4 // .... }

Of course, I can create auxuliary array:

var func_types = new Type[] { typeof(Func<>), typeof(Func<,>) , //...}

But this solution seems too ugly for me

Note: array of types can be long (up to 15) and I want to find elegant solution. Unfortunately, I am creating such type from meta data, written in another language

CSharpie · Accepted Answer · 2016-10-12 12:59:08Z

13

There are static Methods in Expression class for Func and Action that do exaclty what you require

Expression.GetFuncType(Type[] types) Expression.GetActionType(Type[] types)

Further Information here:

GetFuncType

GetActionType

edited Oct 12, 2016 at 12:59

answered Oct 12, 2016 at 12:53

CSharpie

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5 Comments

Alex Aparin Over a year ago

that's brilliant answer!

Lasse V. Karlsen Over a year ago

Then I retract all my comments above about ugly code being needed :) Turns out it's just a question of who's going to have to write that ugly or convoluted code - referencesource.microsoft.com/#System.Core/Microsoft/Scripting/…

Alex Aparin Over a year ago

@CSharpie, Is there some analogue of such method for tuple type?

CSharpie Over a year ago

@АлександрЛысенко It appears not, but i also dont see the point why would someone create a Tuple-Type via reflection. Tuples are designed to be used in a small (local) scope. so they should never leave the body of a method. Anything else tends to be a codesmell in the firstplace.

Alex Aparin Over a year ago

@Lasse V. Karlsen, I thought that implementation will intrigue you)) referencesource.microsoft.com/System.Core/…

Jeroen Mostert · Accepted Answer · 2016-10-12 13:00:38Z

Recall that generic types have names of the form T`N, where N is the number of generic parameters. So all Func delegates have a systematic name that allows us to easily get the type:

Type create_type(Type[] types) { return Type.GetType($"System.Func`{types.Length}").MakeGenericType(types); }

No promises as to how efficient this is, but if you're constructing such types at runtime, that's probably not the primary concern anyway.

Given the existence of Expression.GetFuncType, this answer is of course inferior, but I'm leaving it up for educational purposes (of some sort). This stops working beyond 9 arguments, because System.Func delegates of more than 9 arguments live in System.Core, not mscorlib, and therefore require an assembly-qualified name. The fix is easy (throw in a check and conditionally add System.Core to the name), but unnecessary given the existence of GetFuncType.

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