CS0570 'AvailableItems.GetListAsync(string)' is not supported by the language

I am getting the message above when using a .NET Standard class recently compiled with Visual Studio 2017 15.8.9 and with the Advanced Build Settings, Language version set to C# latest minor version.

The method definition is: public async Task> GetListAsync(String filter)

I tried enabling the Use experimental Roslyn assemblies which says Current Version (up to date) but since that did not resolve the issue I am guessing that is not what I needed to enable. Is there a way to get support for my return of a ValueTuple? This works in Visual Studio and with my unit tests that call it for testing purposes.

Thank you in advance.


  • I created a new .NET Standard project in Visual Studio with language version set to C# latest minor, and modified Class1.cs as follows:
    using System;
    using System.Threading.Tasks;
    namespace ClassLibrary15
    	public class Class1
    		public async Task GetListAsync (String filter)
    			await Task.Delay (100);
    			return filter;
    Then I referenced bin\Debug\netstandard2.0\ClassLibrary15.sll from LINQPad, and ran the following query:

    new Class1().GetListAsync("test")

    It ran without error in LINQPad 5.31 and also LINQPad 5.36.

    Is there anything else I need to do to reproduce this?
  • Your class and method do work just fine. I think it has to do with ValueTuple being returned by my method.

    using System; using System.Collections.Generic; using System.Threading.Tasks; namespace ClassLibrary1 { public class Class1 { public async Task> GetListAsync(String filter) { await Task.Delay(100); List<(String Description, String Code)> list = new List<(String Description, String Code)>(); list.Add(new System.ValueTuple("Test", "Code")); return list; } public async Task GetListAsync2(String filter) { await Task.Delay(100); return filter; } } } }

    This was in a .NET Standard 2.0 Class built in release mode.

    Note that this same code runs in a Program in LINQPad, it just does not run when I build the class with VS 2017 and then reference that dll from LINQPad and try calling GetListAsync. Not sure it helps or not but I also noticed today that the autocompletion in LINQPad doesn't show the GetListAsync method returning the ValueTuple when I dot into the object either. The GetListAsync2 does show up in autocompletion. I am using LINQPad 5.36.03

    Thank you for looking into this and let me know if I can be of more assistance.
  • I'm experiencing the same thing. Apparently it's because the method in the .NET Standard assembly is returning a value tuple (or in my case, an array of value tuple).
  • I can confirm this is still an issue with code compiled in Visual Studio 2019 16.2.1, and the LINQPad beta 5.40.00.
  • I am currently working around this by invoking the method via reflection, and casting within the LINQPad query to an array of value tuples.
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