Aspnetcore: error CS1503: Argument 2: cannot convert from 'method group' to 'EventCallback'

[gurunathancs1991]

Description:

When using EventCallBack<T>, I countered an issue about error CS1503: Argument 2: cannot convert from 'method group' to 'EventCallback'

I have checked other github blogs which related to this, mostly reported that, this has been fixed in preview 7 version. Right now, i have tested with preview 7 build but could not get resolve it.

Here the link which i checked earlier.
https://github.com/aspnet/AspNetCore/issues/10077
https://github.com/aspnet/AspNetCore/issues/8385
https://github.com/aspnet/AspNetCore/pull/10730
https://github.com/aspnet/AspNetCore/issues/10077

Whether any syntax changes required to resolve in preview 7 version?

Screenshot:

Please find my component structure:

Rendering Page [Index razor page]

      @page "/"
      @using GenericEvent.Shared.Models;
      @inject IJSRuntime jsruntime;

    <MyGenComponent Data="@MyData">
        <Events>
        <NonGenericEvents EventTwo="@Event2"></NonGenericEvents>
        <GenericEvents EventOne="@EventOne"></GenericEvents>
         </Events>

    </MyGenComponent>

    @functions {
        public List<string> MyData = new List<string>() { "a", "b", "c", "d" };

        public void Event2(EventArguments args)
       {

       }

       public async Task EventOne(EventArgsData<string> data)
       {
        //dummy call
        await jsruntime.InvokeAsync<string>("call");
       }
      }

MyGenComponent.razor

    @using GenericEvent.Shared.Models;

    @typeparam T;

    <div>
        <ul>
            @ChildContent
        </ul>
    </div>


    @functions {
    [Parameter]
    protected RenderFragment ChildContent { get; set; }

    [Parameter]
    public IEnumerable<T> Data { get; set; }
    }

Events.razor

      @using Microsoft.AspNetCore.Components;

      @inherits ComponentBase



     <CascadingValue Value="@this">
        @ChildContent
    </CascadingValue>

     @functions{

    [Parameter]
    protected RenderFragment ChildContent { get; set; }

    public GenericEvents<object> GenEvents { get; set; }

    public NonGenericEvents NonGenEvents { get; set; }



     }

GenericEvents.razor

      @using Microsoft.AspNetCore.Components;
      @using GenericEvent.Shared.Models;


       @typeparam TValue


      <CascadingValue Value="@this">

     </CascadingValue>


     @functions{

     [Parameter]
    public EventCallback<EventArgsData<TValue>> EventOne { get; set; }

    }

NonGenericEvents.razor

```
@using Microsoft.AspNetCore.Components;
@using GenericEvent.Shared.Models;

<CascadingValue Value="@this">

</CascadingValue>


@functions{

  [Parameter]
  public EventCallback<EventArguments> EventTwo { get; set; }


  }

**To Reproduce**

Clone this Github repository and run the application

https://github.com/gurunathancs1991/BlazorGenericEvents

**We are also expecting the solution for this thread too**

https://github.com/aspnet/AspNetCore/issues/12116

**Additional context**

C:\Users\GurunathanA>dotnet --info
.NET Core SDK (reflecting any global.json):
Version: 3.0.100-preview7-012814
Commit: efad165932

Runtime Environment:
OS Name: Windows
OS Version: 10.0.17763
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Filesdotnet\sdk\3.0.100-preview7-012814\

Host (useful for support):
Version: 3.0.0-preview7-27912-14
Commit: 4da6ee6450

.NET Core SDKs installed:
1.1.13 [C:\Program Filesdotnet\sdk]
1.1.14 [C:\Program Filesdotnet\sdk]
2.1.202 [C:\Program Filesdotnet\sdk]
2.1.502 [C:\Program Filesdotnet\sdk]
2.1.602 [C:\Program Filesdotnet\sdk]
2.1.700 [C:\Program Filesdotnet\sdk]
2.2.202 [C:\Program Filesdotnet\sdk]
2.2.300 [C:\Program Filesdotnet\sdk]
3.0.100-preview7-012814 [C:\Program Filesdotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.6 [C:\Program Filesdotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.9 [C:\Program Filesdotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.11 [C:\Program Filesdotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.3 [C:\Program Filesdotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.5 [C:\Program Filesdotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.6 [C:\Program Filesdotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.9 [C:\Program Filesdotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.11 [C:\Program Filesdotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.3 [C:\Program Filesdotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.5 [C:\Program Filesdotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.0.0-preview4-19216-03 [C:\Program Filesdotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.0.0-preview7.19363.1 [C:\Program Filesdotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 1.0.15 [C:\Program Filesdotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 1.0.16 [C:\Program Filesdotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 1.1.12 [C:\Program Filesdotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 1.1.13 [C:\Program Filesdotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.0.9 [C:\Program Filesdotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.6 [C:\Program Filesdotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.9 [C:\Program Filesdotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.11 [C:\Program Filesdotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.3 [C:\Program Filesdotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.5 [C:\Program Filesdotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.0.0-preview4-27615-11 [C:\Program Filesdotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.0.0-preview7-27912-14 [C:\Program Filesdotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.0.0-preview7-27912-14 [C:\Program Filesdotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET Core runtimes or SDKs:
https://aka.ms/dotnet-download
```

[SQL-MisterMagoo]

A workaround (maybe even the correct way):

<GenericEvents TValue="string" EventOne="@EventOne"></GenericEvents>

[EdCharbeneau]

I agree that this still seems like an issue as creating verbose markup when handling events (without databinding).

Take the InputSelect for example. ValueChanged either requires T to be defined or a lambda expression within ValueChanged. Compounding the problem is ValueExpression which is also required, this is leading to some very clumsy looking markup @(()=>. This isn't going to be the DX people expect when coming from jQuery, WebForms or even React.

<p>Current value: @form.MyProperty</p>

<EditForm Model="form">
    <InputSelect ValueChanged="@DoThing" 
                 T="string"
                 ValueExpression="@(()=> form.MyProperty)">
        <option value="0">0</option>
        <option value="100">100</option>
        <option value="200">200</option>
        <option value="300">300</option>
    </InputSelect>
</EditForm>

@code {
    MyForm form = new MyForm();

    void DoThing(string s)
    {
        form.MyProperty = s;
    }

    public class MyForm
    {
        public string MyProperty { get; set; }
    }
}

   <InputSelect ValueChanged="@((string s) => DoThing(s))"
                 ValueExpression="@(()=> form.MyProperty)">
        <option value="0">0</option>
        <option value="100">100</option>
        <option value="200">200</option>
        <option value="300">300</option>
    </InputSelect>

[pranavkm]

From what I can tell, the problem is that the compiler does not use the the generic type parameter for type inference in method calls. Here's a fairly trivial example:

```C#

public void Foo()
{
Tester(2, TestAction);
// This fails complaining about type inference
Tester(2, CreateDelegate(TestAction));
}

public void TestAction(int x) { }

public void Tester(T value, Action input) { }

public static Action CreateDelegate(Action action) => action;

We effectively get the same code when we code-gen a Razor file like this:

```C#
<InputSelect ValueChanged="ChangedInt" />

@code {
    public void ChangedInt(int value)
    {
    }
}

```C#
using Microsoft.AspNetCore.Components;

namespace razor1.Pages
{
public class Index : Microsoft.AspNetCore.Components.ComponentBase
{
protected override void BuildRenderTree(Microsoft.AspNetCore.Components.RenderTree.RenderTreeBuilder builder)
{
__Blazor.razor1.Pages.Index.TypeInference.CreateInputSelect_0(builder, 0, 1, Microsoft.AspNetCore.Components.EventCallback.Factory.Create(this,
ChangedInt
));
}

    public void ChangedInt(int value)
    {
    }
}

}
namespace __Blazor.razor1.Pages.Index
{
internal static class TypeInference
{
public static void CreateInputSelect_0(global::Microsoft.AspNetCore.Components.RenderTree.RenderTreeBuilder builder, int seq, int __seq0, global::Microsoft.AspNetCore.Components.EventCallback __arg0)
{
builder.OpenComponent>(seq);
builder.AddAttribute(__seq0, "ValueChanged", __arg0);
builder.CloseComponent();
}
}
}

The compiler fails the type inferrence, picks the wrong overload for `EventCallback.Factory.Create` and generates the method group conversion error reported here.

From playing around, one of the ways to solve this would be to modify the code in `TypeInference` to pass the `Action<T>` in and produce the `EventCallback<T>` in there. This works well, but we now have the pain of having to code-gen an overload per overload of `EventCallback.Factory.Create<T>`. Changing the code-gen is going to be fairly involved, so I'm sending it back to triage to consider for a later release.

-----------------------------------------------------------------

Workarounds that have been pointed out by others in the thread that work:

1) Specify the type parameter

(I filed an issue to do a pass over blazor input elements to consider renaming `T` to `TValue` or something slightly better)

2) Use a delegate or perform an explicit cast

```

[mkArtakMSFT]

Thanks for contacting us, @gurunathancs1991.
As @pranavkm has pointed out, you will have to go with one of the provided workarounds here.
Type inference will always have limitations and it's unfortunate that this is one of them.

[srg1204]

EdCharbeneau commented on 23 Jul wrote: but this solution is not working with
ASP.NET Core and Blazor updates in .NET Core 3.0 Preview 9.19424.4

Current value: @form.MyProperty

T="string"
ValueExpression="@(()=> form.MyProperty)">
0
100
200
300

@code {
MyForm form = new MyForm();

void DoThing(string s)
{
    form.MyProperty = s;
}

public class MyForm
{
    public string MyProperty { get; set; }
}

}
ValueExpression="@(()=> form.MyProperty)">
0
100
200
300

[srg1204]

Sorry my mistake:
EdCharbeneau was OK: the solution works very well