Blazor - cannot convert from 'method group' to 'EventCallback' - components

Question

I have a parent component and a child component with an event callback where I want it to pass back a type of T.
The issue I'm having is that I have an error about converting
MethodGroup to EventCallback.
If I convert this to using an Action then it works, but I can't do it async which isn't ideal.
Any ideas what I'm doing wrong?
Parent
<Child DeleteCallback="#OnDelete"></Child>
public async Task OnDelete(T item)
{ ... }
Child
#typeparam T
[Parameter]
public EventCallback<T> DeleteCallback { get; set; }
<a #onclick="() => DeleteCallback.InvokeAsync(item)"></a>
I've added a repo here explaining the problem. Looking at the Issues for Blazor, this should;ve been fixed in 2019. https://github.com/scott-david-walker/EventCallbackError

Answer 1

You were close:
<ChildComponent Item="someModel" T="SomeModel" DeleteCallback="OnDeleteSomeModel" />
#code {
SomeModel someModel = new SomeModel();
void OnDeleteSomeModel(SomeModel someModel)
{
...
}
}
The EventCallback uses a generic type and blazor cannot infer it if you don't pass the type to the component.
This means that if you have a EventCallback<T> you need to pass the value of T to the component e.g. T="SomeType".

Answer 2

In my case I declared a nullable EventCallback? - you can't do that.

Answer 3

I've experienced that the syntax was spot on and still getting this error.
Restarting Visual Studio 2019 solved the problem for me. Cleaning and rebuilding was not enough.

Answer 4

For some reason Visual Studio kept a previous signature I had used. Cleaning, restarting, emptying bin/obj folders etc did not work. I had to rename the method, which worked for me.

Answer 5

The following syntax worked for me:
// in the component (C#):
[Parameter]
public EventCallback<MovingEventArgs> OnMoving { get; set; }
// on the using side (Razor):
OnMoving="(Component.MovingEventArgs o) => OnMoving(o)"
// on the using side (C#):
protected void OnMoving( Component.MovingEventArgs e ) {
}
There seems to have been a change and the documentation is not up to date.
This event is also using custom event arguments.

Answer 6

For anyone fighting this with the MudBlazor MudTable I found this worked use #bind-SelectedItem and don't pass any parameters to the commit function.
<MudTable T="POCO" Items="#MyState.POCOs"
#bind-SelectedItem="_selectedPOCO"
OnCommitEditClick="#(() => commitPOCO())"
CommitEditTooltip="Save Changes?">
#code {
private POCO _selectedPOCO;
private async void commitPOCO()
{
// all the changed values are in _selectedPOCO
...
}

Answer 7

In my case, the problem solved when I defined the #typeparam manually(not by inference)(TItem="int").
<MyComponent TItem="int" OnChange="Component_Changed" />

Answer 8

Okay I just spent hours trying to figure this out. Turns out on my end the problem was I have sync fusion in the project and I had explicitly define the name space of the Event call back arguments which in my case were Microsoft.AspNetCore.Components.ChangeEventArgs in the child component and then creating an async method to pass back data to the parent.

Answer 9

In my case it was a phantom error among tens others that never went away no matter what I do. Meaning I could still build and run a solution without problems, despite so many erorrs.
What helped me was this solution: https://stackoverflow.com/a/66219566/1215913
However, I finally found a solution after a few hours of digging. To
get rid of the phantom errors I closed Visual Studio, deleted the
files in the following folder, and then re-opened the solution:
C:\Users<Username>\AppData\Local\Temp\VSFeedbackIntelliCodeLogs\Suggestions\
The only difference is I removed all of the folders that start with VS (C:\Users\<Username>\AppData\Local\Temp\VS*) since there was no this exact path as #rovert mentions in his post.
This seems to be some common problem with Blazor and IntelliSense. I also often see that auto-formatter fails to correctly format the razor code (text indentations are a mess).

Answer 10

In my case I got the error
Error (active) CS1503 Argument 2: cannot convert from 'method group'
to 'EventCallback'
for ValueChanged with the following code:
<MatAutocompleteList TItem="UseCase" Items="#selectUseCases" ValueChanged=#SelectUseCases>
<ItemTemplate>
#context?.Label
</ItemTemplate>
</MatAutocompleteList>
#code
{
private UseCase SelectUseCases(UseCase useCaseDto)
{
if (useCaseDto != null)
{
//Do stuff
}
return null;
}
}
Changed to private void SelectUseCases(UseCase useCaseDto), removed all returns and the error disappeared.

Answer 11

After trying all other solutions proposed here, including:
Making sure I was providing T=myType
Clean and rebuild
Restart Visual Studio
Rename the method by appending a letter to its name and aligning its binding
The only thing that worked for me was literally deleting the line with the binding, saving, recompiling, then writing it back.
In doing so, I in fact realized that I was typing:
SelectedItemChanged="#SelectedItemsChanged"
instead of:
SelectedItemsChanged="#SelectedItemsChanged"
I was missing an s in the binding. The method itself had a signature receiving a HashSet of my type, so it could only deal with Items, not a single item.