Thank you for contacting Syncfusion support.
We have checked your reported issue based on your provided link in preview 9 and we are able to reproduce the reported issue in our end. Based on the assistance of the Blazor Preview 9, we would suggest you to use onchange event in the "@" sign prefix with the proper argument declaration as like in the below code example.
<EjsCheckBox Label="Change" @onchange="onChange"></EjsCheckBox>
private void onChange(Microsoft.AspNetCore.Components.ChangeEventArgs args)
//onChange Event triggered
For your convenience, we have prepared the sample based on your requirement. Please find the sample link below.
In the meantime, on our online documentation site, we have considered this and corrected it. And
refresh in our next by-weekly release, which is scheduled to be rolled out at the end of this month.
Could you please check the above sample and get back to us, if you need any further assistance on this.