error CS1660: Cannot convert lambda expression to type 'bool' because it is not a delegate type

Question

This has been driving me mad for ages. I took the following piece of working code

<input @onchange="@((ui) => Console.WriteLine(ui.Value))" />

and copied it to a new blazor component project. I then started getting the error

error CS1660: Cannot convert lambda expression to type 'bool' because it is not a delegate type

Answer 1

As this lost me quite a lot of time and the error is very misleading I thought I'd write it up.

The issue is a missing @using add the following in and it all works.

@using Microsoft.AspNetCore.Components.Web

I'm guessing there is some extension method in that namespace that makes the magic work in the razor generated code...

Answer 2

Adding the @using statement unfortunately didn't work for me. Apparently, you have to define the lambda expression directly inside the quotes now, without wrapping it inside @(...), like this:

<input @onchange="changeEventArgs => Console.WriteLine(changeEventArgs.Value)" />

See this reference for examples: https://learn.microsoft.com/en-us/dotnet/architecture/blazor-for-web-forms-developers/components#event-handlers