Initialize objects in Blazor component

Question

With this Blazor component:

@page "/counter"

<h1>Counter</h1>

<p>Current count: @_a.CurrentCount</p>

<button class="btn btn-primary" onclick="@_b.IncrementCount">Click me</button>

@functions {
    private readonly ClassA _a = new ClassA();
    private readonly ClassB _b = new ClassB(_a);

    class ClassA
    {
        public int CurrentCount { get; set; }
    }

    class ClassB
    {
        private readonly ClassA _classA;

        public ClassB(ClassA classA)
        {
            _classA = classA;
        }

        public void IncrementCount() => _classA.CurrentCount++;
    }
}

I get this error:

Error CS0236 A field initializer cannot reference the non-static field, method, or property '__Counter._a'

This thread explain how resolve this error in standard class:

Why can't you use 'this' in member initializers?

But for this, it needs a constructor.

Is it possible to add constructor in Blazor component?

How resolve this error?

This done the job, but I need remove the readonly attribut to initialize _a and _b in OnInit. — vernou
– vernou, Commented Feb 25, 2019 at 23:30
@HenkHolterman, init also was my first idea and, after read OP comment, I guess "code-behind" is the only way to match readonly properties requirement. I elaborate an answer with this. — dani herrera
– dani herrera, Commented Feb 26, 2019 at 11:28

dani herrera · Accepted Answer · 2019-02-26 22:18:04Z

8

To keep classes readonly you should to move to "code-behind". Then you can instantiate classes on constructor:

@page "/counter"
@inherits CounterBase
<h1>Counter</h1>

<p>Current count: @_a.CurrentCount ...

CounterBase.cs

using Microsoft.AspNetCore.Blazor.Components;

namespace YourApp.Pages
{

    public class ClassA
    {
        public int CurrentCount { get; set; }
    }

    public class ClassB
    {
        private readonly ClassA _classA;

        public ClassB(ClassA classA)
        {
            _classA = classA;
        }

        public void IncrementCount() => _classA.CurrentCount++;
    }

    public class CounterBase : BlazorComponent
    {
        protected readonly ClassA _a;
        protected readonly ClassB _b;

        //constructor
        public CounterBase()
        {
            _a = new ClassA();
            _b = new ClassB(_a);
        }
        ...

edited Feb 26, 2019 at 22:18

answered Feb 26, 2019 at 7:34

dani herrera

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7 Comments

vernou Over a year ago

The constructor is invalid. Can you remove the void? And I valid you answer.

dani herrera Over a year ago

lel, of course. Sorry about typo. Any other comment is welcome.

vernou Over a year ago

Now, I have this error : Error CS0122 'CounterBase._a' is inaccessible due to its protection level. If I replace by protected, I can't use readonly. Other idea?

dani herrera Over a year ago

fixed now with public class ? ( or move classA and classB as nested classes ) Maybe nested classes are closer to your initial question.

vernou Over a year ago

It's not possible to access private field in subclass. You need add a getter and use this getter in component. In the constructor, use field and don't declare variables. Can you improve your answer?

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