What is the [Parameter] Attribute in c#

Question

I was watching a tutorial in Blazor. Then I came across this code and I can't seem to find it in the Internet or I think I'm not using the right terms for searching atleast.

@code{
[Parameter]
    public IList<Todo> Todo {get; set;}
}

Is it only exclusive in blazor or it is available in c#. Kindly give some references. Thanks in advance.

Answer 1

This is explained in Create and use ASP.NET Core Razor components, specifically in the Component Parameters section.

[Parameter] is used to mark the component parameters that can be set when the component is used in another page. Borrowing from the doc example this component doesn't have any parameters :

<div class="panel panel-default">
    <div class="panel-heading">@Title</div>
    <div class="panel-body">@ChildContent</div>

    <button class="btn btn-primary" @onclick="OnClick">
        Trigger a Parent component method
    </button>
</div>

@code {
    public string Title { get; set; }

    public RenderFragment ChildContent { get; set; }

    public EventCallback<MouseEventArgs> OnClick { get; set; }
}

Without the [Parameter] attribute, those are just public properties that can't be set from other pages. The following line would be invalid :

<ChildComponent Title="Panel Title from Parent" />

While this :

<div class="panel panel-default">
    <div class="panel-heading">@Title</div>
    <div class="panel-body">@ChildContent</div>

    <button class="btn btn-primary" @onclick="OnClick">
        Trigger a Parent component method
    </button>
</div>

@code {
    [Parameter]
    public string Title { get; set; }

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

    [Parameter]
    public EventCallback<MouseEventArgs> OnClick { get; set; }
}

Allows us to set the parameters whenever we use that component :

<ChildComponent Title="Panel Title from Parent"
                OnClick="@ShowMessage">
    Content of the child component is supplied
    by the parent component.
</ChildComponent>