What is the [Parameter] Attribute in c#

user_4447680 0
Viewed 4320

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.

blazor c#
2 Answers

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>
user_1342040

All attributes in C# have to reference a type defining that attribute somewhere. That Blazor code is still C#.

In this case, I believe it it refers to Microsoft.AspNetCore.Components.ParameterAttribute - the documentation is currently MIA, but that may well improve over time. There's more detail in the Blazor documentation.

In general, if you have the code in front of you (generally a good idea when watching a tutorial, if at all possible) you can hover over the attribute in Visual Studio to see its fully qualified name or navigate to it.

user_226560
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