Component parameters are how parent components pass data down to child components. They are C# properties decorated with the [Parameter] attribute.

Defining Parameters in a Child

// GreetingCard.razor
<div class="card">
    <h2>Hello, @Name!</h2>
    <p>@Message</p>
</div>

@code {
    [Parameter] public string Name { get; set; } = "";
    [Parameter] public string Message { get; set; } = "";
}

Passing Parameters from a Parent

<GreetingCard Name="Alice" Message="Welcome back!" />

CascadingParameters

For passing values deep through the component tree without threading them through every level:

<CascadingValue Value="currentUser">
    @Body
</CascadingValue>

// In a deeply nested child:
[CascadingParameter] public AppUser CurrentUser { get; set; }

Best Practices

• Always give parameters default values to avoid null exceptions
• Mark required parameters with [EditorRequired] for compile-time warnings
• Never mutate parameters directly — use an EventCallback to notify the parent

Reference:

Wikipedia: Component-Based Software Engineering

https://en.wikipedia.org/wiki/Component-based_software_engineering