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Dynamically-rendered ASP.NET Core Razor components

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  • 4 min. za čitanje

By Dave Brock

Use the built-in DynamicComponent component to render components by type.

A DynamicComponent is useful for rendering components without iterating through possible types or using conditional logic. For example, DynamicComponent can render a component based on a user selection from a dropdown list.

In the following example:

  • componentType specifies the type.
  • parameters specifies component parameters to pass to the componentType component.
<DynamicComponent Type="@componentType" Parameters="@parameters" />

@code {
    private Type componentType = ...;
    private IDictionary<string, object> parameters = ...;
}

For more information on passing parameter values, see the Pass parameters section later in this article.

Use the Instance property to access the dynamically-created component instance:

<DynamicComponent Type="@typeof({COMPONENT})" @ref="dc" />

<button @onclick="Refresh">Refresh</button>

@code {
    private DynamicComponent? dc;

    private Task Refresh()
    {
        return (dc?.Instance as IRefreshable)?.Refresh();
    }
}

In the preceding example:

  • The {COMPONENT} placeholder is the dynamically-created component type.
  • IRefreshable is an example interface provided by the developer for the dynamic component instance.

Example

In the following example, a Razor component renders a component based on the user's selection from a dropdown list of four possible values.

User spaceflight carrier selection Shared Razor component to render
Rocket Lab® Shared/RocketLab.razor
SpaceX® Shared/SpaceX.razor
ULA® Shared/UnitedLaunchAlliance.razor
Virgin Galactic® Shared/VirginGalactic.razor

Shared/RocketLab.razor:

<h2>Rocket Lab&reg;</h2>

<p>
    Rocket Lab is a registered trademark of 
    <a href="https://www.rocketlabusa.com/">Rocket Lab USA Inc.</a>
</p>

Shared/SpaceX.razor:

<h2>SpaceX&reg;</h2>

<p>
    SpaceX is a registered trademark of 
    <a href="https://www.spacex.com/">Space Exploration Technologies Corp.</a>
</p>

Shared/UnitedLaunchAlliance.razor:

<h2>United Launch Alliance&reg;</h2>

<p>
    United Launch Alliance and ULA are registered trademarks of
    <a href="https://www.ulalaunch.com/">United Launch Alliance, LLC</a>.
</p>

Shared/VirginGalactic.razor:

<h2>Virgin Galactic&reg;</h2>

<p>
    Virgin Galactic is a registered trademark of 
    <a href="https://www.virgingalactic.com/">Galactic Enterprises, LLC</a>.
</p>

Pages/DynamicComponentExample1.razor:

@page "/dynamiccomponent-example-1"

<h1><code>DynamicComponent</code> Component Example 1</h1>

<p>
    <label>
        Select your transport:
        <select @onchange="OnDropdownChange">
            <option value="">Select a value</option>
            <option value="@nameof(RocketLab)">Rocket Lab</option>
            <option value="@nameof(SpaceX)">SpaceX</option>
            <option value="@nameof(UnitedLaunchAlliance)">ULA</option>
            <option value="@nameof(VirginGalactic)">Virgin Galactic</option>
        </select>
    </label>
</p>

@if (selectedType is not null)
{
    <div class="border border-primary my-1 p-1">
        <DynamicComponent Type="@selectedType" />
    </div>
}

@code {
    private Type? selectedType;

    private void OnDropdownChange(ChangeEventArgs e)
    {
        selectedType = e.Value?.ToString()?.Length > 0 ? 
            Type.GetType($"{APP NAMESPACE}.Shared.{e.Value}") : null;
    }
}

In the preceding example:

  • Component names are used as the option values using the nameof operator, which returns component names as constant strings.
  • The {APP NAMESPACE} placeholder is the namespace of the app (for example, BlazorSample).

Pass parameters

If dynamically-rendered components have component parameters, pass them into the DynamicComponent as an IDictionary<string, object>. The string is the name of the parameter, and the object is the parameter's value.

The following example configures a component metadata object (ComponentMetadata) to supply parameter values to dynamically-rendered components based on the type name. The example is just one of several approaches that you can adopt. Parameter data can also be provided from a web API, a database, or a method. The only requirement is that the approach returns an IDictionary<string, object>.

ComponentMetadata.cs:

public class ComponentMetadata
{
    public string? Name { get; set; }
    public Dictionary<string, object> Parameters { get; set; } = 
        new Dictionary<string, object>();
}

The following RocketLabWithWindowSeat component (Shared/RocketLabWithWindowSeat.razor) has been updated from the preceding example to include a component parameter named WindowSeat to specify if the passenger prefers a window seat on their flight:

Shared/RocketLabWithWindowSeat.razor:

<h2>Rocket Lab&reg;</h2>

<p>
    User selected a window seat: @WindowSeat
</p>

<p>
    Rocket Lab is a trademark of 
    <a href="https://www.rocketlabusa.com/">Rocket Lab USA Inc.</a>
</p>

@code {
    [Parameter]
    public bool WindowSeat { get; set; }
}

In the following example:

  • Only the RocketLabWithWindowSeat component's parameter for a window seat (WindowSeat) receives the value of the Window Seat checkbox.
  • The {APP NAMESPACE} placeholder is the namespace of the app (for example, BlazorSample).
  • The dynamically-rendered components are shared components in the app's Shared folder:
    • Shown in this article section: RocketLabWithWindowSeat (Shared/RocketLabWithWindowSeat.razor)
    • Components shown in the Example section earlier in this article:
      • SpaceX (Shared/SpaceX.razor)
      • UnitedLaunchAlliance (Shared/UnitedLaunchAlliance.razor)
      • VirginGalactic (Shared/VirginGalactic.razor)

Pages/DynamicComponentExample2.razor:

@page "/dynamiccomponent-example-2"

<h1><code>DynamicComponent</code> Component Example 2</h1>

<p>
    <label>
        <input type="checkbox" @bind="WindowSeat" />
        Window Seat (Rocket Lab only)
    </label>
</p>

<p>
    <label>
        Select your transport:
        <select @onchange="OnDropdownChange">
            <option value="">Select a value</option>
            @foreach (var c in components)
            {
                <option value="@c.Key">@c.Value.Name</option>
            }
        </select>
    </label>
</p>

@if (selectedType is not null)
{
    <div class="border border-primary my-1 p-1">
        <DynamicComponent Type="@selectedType" 
            Parameters="@components[selectedType.Name].Parameters" />
    </div>
}

@code {
    private Dictionary<string, ComponentMetadata> components =
        new()
        {
            {
                "RocketLabWithWindowSeat",
                new ComponentMetadata
                {
                    Name = "Rocket Lab with Window Seat",
                    Parameters = new() { { "WindowSeat", false } }
                }
            },
            {
                "VirginGalactic",
                new ComponentMetadata { Name = "Virgin Galactic" }
            },
            {
                "UnitedLaunchAlliance",
                new ComponentMetadata { Name = "ULA" }
            },
            {
                "SpaceX",
                new ComponentMetadata { Name = "SpaceX" }
            }
        };
    private Type? selectedType;
    private bool windowSeat;

    private bool WindowSeat
    {
        get { return windowSeat; }
        set
        {
            windowSeat = value;
            components[nameof(RocketLabWithWindowSeat)].Parameters["WindowSeat"] = 
                windowSeat;
        }
    }

    private void OnDropdownChange(ChangeEventArgs e)
    {
        selectedType = e.Value?.ToString()?.Length > 0 ? 
            Type.GetType($"{APP NAMESPACE}.Shared.{e.Value}") : null;
    }
}

Avoid catch-all parameters

Avoid the use of catch-all parameters. If catch-all parameters are used, every explicit parameter on DynamicComponent effectively is a reserved word that you can't pass to a dynamic child. Any new parameters passed to DynamicComponent are a breaking change, as they start shadowing child component parameters that happen to have the same name. It's unlikely that the caller always knows a fixed set of parameter names to pass to all possible dynamic children.

Trademarks

Rocket Lab is a registered trademark of Rocket Lab USA Inc. SpaceX is a registered trademark of Space Exploration Technologies Corp. United Launch Alliance and ULA are registered trademarks of United Launch Alliance, LLC. Virgin Galactic is a registered trademark of Galactic Enterprises, LLC.