Substitution Component Samples

The Substitution component emulates the Web Forms asp:Substitution control, which was used to inject dynamic content into output-cached pages. In Blazor, this component invokes a Func<HttpContext, string> callback and renders the returned markup.

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Basic Callback

The SubstitutionCallback parameter accepts a function that receives the current HttpContext and returns an HTML string.

Rendered at: 6:04:22 PM

<Substitution SubstitutionCallback="@GetTimestamp" />

@code {
    private string GetTimestamp(HttpContext? context) =>
        $"<em>Rendered at: {DateTime.Now:h:mm:ss tt}</em>";
}

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Using HttpContext

The callback receives the current HttpContext, allowing access to request information such as headers, query strings, or the user agent.

Request: GET /ControlSamples/Substitution

<Substitution SubstitutionCallback="@GetRequestInfo" />

@code {
    private string GetRequestInfo(HttpContext? context)
    {
        if (context == null) return "<em>No HttpContext available</em>";
        var method = context.Request.Method;
        var path = context.Request.Path;
        return $"<span>Request: {method} {path}</span>";
    }
}

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MethodName Property

The MethodName property is preserved for migration reference. In Web Forms, this specified the name of a static callback method. In Blazor, use the SubstitutionCallback parameter instead.

Dynamic content generated at 6:04:22 PM

<Substitution MethodName="GetDynamicContent" SubstitutionCallback="@GetDynamicContent" />

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Web Forms Equivalent

<!-- Web Forms — used inside output-cached pages -->
<%@ OutputCache Duration="60" VaryByParam="None" %>

<asp:Substitution ID="Sub1" runat="server" MethodName="GetCurrentTime" />

<!-- Code-behind -->
public static string GetCurrentTime(HttpContext context)
{
    return DateTime.Now.ToString("h:mm:ss tt");
}