How do I call an asynchronous method from within an ASPX file (not codebehind)?

Question

How do I call an asynchronous method from within an ASPX file (not codebehind)?

272 views Asked by John Wu At 26 December 2024 at 21:03

I have a DLL that contains an async method for obtaining a report. It does all the heavy lifting, including HTML formatting.

namespace MyReportEngine
{
    public class Renderer
    {
        static public async Task<string> GetReportHtml()
        {
            //Magic (not important)
        }
    }
}

I want to create a web site without doing a lot of work. All I want to have to do is create a simple ASPX file that uses the DLL. I tried this:

<%@ Assembly Name="MyAssembly" %>
<%@ Import Namespace="MyAssembly.MyReportEngine" %>
<%@ Page Async="true" %>

<html>
<body>
    <% =await Renderer.GetReportHtml() %>
</body>
</html>

This results in a runtime exception during JIT compilation:

Compiler Error Message: BC36937: 'Await' can only be used when contained within a method or lambda expression marked with the 'Async' modifier.

I've checked the documentation and tutorials like this one but they only seem to deal with adding async calls to the codebehind. There is no information on how to make the call from the ASPX markup itself. Is it possible?

Original Q&A

There are 3 answers

**JobesK** · Answer 1 · 2020-08-26T18:57:55+00:00

JobesK On 26 August 2020 at 18:57

Wrap it in a seperate method then call that method.

**Jeremy Thompson** · Answer 2 · 2020-08-27T11:15:00+00:00

Jeremy Thompson On 27 August 2020 at 11:15

Are you missing Page.RegisterAyncTask?

public void Page_Load(object sender, EventArgs e)
{
    RegisterAsyncTask(new PageAsyncTask(GetReportHtml));
}

Ref: https://www.hanselman.com/blog/TheMagicOfUsingAsynchronousMethodsInASPNET45PlusAnImportantGotcha.aspx

**Paulo Morgado** · Answer 3 · 2020-08-27T09:43:48+00:00

Avoid putting code in the markup.

For your use case, you can use a Literal control:

<%@ Assembly Name="MyAssembly" %>
<%@ Import Namespace="MyAssembly.MyReportEngine" %>
<%@ Page Async="true" %>

<html>
<body>
    <asp:Literal id="ReportHtml" runat="server" />
</body>
</html>

TechQA.

How do I call an asynchronous method from within an ASPX file (not codebehind)?

There are 3 answers

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