Question

With ASP.NET MVC 1.0, .NET 3.5 and C# you can easily pass a method a lambda expression that will do a “Response.Write” some content when it’s executed within the method:

<% Html.SomeExtensionMethod(
    () => { <%
        <p>Some page content<p>
    %> }
) %>

The method signature is similar to this:

public void SomeExtensionMethod(this HtmlHelper helper, Action pageContent)
{
    pageContent();
}

Does anyone know how to peforma a similar call using Lambda expressions in VB.NET using the same method signature shown above?

I’ve tried the following in VB.NET, but it wont work:

<% Html.SomeExtensionMethod(New Action( _
    Function() %> <p>Some Content</p> <% _
    )) %>

I get an exception saying “Expression Expected.”

Can anyone help me with what I’m doing wrong? How do you do this in VB.NET?

Answer 1

VB.NET 9.0 doesn’t support multi-statement lambda expressions or anonymous methods. It also does not support lambda expression that does not return a value meaning that you cannot declare an anonymous function of type Action.

Otherwise this works but is meaningless because the lambda does nothing:

<% Html.SomeExtensionMethod(New Action(Function() "<p>Some Content</p>"))%>

Now let’s take your example:

<% Html.SomeExtensionMethod(New Action( _
    Function() %> <p>Some Content</p> <% _
)) %>

Should be translated to something like this:

Html.SomeExtensionMethod(New Action(Function() Html.ViewContext.HttpContext.Response.Write("<p>Some Content</p>")))

which doesn’t compile in VB.NET 9.0 because your expression doesn’t return a value.

In VB.NET 10 you will be able to use the Sub keyword:

Html.SomeExtensionMethod(New Action(Sub() Html.ViewContext.HttpContext.Response.Write("<p>Some Content</p>")))

which will compile fine.