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Error Cannot use a lambda expression

If you see an error like this:

Cannot use a lambda expression as an argument to a dynamically dispatched operation without first casting it to a delegate or expression tree type at System.Web.Compilation.AssemblyBuilder.Compile() at System.Web.Compilation.BuildProvidersCompiler.PerformBuild() at System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath virtualPath) at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate) at System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate) at System.Web.Compilation.BuildManager.GetVirtualPathObjectFactory(VirtualPath virtualPath, HttpContext context, Boolean allowCrossApp, Boolean throwIfNotFound) at System.Web.Compilation.BuildManager.GetCompiledType(VirtualPath virtualPath) at ToSic.Sxc.Engines.RazorEngine.CreateWebPageInstance() in

It usually means that you tried to write LINQ code like a .First(...) or .Select(...) on an object, and the compiler can't be sure that you tried to write LINQ.

Background: Dynamic Code and Extension Methods

Razor is dynamically compiled code, and many objects like Content are typed as dynamic. Because of this, the compiler can't be sure what's in a dynamic object, and also not what is in a Content.Tags - since this too is regarded as dynamic.

This is why you can't just write Content.Tags.First(), because .First() is an extension method which the compiler must find first - but it can't do that, since it doesn't know that Content.Tags are of the type IEnumerable<...>.

Solution #1 - use AsList(...)

2sxc 10.20 introduces AsList(...) which the compiler knows is an IEnumerable. Unfortunately if the the compiler isn't sure about Content.Tags, then it's also not sure about AsList(Content.Tags). This is a minor inconvenience, since AsList(...) would figure things out, but Razor wants to be sure. So to use AsList() for solving this problem, you'll need to write AsList(Content.Tags as object). That solves it.

Solution #2 - cast as IEnumerable<dynamic>

If you already know it's a list, you can also cast it as an IEnumerable<dynamic>. Since IEnumerable<T> is in the namespace System.Collections.Generic you have 3 options:

Cast with full Namespace

This is what the compiler actually understands - but it's a bit long and hard to read:

var authors = (book.Authors as System.Collections.Generic.IEnumerable<dynamic>)
    .Select(a => a.FirstName + " " + a.LastName);

Cast with @using and IEnumerable<dynamic>

This is the same thing, just nicer to read:

@using System.Collections.Generic;

var authors = (book.Authors as IEnumerable<dynamic>)
    .Select(a => a.FirstName + " " + a.LastName);

Cast with @using Dynlist = ...

This is the same thing, but the nicest, easiest to read method:

@using Dynlist = System.Collections.Generic.IEnumerable<dynamic>;
var authors = (book.Authors as Dynlist)
    .Select(a => a.FirstName + " " + a.LastName);

Shortlink: https://go.2sxc.org/ErrLambda

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