MVC view compilation using wrong .NET Framework

Question

An existing web application is using .NET Framework 4.5.1. The project has the setting MvcBuildViews set to true. The project also uses a Prepend extension method for IEnumerable<T>.

Stuff breaks miraculously after merely installing the .NET Framework 4.7.1 Developer Pack.

Even though the web.config still targets framework 4.5.1:

<system.web>
    <compilation debug="true" targetFramework="4.5.1" />
    <httpRuntime targetFramework="4.5.1" />
    ...

at runtime the view appears to be compiled using 4.7.1, leading to an ambiguous call (because 4.7.1 has its own Prepend extension):

Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.

Compiler Error Message: CS0121: The call is ambiguous between the following methods or properties: 'System.Linq.Enumerable.Prepend(System.Collections.Generic.IEnumerable, TSource)' and 'WebApplication2.EnumerableExtensions.Prepend(System.Collections.Generic.IEnumerable, T)'

How do I solve this issue without upgrading all applications to 4.7.1? It should be possible to use 4.5.1 and 4.7.1 side-by-side, right?!

Reproduction steps:

Create a new ASP.NET Web Application (.NET Framework) project for .NET Framework 4.5.1;
Update package Microsoft.CodeDom.Providers.DotNetCompilerPlatform to version 1.0.8;

Add this class:

public static class EnumerableExtensions
{
    public static IEnumerable<T> Prepend<T>(this IEnumerable<T> source, T item)
    {
        if (source == null)
        {
            throw new ArgumentNullException(nameof(source));
        }

        return Iterator();

        IEnumerable<T> Iterator()
        {
            yield return item;
            foreach (var i in source)
            {
                yield return i;
            }
        }
    }
}

Change About.cshtml to this:

@{
    ViewBag.Title = "About";
    var a = new List<string> { "a", "b" };
    a.Prepend("c");
}
<h2>@ViewBag.Title.</h2>
<h3>@ViewBag.Message</h3>

<p>Use this area to provide additional information.</p>
@foreach (var s in a)
{
    <h4>@s</h4>
}

Run the application from Visual Studio (IIS Express) and see a working About page;
Install .NET Framework 4.7.1 Dev Pack;
Run the application again and see it fail...

Issue reported at Microsoft Developer Community: https://developercommunity.visualstudio.com/content/problem/150969/mvc-view-compilation-using-wrong-framework.html

MVC view compilation using wrong .NET Framework

0 Answers0