Visual Studio 2015 - Adding virtual directory not possible

Alternative if you have solution file ".sln", then follow the steps:

1. Use notepad open the solution file.

2. Find the row starts with Project("{

For example:

Project("{E24C65DC-7377-472B-9ABA-BC803B73C61A}") = "WebProject1", "WebProject1", ......

Then change it to be:

Project("{E24C65DC-7377-472B-9ABA-BC803B73C61A}") = "WebProject1", "http://localhost:12345/WebProject1", ......

And add new property

SlnRelativePath = "C:\WebProject1\"

within ProjectSection(WebsiteProperties)

In example, the ProjectSection from

Project("{E24C65DC-7377-472B-9ABA-BC803B73C61A}") = "WebProject1", "WebProject1", "{6D411253-734E-401A-A049-9620FC55D4FB}"
    ProjectSection(WebsiteProperties) = preProject
        UseIISExpress = "true"
        TargetFrameworkMoniker = ".NETFramework,Version%3Dv4.0"
        Debug.AspNetCompiler.VirtualPath = "/WebProject1"
        Debug.AspNetCompiler.PhysicalPath = "C:\WebProject1\"
        Debug.AspNetCompiler.TargetPath = "PrecompiledWeb\WebProject1\"
        Debug.AspNetCompiler.Updateable = "true"
        Debug.AspNetCompiler.ForceOverwrite = "true"
        Debug.AspNetCompiler.FixedNames = "false"
        Debug.AspNetCompiler.Debug = "True"
        Release.AspNetCompiler.VirtualPath = "/WebProject1"
        Release.AspNetCompiler.PhysicalPath = "C:\WebProject1\"
        Release.AspNetCompiler.TargetPath = "PrecompiledWeb\WebProject1\"
        Release.AspNetCompiler.Updateable = "true"
        Release.AspNetCompiler.ForceOverwrite = "true"
        Release.AspNetCompiler.FixedNames = "false"
        Release.AspNetCompiler.Debug = "False"
    EndProjectSection
EndProject

become this finally:

Project("{E24C65DC-7377-472B-9ABA-BC803B73C61A}") = "WebProject1", "http://localhost:12345/WebProject1", "{6D411253-734E-401A-A049-9620FC55D4FB}"
    ProjectSection(WebsiteProperties) = preProject
        UseIISExpress = "true"
        TargetFrameworkMoniker = ".NETFramework,Version%3Dv4.0"
        Debug.AspNetCompiler.VirtualPath = "/WebProject1"
        Debug.AspNetCompiler.PhysicalPath = "C:\WebProject1\"
        Debug.AspNetCompiler.TargetPath = "PrecompiledWeb\WebProject1\"
        Debug.AspNetCompiler.Updateable = "true"
        Debug.AspNetCompiler.ForceOverwrite = "true"
        Debug.AspNetCompiler.FixedNames = "false"
        Debug.AspNetCompiler.Debug = "True"
        Release.AspNetCompiler.VirtualPath = "/WebProject1"
        Release.AspNetCompiler.PhysicalPath = "C:\WebProject1\"
        Release.AspNetCompiler.TargetPath = "PrecompiledWeb\WebProject1\"
        Release.AspNetCompiler.Updateable = "true"
        Release.AspNetCompiler.ForceOverwrite = "true"
        Release.AspNetCompiler.FixedNames = "false"
        Release.AspNetCompiler.Debug = "False"
        SlnRelativePath = "C:\WebProject1\"
    EndProjectSection
EndProject

Neither of the answers above worked for me, I am still not sure why this works but here is what I did:

Modify the Applicationhost.config file that resides in the project:

Originally it looked like this:

<site name="Website" id="2">
    <application path="/" applicationPool="Clr4IntegratedAppPool">
        <virtualDirectory path="/" physicalPath="C:\...\MyWebsite" />
    </application>

    <bindings>
        <binding protocol="http" bindingInformation="*:51005:localhost" />
    </bindings>
</site>

I had to add another application path entry:

<application path="/mywebsite" applicationPool="Clr4IntegratedAppPool">
    <virtualDirectory path="/" physicalPath="C:\...\MyWebsite" />
</application>

So it ends up looking like this:

<site name="Website" id="2">
    <application path="/" applicationPool="Clr4IntegratedAppPool">
        <virtualDirectory path="/" physicalPath="C:\...\MyWebsite" />
    </application>

    <application path="/mywebsite" applicationPool="Clr4IntegratedAppPool">
        <virtualDirectory path="/" physicalPath="C:\...\MyWebsite" />
    </application>

    <bindings>
        <binding protocol="http" bindingInformation="*:51005:localhost" />
    </bindings>
</site>

FileSystem Web site to IIS Express.

1. Open VS2015 (Right click and select Run as administrator).
2. Click File > New > Web Site...
3. In the New Web Site dialog, click Browse... button.
4. Select Local IIS in the left pane. And select IIS Express Sites in the right pane.
5. There are 4 buttons in right side on top. Click Create New Site button.
6. Give name and click Open.This will select HTTP in Web location.
7. Click OK.
8. Then close the solution.
9. Open applicationhost.config from Users\\Documents\IISExpress\config folder. You can open in the notepad.
10. Find your web site in sites section. And change the physical path inside (virtualDirectory path="/" physicalPath="C:\WebApp\WebSite1"). Write the path of your existing Web Project.
11. Save and close applicationhost.config file.
12. Run VS2015 (Run as administrator).
13. Click File > Open > Web Site...
14. In the Open Web Site dialog, select Local IIS from left pane.
15. Select your site under IIS Express Sites and click Open.
16. Click Web Site menu and New Virtual Directory...
17. Give Alias Name and Folder path and click OK.

Now you can create it without error.