The extern alias 'xxx' was not specified in a /reference option

Question

I have two assemblies that unfortunately define the same type in the same namespace. I'm trying to use a an extern alias to work around the problem. In the Visual Studio IDE I have set the "Aliases" property of the reference to my alias. This is supposed to change the C# compiler command line to be something like this:

/reference:MyAlias=MyAssembly.dll

But it doesn't actually do that. The Visual Studio IDE seems to just ignore the Aliases property setting on the reference. So when I go and add the line extern alias MyAlias; at the top of my C# code file I get the error that the alias was not specified in a /reference option to the compiler. I can't figure out what I am doing wrong. Any ideas?

Please post the build output (especially the line that runs Csc.exe). — Allon Guralnek, Apr 26 '10 at 07:11
The build output can be seen in another question here http://stackoverflow.com/questions/2503232/visual-studio-compiles-wpf-application-twice-during-build. I now believe these issues are related. The build fails because the first compiler line does not include the alias. — Brian Ensink, May 03 '10 at 14:42

orcun · Accepted Answer · 2010-10-25T07:14:19.373

I have the same problem and I was able to reproduce the issue.

It turns out reference aliases are ignored on projects containing xaml files which has an xmlns definition to the output assembly like xmlns:local='clr-namespace:TestProject'.

If you think this is your case as well, please vote up my bug report at Microsoft Connect.

EDIT: There is a suggested workaround in the link above which requires editing the project file manually. In order for this to work, I had to give full path of the assembly. Add the following instructions to the end of your project file:

<Target Name="solveAliasProblem" >
<ItemGroup>
 <ReferencePath Remove="FullPath.dll"/>
 <ReferencePath Include="FullPath.dll">
    <Aliases>ourAlias</Aliases>
 </ReferencePath>
</ItemGroup>
</Target>
<PropertyGroup>
    <CoreCompileDependsOn>solveAliasProblem;$(PrepareResourcesDependsOn)</CoreCompileDependsOn>
</PropertyGroup>

I tried the above code and ended with this new error - Could not load file or assembly '[AliasName]' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. — Nithin Paul, Aug 01 '14 at 10:57
The problem I ran into was that I typed in an alias and clicked away and I didn't realize that it didn't save. I had to click in another field in the properties window to commit it. — The Muffin Man, Mar 14 '15 at 00:05

score 12 · Answer 2 · answered Mar 15 '11 at 17:42

Brian, I had the same problem as you and I figured out how to fix it.

I would do the same thing as you:

Use the properties window to change the alias for the assembly from 'global' to 'MyAlias'
At the top of the file where the aliased assembly is used, put extern alias MyAlias. This must be before any using statements.
Use the alias prefix to use the namespace you want, for example using MyAlias::MyNamespace.

And I would still get the error. After screwing around a bit, I figured out that the way to fix it is to set the referenced dll to a dll that is outside of the solution you are working in. Once I did that, the error message we were both seeing went away, and I was able to continue working on my project.

I hope that helps, happy coding!

score 10 · Answer 3 · answered Jun 20 '11 at 21:29

10

An update: It is a bug. It will be fixed in .Net 4.5.

As seen on the Microsoft Connect bug report.

answered Jun 20 '11 at 21:29

Frederic Gaudet

161
1
5

1

This link is now dead :-( – Ganesh Sittampalam Oct 02 '18 at 06:54
1

also still having the bug in .net 4.6.2 – Matt Jan 31 '19 at 22:02

score 5 · Answer 4 · answered May 10 '10 at 15:08

5

Working for me in VS2008 using these steps:

Use the properties window to change the alias for the assembly from 'global' to 'MyAlias'
At the top of the file where the aliased assembly is used, put extern alias MyAlias. This must be before any using statements.
Use the alias prefix to use the namespace you want, for example using MyAlias::MyNamespace.

answered May 10 '10 at 15:08

andypaxo

6,171
3
38
53

Works in VS2010 fine for me. Might have been fixed in SP1. – Doguhan Uluca Sep 02 '11 at 03:03
2

But for me with no luck still getting error "The extern alias was not specified in a /reference option" – Nithin Paul Aug 01 '14 at 05:27

score 3 · Answer 5 · answered Feb 15 '13 at 20:17

3

If you install .Net Framework 4.5 it will update your MSBuild to Microsoft (R) Build Engine version 4.0.30319.17929 which fixed this issue for my team.

Download .Net Framework 4.5:
http://www.microsoft.com/en-us/download/details.aspx?id=30653

answered Feb 15 '13 at 20:17

Charlie

846
7
21

score 2 · Answer 6 · answered Sep 15 '10 at 16:31

I had exact similar issue, looks like when you clean your solution and build the project, first time MSBuild ignores references and gives error (may be it does not have some assemblies built yet with the references specified), however on second try of the build, it works.

score 2 · Answer 7 · answered Jan 12 '11 at 08:18

2

I had the same problem and it was resolved by doing the following.

Go to properties window of the assembly with the conflict and change the Aliases property from 'global' to 'X' (X - your alias name)

Hope this helps.

answered Jan 12 '11 at 08:18

Kusal Munasinghe

31
1

score 1 · Answer 8 · answered Jun 11 '15 at 22:08

1

Set the project's target framework to .NET 4.5.1.

answered Jun 11 '15 at 22:08

kat

577
4
7

score 1 · Answer 9 · answered Feb 18 '16 at 15:17

myprojectHad the same issue, but the above solutions didn't work my project file looked like

    <Reference Include="AntiXSSLibrary" Condition="'$(TargetFrameworkVersion)' == 'v4.0' or '$(TargetFrameworkVersion)' == 'v4.5'">
  <HintPath>..\Dependencies\Third-party\.net 4\AntiXSSLibrary.dll</HintPath>
  <Aliases>antixss</Aliases>
</Reference>
<Reference Include="AntiXssLibrary">
  <HintPath>..\..\lib\myproject\AntiXssLibrary.dll</HintPath>     
</Reference>

Adding the alias the actual project dll worked. So

    <Reference Include="AntiXSSLibrary" Condition="'$(TargetFrameworkVersion)' == 'v4.0' or '$(TargetFrameworkVersion)' == 'v4.5'">
  <HintPath>..\Dependencies\Third-party\.net 4\AntiXSSLibrary.dll</HintPath>
  <Aliases>antixss</Aliases>
</Reference>
<Reference Include="AntiXssLibrary">
  <HintPath>..\..\lib\myproject\AntiXssLibrary.dll</HintPath>
  <Aliases>antixss</Aliases>
</Reference>

score 1 · Answer 10 · answered Mar 01 '17 at 08:58

1

I had the same problem and I finally found that the .csproj file has target version of 4.5 while the dll which I faced the problem with is targeted to 4.5.2.

I changed my .csproj file target version to 4.5.2 and got successful build.

answered Mar 01 '17 at 08:58

Sudheer

81
7

score 1 · Answer 11 · answered Dec 05 '17 at 16:01

1

I had the same problem, and I could solve it only when I changed the target version to 4.6.1. The versions 4, 4.5, 4.5.1, or 4.6 did not help.

answered Dec 05 '17 at 16:01

JarmoP

331
3
3

score 1 · Answer 12 · answered May 04 '19 at 21:32

Slightly different solution from the one orcun's suggested:

Manually add this to the project csproj file:

  <Target Name="ChangeAliasesOfStrongNameAssemblies" BeforeTargets="FindReferenceAssembliesForReferences;ResolveReferences">
    <ItemGroup>
      <!-- Use the file name, no need to specify the full path. -->
      <ReferencePath Condition="'%(FileName)' == 'StackExchange.Redis.StrongName'">
        <!-- Give the desired alias to the file here -->
        <Aliases>myAlias</Aliases>
      </ReferencePath>
    </ItemGroup>
  </Target>

Than, you can use it by adding the alias on the top of your using list:

extern alias myAlias;
...
var test = new myAlias::Fully.Qualified.NameSpace();

BTW, this one works for both References and PackageReferences!

Solution was found here: https://github.com/NuGet/Home/issues/4989#issuecomment-311042085

score 1 · Answer 13 · answered Aug 10 '21 at 07:08

1

Our Build agent had this issue for a .net core 3.1 app. Fixed it by installing .net 5.0 sdk on the build agent, which upgraded the build engine version and it worked.

answered Aug 10 '21 at 07:08

user16630481

11
1

score 0 · Answer 14 · answered Nov 25 '18 at 01:27

0

Please check your Project Target framework Version. Must be same version.

I change the same version fixed the issue.

enter image description here

answered Nov 25 '18 at 01:27

Jason Yang

11
1

score 0 · Answer 15 · answered Mar 17 '19 at 15:08

0

Had the same issue after installing some NUget packages if the path to my project contains "=" sign. I had a directory like "D:\=Projects..." and renaming it solved the issue.

answered Mar 17 '19 at 15:08

Vladimir

429
7
21

Jon Ediger · Answer 16 · 2021-07-30T20:59:27.760

Using core 3.1 and VS 16.10.4:

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <RuntimeIdentifier>win-x64</RuntimeIdentifier>
    <OutputType>Exe</OutputType>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Hl7.Fhir.STU3" Version="3.4.0">
      <Aliases>Stu3</Aliases>
    </PackageReference>
    <PackageReference Include="Hl7.Fhir.R4" Version="3.4.0">
      <Aliases>R4</Aliases>
    </PackageReference>
  </ItemGroup>

  <ItemGroup>
    <TransitivePackageReference Include="Hl7.Fhir.STU3.Core" Alias="STU3" />
    <TransitivePackageReference Include="Hl7.Fhir.R4.Core" Alias="R4" />
  </ItemGroup>

</Project>

The extern alias 'xxx' was not specified in a /reference option

16 Answers16

Linked