Build error "An item with the same key has already been added."

“dotnet build” throws an error when project.lock.json contains assemblies with the same name in different packages. While this seems logical on one side, it is quite hard to debug these issues.

My biggest concern is that this is a common problem in Microsoft packages as well. Packages like “Microsoft.Net.Http” and “System.Net.Http” contain assemblies with the same name. AFAIK people should move to the newer “System.Net.Http” package but I’m afraid that’s not always under our control. Some 3rd party libraries might still use “Microsoft.Net.Http” and others might already use “System.Net.Http”.

Steps to reproduce

These are two sample projects that fail:

{
  "version": "1.0.0-*",
  "compilationOptions": {
    "warningsAsErrors": true
  },

  "dependencies": {
    "Microsoft.Bcl.Immutable": "1.0.34",
    "Microsoft.AspNet.Mvc": "6.0.0-rc1-final"
  },

  "frameworks": {
    "net46": { }
  }
}

Error: An item with the same key has already been added. Key: (System.Collections.Immutable, neutral, b03f5f7f11d50a3a)

{
  "version": "1.0.0-*",
  "compilationOptions": {
    "warningsAsErrors": true
  },

  "dependencies": {
    "Hyak.Common": "1.1.0",
    "System.Net.Http.Formatting.Extension": "5.2.3"
  },

  "frameworks": {
    "net46": { }
  }
}

Error: An item with the same key has already been added. Key: (System.Net.Http.Extensions, neutral, b03f5f7f11d50a3a)

Expected behavior

I’m not sure if these are exactly the same libraries and a binding redirect would solve the problem. I’ve seen that there are already some other issues (eg dotnet/sdk#4805 ) on that topic.

If it can’t be solved with a binding redirect, maybe it can be resolved by renaming the libraries?

If this can’t be fixed, how should we proceed in such cases? If the vendor of the library doesn’t change his dependencies we might end up in a blocking situation.

Environment data

dotnet --version output:

.NET Command Line Tools (1.0.0-beta-001718)

Product Information:
 Version:     1.0.0-beta-001718
 Commit Sha:  5b0afe7880

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.10586
 OS Platform: Windows
 Runtime Id:  win10-x86

[SergeySagan]

I found another solution that seems to fix this problem, do a:

dotnet restore --no-cache

[hovsepm]

I start hitting this on VS Update 3.

D:\GitHub\azure-sdk-for-net\tools>dotnet --info
.NET Command Line Tools (1.0.0-preview1-002702)

Product Information:
 Version:     1.0.0-preview1-002702
 Commit Sha:  6cde21225e

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.10586
 OS Platform: Windows
 RID:         win10-x64

D:\GitHub\azure-sdk-for-net\tools>NuGet.exe update -self
Checking for updates from https://www.nuget.org/api/v2/.
Currently running NuGet.exe 3.5.0-beta2-1456.
NuGet.exe is up to date.

How to repro:

1. clone https://github.com/hovsepm/azure-sdk-for-net/
2. switch to AutoRest branch
3. open src\ResourceManagement\Authorization\Authorization.sln
4. build

Actual: Error An item with the same key has already been added. Key: Microsoft.Azure.Test.HttpRecorder TestFramework C:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v14.0\DotNet\Microsoft.DotNet.Common.Targets 241

Expected: Build successful.