How to Fix, Could not load file or assembly 'XXX' or one of its dependencies. Strong name signatu...

Fixing 'Could not load file or assembly' Strong Name Signature Errors in .NET

Learn to diagnose and resolve the 'Could not load file or assembly 'XXX' or one of its dependencies. Strong name signature could not be verified' error in C# and ASP.NET applications.

The error message "Could not load file or assembly 'XXX' or one of its dependencies. Strong name signature could not be verified" is a common and often frustrating issue encountered by developers working with .NET applications, particularly in C# and ASP.NET environments. This error typically indicates a problem with the strong-name signing of an assembly, which is a security feature in .NET designed to guarantee the uniqueness and integrity of an assembly. When the runtime attempts to load an assembly and its strong name signature cannot be verified, it throws this exception, preventing the application from starting or functioning correctly.

Understanding Strong-Name Signing

Strong-name signing is a cryptographic process that uses a private key to sign an assembly and a public key to verify it. This process creates a unique identity for the assembly, ensuring that it hasn't been tampered with and that it originates from a trusted publisher. It's crucial for shared components, especially those installed in the Global Assembly Cache (GAC), where multiple applications might rely on the same assembly. The verification process checks if the assembly's hash matches the hash stored in its manifest, and if the publisher's public key matches the one used to sign the assembly. If any of these checks fail, the strong name signature cannot be verified, leading to the error.

flowchart TD
    A[Application Starts]
    B{Load Assembly 'XXX'}
    C[Check Strong Name Signature]
    D{Signature Valid?}
    E[Assembly Loaded Successfully]
    F[Error: Strong Name Signature Could Not Be Verified]

    A --> B
    B --> C
    C --> D
    D -- Yes --> E
    D -- No --> F

Flowchart of Strong Name Signature Verification Process

Common Causes and Solutions

This error can stem from several issues, ranging from incorrect assembly versions to corrupted files or even security policies. Identifying the root cause is key to resolving it. Here are the most common scenarios and their respective solutions:

1. Mismatched Assembly Versions or Corrupted Files

One of the most frequent causes is a mismatch between the version of the assembly your application expects and the version actually present, or a corrupted assembly file. This often happens when updating NuGet packages or deploying new versions of dependencies.

1. Clean and Rebuild Solution

In Visual Studio, go to 'Build' -> 'Clean Solution', then 'Build' -> 'Rebuild Solution'. This ensures all old build artifacts are removed and new ones are generated.

2. Delete Bin/Obj Folders

Manually delete the bin and obj folders from your project directory. These folders contain compiled assemblies and intermediate files that might be stale or corrupted. After deletion, rebuild your solution.

3. Update NuGet Packages

Ensure all NuGet packages are up-to-date and consistent across your solution. Use the NuGet Package Manager to update or reinstall problematic packages.

4. Check web.config or app.config

Look for <bindingRedirect> entries in your web.config or app.config file. These redirects tell the runtime to use a different version of an assembly than what was compiled against. Ensure they are correct and point to the actual version of the assembly you intend to use. Sometimes, automatic binding redirects generated by Visual Studio can be incorrect or outdated.

<configuration>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="Newtonsoft.Json" publicKeyToken="30ad4fe6b2a6aeed" culture="neutral" />
        <bindingRedirect oldVersion="0.0.0.0-13.0.0.0" newVersion="13.0.0.0" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
</configuration>

Example of an assembly binding redirect in web.config

2. Disabling Strong Name Verification (Development Only)

For development and debugging purposes, you can temporarily disable strong name verification for a specific assembly. This is not recommended for production environments as it compromises the security and integrity checks. This method is useful for quickly identifying if the strong name verification itself is the problem, or if there's another underlying issue.

1. Identify the Assembly

Note the full name of the assembly mentioned in the error message (e.g., 'XXX').

2. Use sn.exe to Disable Verification

Open a Developer Command Prompt for Visual Studio as an administrator. Use the sn.exe tool (Strong Name tool) to disable verification for the specific assembly. You'll need its public key token. If you don't know the public key token, you can find it using sn.exe -Tp <assembly_path>.

3. Command to Disable Verification

Execute sn.exe -Vr <assembly_name>,<public_key_token> (e.g., sn.exe -Vr "MyAssembly, 0123456789abcdef").

4. Command to Re-enable Verification

Once debugging is complete, remember to re-enable verification using sn.exe -Vu <assembly_name>,<public_key_token>.

:: To find the public key token of an assembly
sn.exe -Tp "C:\Path\To\Your\Assembly.dll"

:: To disable strong name verification for an assembly
sn.exe -Vr "MyAssembly, 0123456789abcdef"

:: To re-enable strong name verification
sn.exe -Vu "MyAssembly, 0123456789abcdef"

Using sn.exe to manage strong name verification

3. Global Assembly Cache (GAC) Issues

If the problematic assembly is intended to be in the GAC, issues can arise if the version in the GAC is different from what your application expects, or if the GAC entry is corrupted.

1. Check GAC Contents

Navigate to C:\Windows\Microsoft.NET\assembly (for .NET 4.x) or C:\Windows\assembly (for .NET 2.0/3.5) and locate the assembly. Verify its version and public key token.

2. Uninstall and Reinstall from GAC

If you suspect a GAC issue, try uninstalling the assembly from the GAC using gacutil.exe /u <assembly_name> and then reinstalling it with gacutil.exe /i <assembly_path>.

:: Uninstall an assembly from the GAC
gacutil.exe /u "MyAssembly, Version=1.0.0.0, Culture=neutral, PublicKeyToken=0123456789abcdef"

:: Install an assembly into the GAC
gacutil.exe /i "C:\Path\To\MyAssembly.dll"

Managing assemblies in the GAC using gacutil.exe

4. Security Policy and Publisher Policy Files

In rare cases, security policies or publisher policy files might interfere with assembly loading. Publisher policy files are used to redirect assembly references from one version of a strong-named assembly to another. While less common now, they can still cause issues.

If you suspect a publisher policy issue, you might need to investigate the machine.config file or specific policy files on the system. However, this is an advanced scenario and usually not the first place to look.