Unable to load DLL 'Microsoft.Data.SqlClient.SNI.dll'

[adrianm64]

Describe the bug

I have a windows service which works fine for maybe a week and then it suddenly gets exception DllNotFoundException: Unable to load DLL 'Microsoft.Data.SqlClient.SNI.dll' in SqlConnection.Open

Retrying the operation gives same exception.

Restarting the service fixes the problem for another week or so.

Have seen this in 2 different applications running on different servers.
One application is busy during working hours, the other only accesses the database maybe 10 times a day.

• TypeInitializationException: The type initializer for 'Microsoft.Data.SqlClient.TdsParser' threw an exception.
  TypeName = "Microsoft.Data.SqlClient.TdsParser"
• TypeInitializationException: The type initializer for 'Microsoft.Data.SqlClient.SNILoadHandle' threw an exception.
  TypeName = "Microsoft.Data.SqlClient.SNILoadHandle"
• DllNotFoundException: Unable to load DLL 'Microsoft.Data.SqlClient.SNI.dll' or one of its dependencies: The specified module could not be found. (0x8007007E)
  TypeName = ""
  at Microsoft.Data.SqlClient.SNINativeMethodWrapper.SNIInitialize(IntPtr pmo)
  at Microsoft.Data.SqlClient.SNINativeMethodWrapper.SNIInitialize()
  at Microsoft.Data.SqlClient.SNILoadHandle..ctor()
  at Microsoft.Data.SqlClient.SNILoadHandle..cctor()
  at Microsoft.Data.SqlClient.TdsParserStateObjectFactory.get_EncryptionOptions()
  at Microsoft.Data.SqlClient.TdsParser..cctor()
  at Microsoft.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, UInt32 waitForMultipleObjectsTimeout, Boolean allowCreate, Boolean onlyOneCheckConnection, DbConnectionOptions userOptions, DbConnectionInternal& connection)
  at Microsoft.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, TaskCompletionSource`1 retry, DbConnectionOptions userOptions, DbConnectionInternal& connection)
  at Microsoft.Data.ProviderBase.DbConnectionFactory.TryGetConnection(DbConnection owningConnection, TaskCompletionSource`1 retry, DbConnectionOptions userOptions, DbConnectionInternal oldConnection, DbConnectionInternal& connection)
  at Microsoft.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)
  at Microsoft.Data.ProviderBase.DbConnectionClosed.TryOpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)
  at Microsoft.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource`1 retry, SqlConnectionOverrides overrides)
  at Microsoft.Data.SqlClient.SqlConnection.Open(SqlConnectionOverrides overrides)
  at Microsoft.Data.SqlClient.SqlConnection.Open()

To reproduce

Expected behavior

Expect no exception.

Further technical details

Microsoft.Data.SqlClient version: 5.2.2
.NET target: net 8.0
SQL Server version: SQL Server 2019
Operating system: Windows Server 2019

Edit: Sorry, forgot an important detail!

Both applications use "Produce single file" and has <IncludeNativeLibrariesForSelfExtract>true</IncludeNativeLibrariesForSelfExtract>
Could it be that the self-extract can only load a native assembly once?

Additional context
Looked at the troubleshooting guide but it just says "Microsoft.Data.SqlClient doesn't manage loading/unloading of this library in .NET Core.".
What manages the loading/unloading and why does it get unloaded while the application is running?

[David-Engel]

This is very strange behavior. It makes me suspect some kind of security software interfering with file access. Just something to look for.

[cheenamalhotra]

Looked at the troubleshooting guide but it just says "Microsoft.Data.SqlClient doesn't manage loading/unloading of this library in .NET Core.".
What manages the loading/unloading and why does it get unloaded while the application is running?

In .NET Runtime, DLLs are loaded by the runtime itself, based on the RID configured in client application. Read more here: https://learn.microsoft.com/en-us/dotnet/core/rid-catalog#loading-assets-for-a-specific-os-version-or-distribution

Supported RIDs can be located in the NuGet package: https://nuget.info/packages/Microsoft.Data.SqlClient.SNI.runtime/6.0.2

You can try providing this in your csproj:

    <RuntimeIdentifier>win-x64</RuntimeIdentifier>

[wstaelens]

Error: 'System.TypeInitializationException: The type initializer for 'Microsoft.Data.SqlClient.TdsParser' threw an exception.
---> System.TypeInitializationException: The type initializer for 'Microsoft.Data.SqlClient.SNILoadHandle' threw an exception.
---> System.DllNotFoundException: Unable to load DLL 'Microsoft.Data.SqlClient.SNI.dll' or one of its dependencies: The media is write protected. (0x80070013)
   at Microsoft.Data.SqlClient.SNINativeMethodWrapper.SNIInitialize(IntPtr pmo)
   at Microsoft.Data.SqlClient.SNINativeMethodWrapper.SNIInitialize(IntPtr pmo)
   at Microsoft.Data.SqlClient.SNINativeMethodWrapper.SNIInitialize()
   at Microsoft.Data.SqlClient.SNILoadHandle..ctor()
   at Microsoft.Data.SqlClient.SNILoadHandle..ctor()
   at Microsoft.Data.SqlClient.SNILoadHandle..cctor()
   --- End of inner exception stack trace ---
   at Microsoft.Data.SqlClient.TdsParserStateObjectFactory.get_EncryptionOptions()
   at Microsoft.Data.SqlClient.TdsParser..cctor()
   --- End of inner exception stack trace ---
   at Microsoft.Data.SqlClient.TdsParser..ctor(Boolean MARS, Boolean fAsynchronous)
   at Microsoft.Data.SqlClient.SqlInternalConnectionTds.LoginNoFailover(ServerInfo serverInfo, String newPassword, SecureString newSecurePassword, Boolean redirectedUserInstance, SqlConnectionString connectionOptions, SqlCredential credential, TimeoutTimer timeout)
   at Microsoft.Data.SqlClient.SqlInternalConnectionTds.OpenLoginEnlist(TimeoutTimer timeout, SqlConnectionString connectionOptions, SqlCredential credential, String newPassword, SecureString newSecurePassword, Boolean redirectedUserInstance)
   at Microsoft.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, SqlCredential credential, Object providerInfo, String newPassword, SecureString newSecurePassword, Boolean redirectedUserInstance, SqlConnectionString userConnectionOptions, SessionData reconnectSessionData, Boolean applyTransientFaultHandling, String accessToken, DbConnectionPool pool, Func`3 accessTokenCallback)
   at Microsoft.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, DbConnectionPoolKey poolKey, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection, DbConnectionOptions userOptions)
   at Microsoft.Data.ProviderBase.DbConnectionFactory.CreateNonPooledConnection(DbConnection owningConnection, DbConnectionPoolGroup poolGroup, DbConnectionOptions userOptions)
   at Microsoft.Data.ProviderBase.DbConnectionFactory.TryGetConnection(DbConnection owningConnection, TaskCompletionSource`1 retry, DbConnectionOptions userOptions, DbConnectionInternal oldConnection, DbConnectionInternal& connection)
   at Microsoft.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)
   at Microsoft.Data.ProviderBase.DbConnectionClosed.TryOpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource`1 retry, DbConnectionOptions userOptions)
   at Microsoft.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource`1 retry, SqlConnectionOverrides overrides)
   at Microsoft.Data.SqlClient.SqlConnection.Open(SqlConnectionOverrides overrides)
   at Microsoft.Data.SqlClient.SqlConnection.Open()
   at DevExpress.Xpo.DB.MSSqlConnectionProvider.CreateDataBase(IDbConnection conn)'
---> System.TypeInitializationException: The type initializer for 'Microsoft.Data.SqlClient.TdsParser' threw an exception.
---> System.TypeInitializationException: The type initializer for 'Microsoft.Data.SqlClient.SNILoadHandle' threw an exception.
---> System.DllNotFoundException: Unable to load DLL 'Microsoft.Data.SqlClient.SNI.dll' or one of its dependencies: The media is write protected. (0x80070013)
   at Microsoft.Data.SqlClient.SNINativeMethodWrapper.SNIInitialize(IntPtr pmo)
   at Microsoft.Data.SqlClient.SNINativeMethodWrapper.SNIInitialize(IntPtr pmo)
   at Microsoft.Data.SqlClient.SNINativeMethodWrapper.SNIInitialize()
   at Microsoft.Data.SqlClient.SNILoadHandle..ctor()
   at Microsoft.Data.SqlClient.SNILoadHandle..ctor()
   at Microsoft.Data.SqlClient.SNILoadHandle..cctor()

System.DllNotFoundException: Unable to load DLL 'Microsoft.Data.SqlClient.SNI.dll' or one of its dependencies: The media is write protected. (0x80070013)

similar situation today .net 9, windows server 2019, sql server 2019. web service (blazor server/signalr) completely frozen and seen in logs issues with SqlClient.SNI.dll
app was also working perfectly for several days.

No idea what is/was causing this.

[gabriel-aquino-jr]

I had the same error in a .NET Framework application that was mistakenly using Microsoft.Data.SqlClient. I removed the reference and used System.Data.SqlClient instead, and it worked.

[adrianm64]

You can try providing this in your csproj:

    <RuntimeIdentifier>win-x64</RuntimeIdentifier>

I don't have it in csproj but in the publishprofile. Does that make any difference?

The applications where it originally happened haven't had the problem for several weeks now. 2 other applications had the problem last week. One of them had it twice a couple of hours apart.

The randomness makes it difficult for me to "try" things and know if it helps.

[cheenamalhotra]

The applications where it originally happened haven't had the problem for several weeks now. 2 other applications had the problem last week. One of them had it twice a couple of hours apart.

That clearly doesn't seem like a library problem to me if you haven't changed your application code and dependent driver version.

What else changed recently can you elaborate? Did you update .NET Runtime recently or anything else that is contributing to .NET runtime assembly resolution issues?

[cheenamalhotra]

I had the same error in a .NET Framework application that was mistakenly using Microsoft.Data.SqlClient. I removed the reference and used System.Data.SqlClient instead, and it worked.

Could you elaborate or provide us a repro of your application structure where SNI.dll is not loading? We are gathering reports and usecases where customers are facing challenges migrating their apps off SDS to MDS, and this looks like one of those. If you can provide us a repro, that way we can help with solution projects right away.

[wstaelens]

happens in production, no repro. Only thing that happened in production is windows server 2019 updates and .net (hosting) runtime updates.