SQL Connection Error: System.Data.SqlClient.SqlException (0x80131904)

I am seeing this in several situations and it is intermittent in our web based application connecting to SQL 2008 R2 serve back end. Users are coming across a point 2 point connection and seeing this on and off. Thought it was bandwidth issues until I started seeing it on terminal servers that are on the same core switch as this SQL server. I have checked remote connection enabled, Port 1433 is set correctly in Configuration for TCP/IP and the only thing I see that could be a cause is the timeout setting is set to 100000 in the remote connections rather than unlimited.

The error is

System.Data.SqlClient.SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server) --->

System.ComponentModel.Win32Exception (0x80004005): The network path was not found at

System.Data.SqlClient.SqlInternalConnection.OnError(SqlException
exception, Boolean breakConnection, Action`1 wrapCloseInAction) at
System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject
stateObj, Boolean callerHasConnectionLock, Boolean asyncClose) at
System.Data.SqlClient.TdsParser.Connect(ServerInfo serverInfo,
SqlInternalConnectionTds connHandler, Boolean ignoreSniOpenTimeout,
Int64 timerExpire, Boolean encrypt, Boolean trustServerCert, Boolean
integratedSecurity, Boolean withFailover) at
System.Data.SqlClient.SqlInternalConnectionTds.AttemptOneLogin(ServerInfo
serverInfo, String newPassword, SecureString newSecurePassword,
Boolean ignoreSniOpenTimeout, TimeoutTimer timeout, Boolean
withFailover) at
System.Data.SqlClient.SqlInternalConnectionTds.LoginNoFailover(ServerInfo
serverInfo, String newPassword, SecureString newSecurePassword,
Boolean redirectedUserInstance, SqlConnectionString connectionOptions,
SqlCredential credential, TimeoutTimer timeout) at
System.Data.SqlClient.SqlInternalConnectionTds.OpenLoginEnlist(TimeoutTimer
timeout, SqlConnectionString connectionOptions, SqlCredential
credential, String newPassword, SecureString newSecurePassword,
Boolean redirectedUserInstance) at
System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity
identity, SqlConnectionString connectionOptions, SqlCredential
credential, Object providerInfo, String newPassword, SecureString
newSecurePassword, Boolean redirectedUserInstance, SqlConnectionString
userConnectionOptions, SessionData reconnectSessionData) at
System.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions
options, DbConnectionPoolKey poolKey, Object poolGroupProviderInfo,
DbConnectionPool pool, DbConnection owningConnection,
DbConnectionOptions userOptions) at
System.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(DbConnectionPool
pool, DbConnection owningObject, DbConnectionOptions options,
DbConnectionPoolKey poolKey, DbConnectionOptions userOptions) at
System.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnection
owningObject, DbConnectionOptions userOptions, DbConnectionInternal
oldConnection) at
System.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnection
owningObject, DbConnectionOptions userOptions, DbConnectionInternal
oldConnection) at
System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection
owningObject, UInt32 waitForMultipleObjectsTimeout, Boolean
allowCreate, Boolean onlyOneCheckConnection, DbConnectionOptions
userOptions, DbConnectionInternal& connection) at
System.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection
owningObject, TaskCompletionSource`1 retry, DbConnectionOptions
userOptions, DbConnectionInternal& connection) at
System.Data.ProviderBase.DbConnectionFactory.TryGetConnection(DbConnection
owningConnection, TaskCompletionSource`1 retry, DbConnectionOptions
userOptions, DbConnectionInternal oldConnection, DbConnectionInternal&
connection) at
System.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection
outerConnection, DbConnectionFactory connectionFactory,
TaskCompletionSource`1 retry, DbConnectionOptions userOptions) at
System.Data.ProviderBase.DbConnectionClosed.TryOpenConnection(DbConnection
outerConnection, DbConnectionFactory connectionFactory,
TaskCompletionSource`1 retry, DbConnectionOptions userOptions) at
System.Data.SqlClient.SqlConnection.TryOpenInner(TaskCompletionSource`1
retry) at
System.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource`1
retry) at System.Data.SqlClient.SqlConnection.Open() at
System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.<>c__DisplayClass1.b__0()
at
System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute[TResult](Func`1
operation) at
System.Data.Entity.SqlServer.DefaultSqlExecutionStrategy.Execute(Action
operation) at
System.Data.Entity.Core.EntityClient.EntityConnection.Open()
ClientConnectionId:00000000-0000-0000-0000-000000000000

i had the same issue. go to Sql Server Configuration management->SQL Server network config->protocols for 'servername' and check named pipes is enabled.

I had the same issue.

Make sure that In SQL Server configuration --> SQL Server Services --> SQL Server Agent is enable

This solved my problem

Anyone who has this error, especially on Azure, try adding "tcp:" to the db-server-name in your connection string in your application. This forces the sql client to communicate with the db using tcp. I'm assuming the connection is UDP by default and there can be intermittent connection issues

I meet the same exception, it throws message like "Database 'YYYY' cannot be opened. It is in the middle of a restore." This is expected behavior for me, the sql server is actually in the middle of restore.

The error number is 927. For all the error numbers, refer to https://docs.microsoft.com/en-us/previous-versions/sql/sql-server-2008-r2/cc645603(v=sql.105)?redirectedfrom=MSDN

how to catch this type of sql exception:

try
{
    await db.ExecuteProcAsync(...);
}
catch (SqlException e) when (e.Number == 927)
{
    ...
}