SQL Server can be accessed using several technologies, each of which has different connection string particulars. Connection strings are provider/driver specific so one first needs to decide on a client API before formulating the proper string can be created.
All connection strings share the same basic format, name/value pairs separated by semicolons, but the actual connection string keywords may vary by provider. Which keywords are required or optional also vary by provider and providers often share the same keywords (or provide synonyms) to minimize the connection string changes when switching between different providers. Most connection string keywords are optional and need to be specified only when the default is not appropriate. Connection string values should be enclosed in single or double quotes when the value may include a semicolon or equal sign (e.g. Password=”a&==b=;1@23″)
The purpose of a connection string is to supply a SQL Server provider/driver with the information needed to establish a connection to a SQL Server instance and may also be used to specify other configuration values, such as whether connection pooling is used. At the end of the day, the provider/driver needs to know at least:
One typically uses the .Net Framework Provider for SQL Server (abbreviated to SqlClient here) in managed code and a SQL Server OLE DB provider or ODBC driver from unmanaged code. It is possible to use OLE DB or ODBC for SQL Server data access in managed code but there is seldom a reason to do so since SqlClient offers high-performance access to SQL Server natively.
The SqlConnectionStringBuilder class provides a programmatic way to build connection strings needed by SqlConnection class. The nice thing about SqlConnectionStringBuilder is that it provides IntelliSense and avoids connection string typos. It should always be used when constructing connection strings based in user input (e.g. user id and password prompt). But you still need to know which connection string properties (keywords) you need to set along with the default values. The examples here apply regardless of whether or not you use yjr SqlConnectionStringBuilder class.
SqlClient Connection String Keyword Examples
Unlike other providers, there is no “Provider” or “Driver” connection string keyword in a SqlClient connection string. The .Net Framework Provider for SQL Server is implicit with a SqlConnection class so it is redundant to also specify the provider.
I’ll start with the minimal keyword(s) needed. The minimal SqlClient connection string need only specify the authentication method. The example below specifies Windows authentication using “Integrated Security=SSPI”. This connection string will connect the default instance on the local machine using Windows authentication under the current process Windows security credentials.
Listing 1: Connect to local default instance using Windows authentication
To connect to the local default instance using SQL authentication, just specify the credentials using the “User ID” and “Password” keywords instead of “Integrated Security=SSPI” keyword. SQL authentication is the default when “Integrated Security” or “Trused_Connection” keyword is not specified. Although I commonly see “Persist Security Info=False” also specified (a best practice from a security perspective), that is the default setting and may be omitted. Be aware that you should encrypt connection strings (or passwords in general) stored in configuration files when using SQL authentication.
Listing 2: Connect to local default instance using SQL authentication
One often connects to a remote SQL Server. Along with the authentication method, add the Data Source keyword to specify the desired SQL Server name or network address.
Data Source=SQLSERVERNAME;Integrated Security=SSPI
Listing 3: Connect to default instance on host SQLSERVERNAME using Windows authentication
Data Source=SQLSERVERNAME;User ID=MyLogin;Password=MiP@ssw0rd
Listing 4: Connect to instance on host SQLSERVERNAME using SQL authentication
Note that these same connection strings may be used to connect locally or remotely. Personally, I recommend always specifying the Data Source even when connecting locally. This makes it easy to move the application to another machine using with the same configuration and helps avoid oversights.
It is usually best to let SqlClient determine the appropriate network library to use rather than an explicit specification. SqlClient will figure out the appropriate network library based on the specified Data Source value. When you connect to a local instance using an unqualified name (or the value “(local)”), Shared Memory is used by default. SqlClient will use TCP/IP if a FQDN (e.g. SQLSERVERNAME.MyDOMAIN.COM) or IP address is specified regardless of whether the instance is local or remote. Since TCP/IP is most commonly used nowadays, I’ll focus on TCP/IP in this article and use a FQDN in the subsequent examples to avoid ambiguity.
It is often desirable to specify the initial database context in the connection sting. If omitted, the default database of the authenticated account is used. This is accomplished using either the “Initial Catalog” or “Database” keyword. I suggest always including the “Initial Catalog” keyword.
Data Source=SQLSERVERNAME.MYDOMAIN.COM;Integrated Security=SSPI;Initial Catalog=MyDatabase
Listing 4: Connect to default instance on host SQLSERVERNAME using Windows authentication with initial database context of MyDatabase
The connection strings I’ve shown so far assume the target is a default SQL Server instance listening on port 1433. One can run multiple instances of SQL Server on the same host using the named instance feature. If your target database instance is a named instance, SqlClient will also need to know the instance name or instance port number. The instance name can be specified by appending a backslash and instance name to the Data Source value:
Data Source=SQLSERVERNAME.MYDOMAIN.COM\MYINSTANCE;Integrated Security=SSPI;Initial Catalog=MyDatabase
Listing 5: Connect to named instance on host SQLSERVERNAME using Windows authentication with initial database context of MyDatabase
As an aside, I often see connectivity problems with named instances due to oversights in the SQL Server configuration. When an instance name is specified, SqlClient interrogates the SQL Server Brower service on the SQL Server host to determine the instance port (or named pipe name). The SQL Server Brower service is disabled by default so you need to enable and start it in order to connect by the instance name. This can be done using the SQL Server Configuration Manager tool. Also, since the SQL Server Brower service communicates over UDP port 1434, that port must be allowed through firewalls.
You can specify a port number instead of instance name to directly to a named instance (or to a default instance listing on a non-standard port). The port may be specified by appending a comma and port number to the data source value. The needed port number can be ascertained from the SQL Server Configuration Manager tool.
Data Source=SQLSERVERNAME.MYDOMAIN.COM,60086;Integrated Security=SSPI;Initial Catalog=MyDatabase
Listing 6: Connect to instance on host SQLSERVERNAME listening on port 60086 using Windows authentication with initial database context of MyDatabase
In addition to the “Data Source”, “Initial Catalog” and “Integrated Security” (or “User Id” and “Password”) keywords I’ve discussed so far, I recommend that “Application Name” also be specified. The specified string is helps identify the application when monitoring activity on the database server. This is especially useful when an application server or client hosts multiple applications.
Data Source=SQLSERVERNAME.MYDOMAIN.COM;Integrated Security=SSPI;Initial Catalog=MyDatabase;Application Name=Connection String Example
Listing 7: Connect to default instance on host SQLSERVERNAME using Windows authentication with initial database context of MyDatabase with application name specification
In my opinion, the many other keywords are noise unless the default values are inappropriate for your environment.
You can get by nicely in most cases with only the 4 or 5 SqlClient connection string keywords I’ve discussed here. I suggest you establish a connection string standard that includes the “Data Source”, “Initial Catalog”, “Application Name” keywords plus the authentication method, “Integrated Security=SSPI” or “User Id” and “Password”.