ASP.NET: How to create a connection from a web.con

问题:

How do you construct a DbConnection based on a provider name?

Sample provider names

• System.Data.SqlClient
• System.Data.OleDb
• System.Data.Odbc
• FirebirdSql.Data.FirebirdClient

i have connection strings stored in my IIS server's web.config file:

<connectionStrings>
  <add name="development"
        connectionString="Provider = IBMDA400; Data Source = MY_SYSTEM_NAME; User Id = myUsername; Password = myPassword;" 
        providerName="System.Data.OleDb" />
  <add name="live" 
        connectionString="usd=sa;pwd=password;server=deathstar;" 
        providerName="System.Data.Odbc" />
  <add name="testing" 
        connectionString="usd=sa;pwd=password;server=deathstar;" 
        providerName="System.Data.SqlClient" />
  <add name="offline"
        connectionString="Server=localhost;User=SYSDBA;Password=masterkey;Charser=NONE;Database=c:\data\mydb.fdb"
        providerName="FirebirdSql.Data.FirebirdClient"/>

You can see they all use different providers. When it comes time for me to create a connection, i have to know what kind of DbConnection to create, e.g.:

• SqlConnection
• OleDbConnection
• OdbcConnection
• FbConnection

The connectionStrings entries contains a providerName, but these aren't the names of DbConnection descendant classes, but appear to be a namespace

How do i turn construct a DbConnection based on a string providerName?

---

public DbConnection GetConnection(String connectionName)
{
    //Get the connectionString infomation
    ConnectionStringSettings cs = 
          ConfigurationManager.ConnectionStrings[connectionName];
    if (cs == null)
       throw new ConfigurationException("Invalid connection name \""+connectionName+"\");

    //Create a connection based on the provider
    DbConnection conn = new DbConnection();

}

回答1:

If you go this route, I think you'll want to use the DbProviderFactories class to get a DbProviderFactory that you can use to construct the connection. I haven't tried this code out, but I think it will work. It's possible that you may need to look up the provider name using the GetFactoryClasses method on the DbProviderFactories class and use the InvariantName.

public DbConnection GetConnection(String connectionName)
{
   //Get the connection string info from web.config
   ConnectionStringSettings cs= 
         ConfigurationManager.ConnectionStrings[connectionName];

   //documented to return null if it couldn't be found
   if (cs == null)
      throw new ConfigurationErrorsException("Invalid connection name \""+connectionName+"\"");

   //Get the factory for the given provider (e.g. "System.Data.SqlClient")
   DbProviderFactory factory = 
         DbProviderFactories.GetFactory(cs.ProviderName);

   //Undefined behaviour if GetFactory couldn't find a provider.
   //Defensive test for null factory anyway
   if (factory == null)
      throw new Exception("Could not obtain factory for provider \""+cs.ProviderName+"\"");

   //Have the factory give us the right connection object      
   DbConnection conn = factory.CreateConnection();

   //Undefined behaviour if CreateConnection failed
   //Defensive test for null connection anyway
   if (conn == null)
      throw new Exception("Could not obtain connection from factory");

   //Knowing the connection string, open the connection
   conn.ConnectionString = cs.ConnectionString;
   conn.Open()

   return conn;
}

回答2:

Check out this Hanselman blog on adding custom build types for different connection strings names, it looks like it may fit what you want to accomplish in a different way than using the provider types.

回答3:

If the providerName for the particular connection name (dev, test, prod) never changes why cant you do a switch on the connectionName param for your method and set the providerName instance that way?