Recently I needed the ability to modify the current database for an application at runtime.  There are lots of ways of doing this, for example the IDbConnection interface defines the ChangeDatabase method allowing you to do just that.  Alternatively, you will have abstracted away your connection object behind a factory or inject it in using your favourite IoC tool.

However, I was faced with some old code that created the SqlConnection object as needed in hundreds of different places, and didn’t have the opportunity to go through and replace all of these references, so looked at modifying the ConfigurationManager.ConnectionStrings collection directly.  I thought that would be easy enough, but the base ConfigurationElement class has a read-only flag preventing modification.  There’s always a way though… as long as you use reflection you can indeed modify the connection string!

//Update the readonly flag to false, using reflection:
var settings = ConfigurationManager.ConnectionStrings["MyConnectionName"];
var fieldInfo = typeof(ConfigurationElement).GetField("_bReadOnly", BindingFlags.Instance | BindingFlags.NonPublic);
fieldInfo.SetValue(settings, false);

//Create a connection string builder as it makes it easy to modify just the DB name:
var builder = new SqlConnectionStringBuilder(ConfigurationManager.ConnectionStrings["MyConnectionName"].ConnectionString);
builder.InitialCatalog = dbName;  //You can also change, server, user, password, etc here, if required
//Update the connection string setting:
settings.ConnectionString = builder.ConnectionString;

Any new connections created after this will use the new connection string!  In my case, only the database name needed to be changed, so I only set the InitialCatalog, but you can set anything else you need as well.

Note that this is NOT a sensible way to do things – accessing private data can break in future releases or cause unintended side-effects.  In my case however, this code was only used for debug builds (and wrapped in #if DEBUG…) so it was good enough, YMMV.