Today we’ll talk about how we can get a list of locally installed SQL Server instances on a system.   This information is stored in the registry at the following location:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\Instance Names\SQL

EDIT:

If you are looking for 32 bit instances on a 64 bit OS, you will need to look here:

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Microsoft SQL Server\Instance Names\SQL

All we need to do is query this registry key to get the list of instances installed on a machine.   On my personal machine, I have the following instances installed:

SQLEXPRESS   
MSSQLSERVER   
CRASH   
DEBUG   
MICROSOFT##SSEE
SQL2005  

Below are a few ways we can get this information for 64 bit instances on 64 bit Windows:

Command shell “reg.exe” utility:

reg query "HKLM\Software\Microsoft\Microsoft SQL Server\Instance Names\SQL"

In PowerShell:

Get-ItemProperty 'HKLM:\Software\Microsoft\Microsoft SQL Server\Instance Names\SQL'

In .NET:

RegistryKey baseKey = RegistryKey.OpenBaseKey(RegistryHive.LocalMachine, RegistryView.Registry64);
RegistryKey key = baseKey.OpenSubKey(@"SOFTWARE\Microsoft\Microsoft SQL Server\Instance Names\SQL");

foreach (string s in key.GetValueNames())
{
    MessageBox.Show(s);
}

key.Close();
baseKey.Close();

If you need to read the registry from another computer, then the RegistryKey class has a “OpenRemoteBaseKey” static method that you can use. 

- Jay