An utility class designed to work with SQL LocalDb instances inside of integration tests.

LocalDbPrivateInstance.cs

using System;
using System.Data.SqlClient;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;
using Microsoft.Win32;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using System.IO;

namespace HackedBraind.Testing.Utilities
{
	internal class LocalDbPrivateInstance : IDisposable
	{
		#region Fields

		private static IntPtr LocalDbDllHandle;
		private static string LocalDbVersion;
		private static LocalDbCreateInstanceFunction LocalDbCreateInstance;
		private static LocalDbDeleteInstanceFunction LocalDbDeleteInstance;
		private static LocalDbStopInstanceFunction LocalDbStopInstance;
		private static LocalDbFormatMessageFunction LocalDbFormatMessage;
		private static LocalDbGetInstanceInfoFunction LocalDbGetInstanceInfo;

		private readonly string instanceName;

		#endregion

		#region Constructor/Finalizer

		static LocalDbPrivateInstance()
		{
			LocalDbPrivateInstance.LoadSQLUserInstanceLibrary();
			LocalDbPrivateInstance.BindSQLUserInstancePInvokeMethods();
		}

		~LocalDbPrivateInstance()
		{
			this.Dispose(false);
		}

		private LocalDbPrivateInstance(string instanceName)
		{
			this.instanceName = instanceName;
		}

		#endregion

		#region Type specific methods

		public static LocalDbPrivateInstance CreateInstance(string instanceName)
		{
			return LocalDbPrivateInstance.CreateInstance(instanceName, true);
		}

		public static LocalDbPrivateInstance CreateInstance(string instanceName, bool deleteIfAlreadyExists)
		{
			if(deleteIfAlreadyExists)
			{
				LocalDbPrivateInstance.DeleteIfAlreadyExists(instanceName);
			}
			
			int createInstanceResult = LocalDbPrivateInstance.LocalDbCreateInstance(LocalDbPrivateInstance.LocalDbVersion, instanceName, 0);

			if(createInstanceResult != 0)
			{
				throw new Exception("An error occurred creating the local DB intance: " + LocalDbPrivateInstance.FormatMessage(createInstanceResult));
			}

			return new LocalDbPrivateInstance(instanceName);
		}

		public SqlConnection CreateConnection(string connectionString)
		{
			SqlConnectionStringBuilder sqlConnectionStringBuilder = new SqlConnectionStringBuilder(connectionString);

			sqlConnectionStringBuilder.DataSource = "(LocalDb)\\" + this.instanceName;

			return new SqlConnection(sqlConnectionStringBuilder.ToString());
		}

		public void AttachDatabases(IEnumerable<KeyValuePair<string, string>> namesAndPaths)
		{
			using(SqlConnection connection = this.CreateConnection("Database=master;MultipleActiveResultSets=true;"))
			{
				connection.Open();

				foreach(KeyValuePair<string, string> nameAndPath in namesAndPaths)
				{
					LocalDbPrivateInstance.AttachDatabase(connection, nameAndPath.Key, nameAndPath.Value);
				}
			}
		}

		public void AttachDatabase(string name, string path)
		{
			using(SqlConnection connection = this.CreateConnection("Database=master;MultipleActiveResultSets=true;"))
			{
				connection.Open();

				LocalDbPrivateInstance.AttachDatabase(connection, name, path);
			}
		}

		public void DetatchAllDatabases()
		{
			using(SqlConnection connection = this.CreateConnection("Database=master;MultipleActiveResultSets=true;"))
			{
				connection.Open();

				SqlCommand getDatabasesCommand = connection.CreateCommand();
				getDatabasesCommand.CommandText = "SELECT name FROM sys.databases WHERE owner_sid <> 0x01";

				SqlCommand detatchDatabaseCommand = connection.CreateCommand();
				detatchDatabaseCommand.CommandText = "EXEC master.dbo.sp_detach_db @dbname = @DatabaseName, @keepfulltextindexfile=N'false'";
				SqlParameter detatchDatabaseNameParameter = detatchDatabaseCommand.Parameters.Add("@DatabaseName", System.Data.SqlDbType.NVarChar);

				using(SqlDataReader databaseReader = getDatabasesCommand.ExecuteReader())
				{
					while(databaseReader.Read())
					{
						detatchDatabaseNameParameter.Value = databaseReader[0];

						detatchDatabaseCommand.ExecuteNonQuery();
					}
				}
			}
		}

		#endregion

		#region IDisposable implementation

		public void Dispose()
		{
			this.Dispose(true);
		}

		#endregion

		#region Helper methods

		private void Dispose(bool disposing)
		{
			if(disposing)
			{
				GC.SuppressFinalize(this);
			}

			int stopInstanceResult = LocalDbPrivateInstance.LocalDbStopInstance(this.instanceName, 0);

			if(stopInstanceResult != 0)
			{
				throw new Exception("An error occurred stopping the local DB intance: " + LocalDbPrivateInstance.FormatMessage(stopInstanceResult));
			}

			int deleteInstanceResult = LocalDbPrivateInstance.LocalDbDeleteInstance(this.instanceName, 0);

			if(stopInstanceResult != 0)
			{
				throw new Exception("An error occurred deleting the local DB intance: " + LocalDbPrivateInstance.FormatMessage(deleteInstanceResult));
			}
		}

		private static string FormatMessage(int hresult)
		{
			int messageBufferLength = 1024;
			StringBuilder stringBuilder = new StringBuilder(messageBufferLength);

			int formatMessageResult = LocalDbPrivateInstance.LocalDbFormatMessage(hresult, 0, 1033, stringBuilder, ref messageBufferLength);

			if(formatMessageResult != 0)
			{
				throw new Exception(string.Format("An error occurred formatting the message for result {0}: {1}", hresult, formatMessageResult));
			}

			return stringBuilder.ToString();
		}

		private static void LoadSQLUserInstanceLibrary()
		{
			RegistryKey localDbInstalledVersionsRegistryKey = Registry.LocalMachine.OpenSubKey(@"SOFTWARE\Microsoft\Microsoft SQL Server Local DB\Installed Versions");

			string latestVersionSubKeyName = localDbInstalledVersionsRegistryKey.GetSubKeyNames().Last();

			RegistryKey latestVersionRegistryKey = localDbInstalledVersionsRegistryKey.OpenSubKey(latestVersionSubKeyName);

			string latestVersionDllPath = (string)latestVersionRegistryKey.GetValue("InstanceApiPath");

			IntPtr latestVersionDllHandle = LocalDbPrivateInstance.LoadLibrary(latestVersionDllPath);

			if(latestVersionDllHandle == IntPtr.Zero)
			{
				throw new Exception(string.Format("LoadLibrary of \"{0}\" failed: {1}", latestVersionDllPath, Marshal.GetLastWin32Error()));
			}

			LocalDbPrivateInstance.LocalDbVersion = latestVersionSubKeyName;
			LocalDbPrivateInstance.LocalDbDllHandle = latestVersionDllHandle;
		}

		private static void AttachDatabase(SqlConnection connection, string name, string path)
		{
			using(SqlCommand command = connection.CreateCommand())
			{
				command.CommandText = string.Format("CREATE DATABASE {0} ON (FILENAME = '{1}') FOR ATTACH", name, path);

				command.ExecuteNonQuery();
			}
		}

		private static void BindSQLUserInstancePInvokeMethods()
		{
			LocalDbPrivateInstance.LocalDbCreateInstance = (LocalDbCreateInstanceFunction)Marshal.GetDelegateForFunctionPointer(LocalDbPrivateInstance.GetProcAddress(LocalDbPrivateInstance.LocalDbDllHandle, "LocalDBCreateInstance"), typeof(LocalDbCreateInstanceFunction));
			LocalDbPrivateInstance.LocalDbDeleteInstance = (LocalDbDeleteInstanceFunction)Marshal.GetDelegateForFunctionPointer(LocalDbPrivateInstance.GetProcAddress(LocalDbPrivateInstance.LocalDbDllHandle, "LocalDBDeleteInstance"), typeof(LocalDbDeleteInstanceFunction));
			LocalDbPrivateInstance.LocalDbStopInstance = (LocalDbStopInstanceFunction)Marshal.GetDelegateForFunctionPointer(LocalDbPrivateInstance.GetProcAddress(LocalDbPrivateInstance.LocalDbDllHandle, "LocalDBStopInstance"), typeof(LocalDbStopInstanceFunction));
			LocalDbPrivateInstance.LocalDbFormatMessage = (LocalDbFormatMessageFunction)Marshal.GetDelegateForFunctionPointer(LocalDbPrivateInstance.GetProcAddress(LocalDbPrivateInstance.LocalDbDllHandle, "LocalDBFormatMessage"), typeof(LocalDbFormatMessageFunction));
			LocalDbPrivateInstance.LocalDbGetInstanceInfo = (LocalDbGetInstanceInfoFunction)Marshal.GetDelegateForFunctionPointer(LocalDbPrivateInstance.GetProcAddress(LocalDbPrivateInstance.LocalDbDllHandle, "LocalDBGetInstanceInfo"), typeof(LocalDbGetInstanceInfoFunction));
		}

		private static void DeleteIfAlreadyExists(string instanceName)
		{
			int stopInstanceResult = LocalDbPrivateInstance.LocalDbStopInstance(instanceName, 1);

			if(stopInstanceResult == 0
					||
				stopInstanceResult == -1983577849)
			{
				int deleteInstanceResult = LocalDbPrivateInstance.LocalDbDeleteInstance(instanceName, 0);

				if(deleteInstanceResult != 0
						&&
					deleteInstanceResult != -1983577849)
				{
					throw new Exception("An error occurred attempting to delete an already existing instance: " + LocalDbPrivateInstance.FormatMessage(deleteInstanceResult));
				}
			}
			else
			{
				throw new Exception("An error occurred attempting to stop an already existing instance: " + LocalDbPrivateInstance.FormatMessage(stopInstanceResult));
			}
		}

		#endregion

		#region P/Invoke imports

		[UnmanagedFunctionPointer(CallingConvention.Cdecl, CharSet = CharSet.Auto, SetLastError = true)]
		private delegate int LocalDbCreateInstanceFunction(string version, string instanceName, int flags);
		[UnmanagedFunctionPointer(CallingConvention.Cdecl, CharSet = CharSet.Auto, SetLastError = true)]
		private delegate int LocalDbDeleteInstanceFunction(string instanceName, int flags);
		[UnmanagedFunctionPointer(CallingConvention.Cdecl, CharSet = CharSet.Auto, SetLastError = true)]
		private delegate int LocalDbStopInstanceFunction(string instanceName, int flags);
		[UnmanagedFunctionPointer(CallingConvention.Cdecl, CharSet = CharSet.Auto, SetLastError = true)]
		private delegate int LocalDbFormatMessageFunction(int hresult, int flags, int languageId, StringBuilder messageBuffer, ref int messageBufferLength);

		[DllImport("kernel32", CharSet = CharSet.Ansi, ExactSpelling = true, SetLastError = true)]
		private static extern IntPtr GetProcAddress(IntPtr hModule, string procName);

		[DllImport("kernel32", SetLastError = true, CharSet = CharSet.Unicode)]
		private static extern IntPtr LoadLibrary(string lpFileName);

		#endregion

	}

	internal static class TypeExtensions
	{
		public static void AttachDeployedDatabases(this LocalDbPrivateInstance dbInstance, Type testType)
		{
			IEnumerable<KeyValuePair<string, string>> deployedMdfDetails = from dia in testType.GetCustomAttributes(typeof(DeploymentItemAttribute), true).Cast<DeploymentItemAttribute>()
																		   where dia.Path.EndsWith(".mdf")
																		   let fileName = Path.GetFileName(dia.Path)
																		   select new KeyValuePair<string, string>(Path.GetFileNameWithoutExtension(fileName), Path.GetFullPath(Path.Combine(dia.OutputDirectory, fileName)));

			dbInstance.AttachDatabases(deployedMdfDetails);
		}
	}
}

_Detatch_AllDatabases()" should be DetachAllDatabases()" (no t)