What's the best way to test SQL Server connection programmatically?

I need to develop a single routine that will be fired each 5 minutes to check if a list of SQL Servers (10 to 12) are up and running.

Is there a way to simply "ping" a SQL Server from C# one with minimal code and sql operational requirements?

I have had a difficulty with the EF when the connection the server is stopped or paused, and I raised the same question. So for completeness to the above answers here is the code.

/// <summary>
/// Test that the server is connected
/// </summary>
/// <param name="connectionString">The connection string</param>
/// <returns>true if the connection is opened</returns>
private static bool IsServerConnected(string connectionString)
{
    using (SqlConnection connection = new SqlConnection(connectionString))
    {
        try
        {
            connection.Open();
            return true;
        }
        catch (SqlException)
        {
            return false;
        }
    }
}

Execute SELECT 1 and check if ExecuteScalar returns 1.

See the following project on GitHub: https://github.com/ghuntley/csharp-mssql-connectivity-tester

try
{
    Console.WriteLine("Connecting to: {0}", AppConfig.ConnectionString);
    using (var connection = new SqlConnection(AppConfig.ConnectionString))
    {
        var query = "select 1";
        Console.WriteLine("Executing: {0}", query);

        var command = new SqlCommand(query, connection);

        connection.Open();
        Console.WriteLine("SQL Connection successful.");

        command.ExecuteScalar();
        Console.WriteLine("SQL Query execution successful.");
    }
}
catch (Exception ex)
{
    Console.WriteLine("Failure: {0}", ex.Message);
}

Wouldn't establishing a connection to the database do this for you? If the database isn't up you won't be able to establish a connection.

Look for an open listener on port 1433 (the default port). If you get any response after creating a tcp connection there, the server's probably up.

---

You know, I first wrote this in 2010. Today, I'd just try to actually connect to the server.

For what Joel Coehorn suggested, have you already tried the utility named tcping. I know this is something you are not doing programmatically. It is a standalone executable which allows you to ping every specified time interval. It is not in C# though. Also..I am not sure If this would work If the target machine has firewall..hmmm..

[I am kinda new to this site and mistakenly added this as a comment, now added this as an answer. Let me know If this can be done here as I have duplicate comments (as comment and as an answer) here. I can not delete comments here.]

public static class SqlConnectionExtension
{
    #region Public Methods

    public static bool ExIsOpen(
        this SqlConnection connection, MessageString errorMsg = null)
    {
        if (connection == null) { return false; }
        if (connection.State == ConnectionState.Open) { return true; }

        try
        {
            connection.Open();
            return true;
        }
        catch (Exception ex) { errorMsg?.Append(ex.ToString()); }
        return false;
    }

    public static bool ExIsReady(
        this SqlConnection connction, MessageString errorMsg = null)
    {
        if (connction.ExIsOpen(errorMsg) == false) { return false; }
        try
        {
            using (var command = new SqlCommand("select 1", connction))
            { return ((int)command.ExecuteScalar()) == 1; }
        }
        catch (Exception ex) { errorMsg?.Append(ex.ToString()); }
        return false;
    }

    #endregion Public Methods
}

public class MessageString : IDisposable
{
	#region Protected Fields

	protected StringBuilder _messageBuilder = new StringBuilder();

	#endregion Protected Fields

	#region Public Constructors

	public MessageString()
	{
	}

	public MessageString(int capacity)
	{
		_messageBuilder.Capacity = capacity;
	}

	public MessageString(string value)
	{
		_messageBuilder.Append(value);
	}

	#endregion Public Constructors

	#region Public Properties

	public int Length {
		get { return _messageBuilder.Length; }
		set { _messageBuilder.Length = value; }
	}

	public int MaxCapacity {
		get { return _messageBuilder.MaxCapacity; }
	}

	#endregion Public Properties

	#region Public Methods

	public static implicit operator string(MessageString ms)
	{
		return ms.ToString();
	}

	public static MessageString operator +(MessageString ms1, MessageString ms2)
	{
		MessageString ms = new MessageString(ms1.Length + ms2.Length);
		ms.Append(ms1.ToString());
		ms.Append(ms2.ToString());
		return ms;
	}

	public MessageString Append<T>(T value) where T : IConvertible
	{
		_messageBuilder.Append(value);
		return this;
	}

	public MessageString Append(string value)
	{
		return Append<string>(value);
	}

	public MessageString Append(MessageString ms)
	{
		return Append(ms.ToString());
	}

	public MessageString AppendFormat(string format, params object[] args)
	{
		_messageBuilder.AppendFormat(CultureInfo.InvariantCulture, format, args);
		return this;
	}

	public MessageString AppendLine()
	{
		_messageBuilder.AppendLine();
		return this;
	}

	public MessageString AppendLine(string value)
	{
		_messageBuilder.AppendLine(value);
		return this;
	}

	public MessageString AppendLine(MessageString ms)
	{
		_messageBuilder.AppendLine(ms.ToString());
		return this;
	}

	public MessageString AppendLine<T>(T value) where T : IConvertible
	{
		Append<T>(value);
		AppendLine();
		return this;
	}

	public MessageString Clear()
	{
		_messageBuilder.Clear();
		return this;
	}

	public void Dispose()
	{
		_messageBuilder.Clear();
		_messageBuilder = null;
	}

	public int EnsureCapacity(int capacity)
	{
		return _messageBuilder.EnsureCapacity(capacity);
	}

	public bool Equals(MessageString ms)
	{
		return Equals(ms.ToString());
	}

	public bool Equals(StringBuilder sb)
	{
		return _messageBuilder.Equals(sb);
	}

	public bool Equals(string value)
	{
		return Equals(new StringBuilder(value));
	}

	public MessageString Insert<T>(int index, T value)
	{
		_messageBuilder.Insert(index, value);
		return this;
	}

	public MessageString Remove(int startIndex, int length)
	{
		_messageBuilder.Remove(startIndex, length);
		return this;
	}

	public MessageString Replace(char oldChar, char newChar)
	{
		_messageBuilder.Replace(oldChar, newChar);
		return this;
	}

	public MessageString Replace(string oldValue, string newValue)
	{
		_messageBuilder.Replace(oldValue, newValue);
		return this;
	}

	public MessageString Replace(char oldChar, char newChar, int startIndex, int count)
	{
		_messageBuilder.Replace(oldChar, newChar, startIndex, count);
		return this;
	}

	public MessageString Replace(string oldValue, string newValue, int startIndex, int count)
	{
		_messageBuilder.Replace(oldValue, newValue, startIndex, count);
		return this;
	}

	public override string ToString()
	{
		return _messageBuilder.ToString();
	}

	public string ToString(int startIndex, int length)
	{
		return _messageBuilder.ToString(startIndex, length);
	}

	#endregion Public Methods
}

Similar to the answer offered by Andrew, but I use:

Select GetDate() as CurrentDate

This allows me to see if the SQL Server and the client have any time zone difference issues, in the same action.

Here is my version based on the @peterincumbria answer:

using var scope = _serviceProvider.CreateScope();
var dbContext = scope.ServiceProvider.GetRequiredService<AppDbContext>();
return await dbContext.Database.CanConnectAsync(cToken);

I'm using Observable for polling health checking by interval and handling return value of the function. try-catch is not needed here because:

I normally do this by open a connection but I had some cases where a simple test via Open caused a AccessViolationException

using (SqlConnection db = new SqlConnection(conn))
{    
  db.Open(); // -- Access Violation caused by invalid Server in Connection String
}

So I did a TCP check before the open like recommanded by Joel Coehoorn. C# Code for this may be:

string targetAddress = "";
try
{
  targetAddress = GetServerFromConnectionString();
  IPAddress ipAddress = Dns.GetHostEntry(targetAddress).AddressList[0];
  IPEndPoint ipEndPoint = new IPEndPoint(ipAddress, 1433);

  using (TcpClient tcpClient = new TcpClient())
  {
       tcpClient.Connect(ipEndPoint);           
  }
            
}
catch (Exception ex)
{
    LogError($"TestViaTcp to server {targetAddress} failed '{ex.GetType().Name}': {ex.Message}");
}