I'm getting a NullReference Exception

When you get an error like that, always step-through your code to see what is going on. Put a breakpoint before the error occurs and then step-through it.

In this case, I think you would find that either context.Request.QueryString["pageType"] does not equal "tour" or that context.Request.QueryString["type"] does not equal "Type1", "Type2", or "Type3" which means that selcmd is never being set.

So, you would then want to figure out what QueryString["type"] or QueryString["pageType"] is being set to and go from there.

Also, as an FYI to how you're creating your commands, I would have a look here: http://www.csharp-station.com/Tutorials/AdoDotNet/Lesson06.aspx[^]. You want to avoid setting a command using a straight variable as you have here:
"select * from tour where tourid='" + context.Request.QueryString["tourid"] + "'".

Your code looks like a Script-Kiddies dream.. ;)
Please (as stated in answers before) NEVER use QueryString variables directly in a SQL Query!

About your Problem, I don't see why the Exception occures, but it seems that some parts of your source is missing.

Anyway, I tried to clean the code up a little bit, could you try it this way (perhaps you have to modify my source a bit according your surroundings.

public IList<Tour> GetTourData(HttpContext context)
{
	// Init output
	var tourOutput = new List<Tour>();

	// Get all input variables out of the QueryString
	string pageType = context.Request.QueryString["pageType"];
	string type = context.Request.QueryString["type"];
	string tourId = context.Request.QueryString["tourid"];

	// Get the connection string
	string connectionString = ConfigurationManager.ConnectionStrings["smartwaydbConnectionString"].ConnectionString;

	// Initialize the SQL Connection and define the SqlCommand and SqlDataReader
	SqlConnection conn = new SqlConnection(connectionString);
	SqlCommand selcmd = null;
	SqlDataReader rdr = null;
			
	if (pageType == "tour")
	{
		// Build the SqlCommand according the type (use sql variables for the input! => SQL Injection!)
		switch (type)
		{
			case "Type1":
			case "Type2":
				selcmd = new SqlCommand("select * from tour where tourid = @tourid", conn);
				break;
			case "Type3":
				selcmd = new SqlCommand("select * from tour where popularimage IS NOT NULL and tourid = @tourid", conn);
				break;
		}
	}

	// Proceed if the command was correctly assigned
	if (selcmd != null)
	{
		// Add the @tourid parameter
		selcmd.Parameters.Add("@tourid", SqlDbType.Int).Value = Convert.ToInt32(tourId);

		conn.Open();

		using (rdr = selcmd.ExecuteReader())
		{
			if (rdr.HasRows)
			{
				while (rdr.Read())
				{
					// Create a new Tour object
					var tour = new Tour();

					// Add all data from the SqlDataReader rdr
					// ...

					// Add the new Tour object to the output list (it is returning more than one result, right?)
					tourOutput.Add(tour);
				}
			}
		}

		conn.Close();
	}

	return tourOutput;
}

As explanation:
The Tour Class is a class which holds the tour data you get from the reader. If you want to process it in a DataTable you have to change the object-reading and the output parameter to make this working.

Hope this helps to get you forward.

And once again, PLEASE NEVER use QueryString Parameters directly in a SQL Statement!! Encapsulate them at least in a Sql Parameter attached to the query (better use a Stored Procedure directly on the SQL Server).

Best regards and have a nice day,
Stops