You can use a library called ExcelLibrary. It's a free, open source library posted on Google Code:
ExcelLibrary
This looks to be a port of the PHP ExcelWriter that you mentioned above. It will not write to the new .xlsx format yet, but they are working on adding that functionality in.
It's very simple, small and easy to use. Plus it has a DataSetHelper that lets you use DataSets and DataTables to easily work with Excel data.
ExcelLibrary seems to still only work for the older Excel format (.xls files), but may be adding support in the future for newer 2007/2010 formats.
You can also use EPPlus, which works only for Excel 2007/2010 format files (.xlsx files). There's also NPOI which works with both.
There are a few known bugs with each library as noted in the comments. In all, EPPlus seems to be the best choice as time goes on. It seems to be more actively updated and documented as well.
Also, as noted by @АртёмЦарионов below, EPPlus has support for Pivot Tables and ExcelLibrary may have some support (Pivot table issue in ExcelLibrary)
Here are a couple links for quick reference:
ExcelLibrary - GNU Lesser GPL
EPPlus - GNU (LGPL) - No longer maintained
EPPlus 5 - Polyform Noncommercial - Starting May 2020
NPOI - Apache License
Here some example code for ExcelLibrary:
Here is an example taking data from a database and creating a workbook from it. Note that the ExcelLibrary code is the single line at the bottom:
//Create the data set and table
DataSet ds = new DataSet("New_DataSet");
DataTable dt = new DataTable("New_DataTable");
//Set the locale for each
ds.Locale = System.Threading.Thread.CurrentThread.CurrentCulture;
dt.Locale = System.Threading.Thread.CurrentThread.CurrentCulture;
//Open a DB connection (in this example with OleDB)
OleDbConnection con = new OleDbConnection(dbConnectionString);
con.Open();
//Create a query and fill the data table with the data from the DB
string sql = "SELECT Whatever FROM MyDBTable;";
OleDbCommand cmd = new OleDbCommand(sql, con);
OleDbDataAdapter adptr = new OleDbDataAdapter();
adptr.SelectCommand = cmd;
adptr.Fill(dt);
con.Close();
//Add the table to the data set
ds.Tables.Add(dt);
//Here's the easy part. Create the Excel worksheet from the data set
ExcelLibrary.DataSetHelper.CreateWorkbook("MyExcelFile.xls", ds);
Creating the Excel file is as easy as that. You can also manually create Excel files, but the above functionality is what really impressed me.
Best Answer
No. There is a lot of confusion over ClickOnce and prerequisites.
ClickOnce does nothing with prereqs. Nothing. ClickOnce copies files from a server to a client's machine and keeps those files in sync when the server files are updated - that's it. It can't put things in the GAC, register dlls, install msi files, etc.
Where the confusion comes in is when you deploy with Visual Studio. VS does several things for you when you publish that really have nothing to do with ClickOnce. For one, it creates a nice html page for you with some links to your deployment. Also, it lets you pick from several prereqs and will create a bootstrapper exe for you. I'm assuming you did this for the Crystal Reports install. The bootstrapper is just a simple way to manage multiple prereqs. Rather than telling your user to install X, then Y, then Z before installing your app, the bootstrapper makes those three installs "seamless" so they appear as if they are one big install. It can also skip prereq installs if the user already has it.
If you pay attention on your html page that VS generates, you can see two links. One to your .application file (the ClickOnce deployment) and one to the bootstrapper exe. You are pretty much at the mercy of the user and must rely on them to read the page and run the bootstrapper if they need to. The one exception to that is the .Net Framework since the html page can check for that through the UserAgent string.
If they don't run the bootstrapper they'll get errors much like the one you're asking about.
One possible solution is to write code to check if it's installed. It could check the registry or the "Program Files" folder; whatever you need to do to make sure it's installed. Then exit gracefully and inform the user if they don't have it. That will work as long as you do the check before you try to load and use the Crystal assemblies.
Good luck! Please respond if you come up with a better solution.