C# – Associate File Extension with Application

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EPPlus - GNU (LGPL) - No longer maintained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.

For the moment it is still not supported out of the box by Roslyn compiler ...

Until now, the extension properties were not seen as valuable enough to be included in the previous versions of C# standard. C# 7 and C# 8.0 have seen this as proposal champion but it wasn't released yet, most of all because even if there is already an implementation, they want to make it right from the start.

But it will ...

There is an extension members item in the C# 7 work list so it may be supported in the near future. The current status of extension property can be found on Github under the related item.

However, there is an even more promising topic which is the "extend everything" with a focus on especially properties and static classes or even fields.

Moreover you can use a workaround

As specified in this article, you can use the TypeDescriptor capability to attach an attribute to an object instance at runtime. However, it is not using the syntax of the standard properties.
It's a little bit different from just syntactic sugar adding a possibility to define an extended property like
string Data(this MyClass instance) as an alias for extension method
string GetData(this MyClass instance) as it stores data into the class.

I hope that C#7 will provide a full featured extension everything (properties and fields), however on that point, only time will tell.

And feel free to contribute as the software of tomorrow will come from the community.

Update: August 2016

As dotnet team published what's new in C# 7.0 and from a comment of Mads Torgensen:

Extension properties: we had a (brilliant!) intern implement them over the summer as an experiment, along with other kinds of extension members. We remain interested in this, but it’s a big change and we need to feel confident that it’s worth it.

It seems that extension properties and other members, are still good candidates to be included in a future release of Roslyn, but maybe not the 7.0 one.

Update: May 2017

The extension members has been closed as duplicate of extension everything issue which is closed too. The main discussion was in fact about Type extensibility in a broad sense. The feature is now tracked here as a proposal and has been removed from 7.0 milestone.

Update: August, 2017 - C# 8.0 proposed feature

While it still remains only a proposed feature, we have now a clearer view of what would be its syntax. Keep in mind that this will be the new syntax for extension methods as well:

public interface IEmployee 
{
    public decimal Salary { get; set; }
}

public class Employee
{
    public decimal Salary { get; set; }
}

public extension MyPersonExtension extends Person : IEmployee
{
    private static readonly ConditionalWeakTable<Person, Employee> _employees = 
        new ConditionalWeakTable<Person, Employee>();


    public decimal Salary
    {
        get 
        {
            // `this` is the instance of Person
            return _employees.GetOrCreate(this).Salary; 
        }
        set 
        {
            Employee employee = null;
            if (!_employees.TryGetValue(this, out employee)
            {
                employee = _employees.GetOrCreate(this);
            }
            employee.Salary = value;
        }
    }
}

IEmployee person = new Person();
var salary = person.Salary;

Similar to partial classes, but compiled as a separate class/type in a different assembly. Note you will also be able to add static members and operators this way. As mentioned in Mads Torgensen podcast, the extension won't have any state (so it cannot add private instance members to the class) which means you won't be able to add private instance data linked to the instance. The reason invoked for that is it would imply to manage internally dictionaries and it could be difficult (memory management, etc...). For this, you can still use the TypeDescriptor/ConditionalWeakTable technique described earlier and with the property extension, hides it under a nice property.

Syntax is still subject to change as implies this issue. For example, extends could be replaced by for which some may feel more natural and less java related.

Update December 2018 - Roles, Extensions and static interface members

Extension everything didn't make it to C# 8.0, because of some of drawbacks explained as the end of this GitHub ticket. So, there was an exploration to improve the design. Here, Mads Torgensen explains what are roles and extensions and how they differs:

Roles allow interfaces to be implemented on specific values of a given type. Extensions allow interfaces to be implemented on all values of a given type, within a specific region of code.

It can be seen at a split of previous proposal in two use cases. The new syntax for extension would be like this:

public extension ULongEnumerable of ulong
{
    public IEnumerator<byte> GetEnumerator()
    {
        for (int i = sizeof(ulong); i > 0; i--)
        {
            yield return unchecked((byte)(this >> (i-1)*8));
        }
    }
}

then you would be able to do this:

foreach (byte b in 0x_3A_9E_F1_C5_DA_F7_30_16ul)
{
    WriteLine($"{e.Current:X}");
}

And for a static interface:

public interface IMonoid<T> where T : IMonoid<T>
{
    static T operator +(T t1, T t2);
    static T Zero { get; }
}

Add an extension property on int and treat the int as IMonoid<int>:

public extension IntMonoid of int : IMonoid<int>
{
    public static int Zero => 0;
}