Shawn asked me in my last post about GroupByMany how to use it dynamically.

The answer is not easy. So here is a new post to answer to this interesting question.

First you must learn a few things about being dynamic with linq.
There is a very good sample from the C# team (actually we can consider it as a library provided as a sample). ScottGu has post about it here:

One of the power of linq is to be checked at compile-time. You must be aware that using this kind of solution may raise some exceptions at runtime.

Now I have added the Dynamic.cs file to my project and I can use a very useful method: DynamicExpression.ParseLambda.

Here is a new GroupByMany extension method which accepts a list of strings instead of delegates to define all the groupings.
The goal is to call the first GroupByMany method. To do this we have to go from a single string defining the name of a property in the TElement class to a Func<TElement, object>.
We could do this building the expression manually but DynamicExpression.ParseLambda will do it in just a single line.

public static class MyEnumerableExtensions { public static IEnumerable<GroupResult> GroupByMany<TElement>( this IEnumerable<TElement> elements, params string[] groupSelectors) { var selectors = new List<Func<TElement, object>>(groupSelectors.Length); foreach (var selector in groupSelectors) { LambdaExpression l = DynamicExpression.ParseLambda( typeof(TElement), typeof(object), selector); selectors.Add((Func<TElement, object>) l.Compile()); } return elements.GroupByMany(selectors.ToArray()); } ...
Now we can write directly:
DataContext = customers.GroupByMany("Country", "City");

instead of :
DataContext = customers.GroupByMany(c => c.Country, c => c.City);

The full project is attached to this post. (VS2008.)