Fluent interfaces are easily supported in C# by using `return` `this`.

In Linq, extension methods are used to accomplish the same thing.  Extension methods over `IQueryable` or `IEnumerable` are easily chained together.

Cascades, as described by the page at the link you provided, seem to be merely a subtle refinement of an ordinary Fluent Interface where `return` `this` is not required to make it work, as all method calls beginning with `..` refer to the first returned result:


    query('#my-form').query('button')
        ..classes.add('toggle')
        ..text = 'Click Me!'
        ..labels.add(toggleLabel);

Does C# support this?  No, not exactly.  But you can still write the following equivalent code:

    var result = query('#my-form').query('button')
    result.classes.add('toggle')
    result.text = 'Click Me!'
    result.labels.add(toggleLabel);

and it is only slightly more verbose (and arguably easier to read).

As to the question, "Were they ever considered for C#," you would have to ask Anders Hejlsberg or Eric Lippert about that.  My guess is that they view it as superfluous syntactic sugar, which in fact it is.