Is there a generic constraint I could use for the + operator?

Question

There are no such devices in C#. A few options are available, though:

in C# 4.0 and .NET 4.0 (or above), use dynamic, which supports + but offers no compile time checking
in .NET 3.5 (or above), MiscUtil offers an Operator class which makes operators available as methods – again, without any compile-time checking

So either:

return (dynamic)left.Evaluate(context) + (dynamic)right.Evaluate(context);

or

return Operator.Add(left.Evaluate(context), right.Evaluate(context));