Functional Programming in C#: Classic Programming Techniques for Modern Projects

.NET 4.0 SPECIFICS

As mentioned earlier, .NET 4.0 has extended the supported language elements for expression trees, but the C# language hasn’t seen any corresponding extensions yet. As a result, expression trees can currently be constructed through helper functions of the Expression class in .NET 4.0, which don’t have a syntactic representation in the language itself. It’s possible to implement a function or a lambda expression with the same functionality as any given expression tree, but the automatic conversion into an expression tree doesn’t work.

For instance, here is a simple function to calculate a factorial using an iterative algorithm:

static int Fact(int x) {

  int result = 1;

  for (int m = 2; m <= x; m++)

    result *= m;

  return result;

}

As a lambda expression, this is what the function looks like:

Func<int, int> fact = x => {

  int result = 1;

  for (int m = 2; m <= x; m++)

    result *= m;

  return result;

};

But that is not possible and results in the compilation error “A lambda expression with a statement body cannot be converted to an expression tree.”

Expression<Func<int, int>> factExpression = x => {

  int result = 1;

  for (int m = 2; m <= x; m++)

    result *= m;

  return result;

};

Constructing the expression tree for this simple algorithm is quite complicated. Here’s the function that does it:

static Func<int, int> ConstructRuntimeFact( ) {

  var param = Expression.Parameter(typeof(int), "x");

  var resultVar = Expression.Variable(typeof(int), "result"); ...