Dynamic Programming in C# - Effective C# - page 232

var adder = Expression .Lambda< Func <T1, T2, TResult>>(
body, leftOperand, rightOperand);
return adder.Compile()(left, right);
}
This method looks very similar to the previous version; it just enables you
to call it with different types for the left and the right operand. The only
downside is that you need to specify all three parameter types whenever
you call this version:
int sum2 = AddExpression< int , int , int >( 5 , 7 );
However, because you specify all three parameters, expressions with dis-
similar parameters work:
DateTime nextWeek= AddExpression< DateTime , TimeSpan ,
DateTime >(
DateTime .Now, TimeSpan .FromDays( 7 ));
It's time to address the other nagging issue. The code, as I have shown so
far, compiles the expression into a delegate every time the AddExpression()
method is called. That's quite inefficient, especially if you end up execut-
ing the same expression repeatedly. Compiling the expression is expen-
sive, so you should cache the compiled delegate for future invocations.
Here's a first pass at that class:
// dangerous but working version
public static class BinaryOperator <T1, T2, TResult>
{
static Func <T1, T2, TResult> compiledExpression;
public static TResult Add(T1 left, T2 right)
{
if (compiledExpression == null )
createFunc();
return compiledExpression(left, right);
}
private static void createFunc()
{
var leftOperand = Expression .Parameter( typeof (T1),
"left" );