Imagine you weren't working with expressions, but with delegates. Then you could write Or
like this:
public static Func<T, bool> Or<T>(this Func<T, bool> expr1, Func<T, bool> expr2)
{
return x => expr1(x) || expr2(x);
}
You create a new delegate that invokes the two delegates, combined using ||
. When you rewrite this to use expressions, invoking a delegate turns into Expression.Invoke()
:
public static Expression<Func<T, bool>> Or<T>(
this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2)
{
var parameter = Expression.Parameter(typeof(T), "x");
var invokedExpr1 = Expression.Invoke(expr1, parameter);
var invokedExpr2 = Expression.Invoke(expr2, parameter);
return Expression.Lambda<Func<T, bool>>(
Expression.OrElse(invokedExpr1, invokedExpr2), parameter);
}
The reason why the actual Or
isn't written like this is (most likely) an optimization: you don't have to invoke both expressions, you can reuse the body and parameter from one of them. (But you can't reuse both of them, because they have different parameters.)