PredicateBuilder methods clarification

I lo开发者_C百科oked through PredicateBuilder sources and its' implementation makes me curious. Let's look at Or method implementation:

public static Expression<Func<T, bool>> Or<T> (this Expression<Func<T, bool>> expr1,
                                                      Expression<Func<T, bool>> expr2)
  {
    var invokedExpr = Expression.Invoke (expr2, expr1.Parameters.Cast<Expression> ());
    return Expression.Lambda<Func<T, bool>>
          (Expression.OrElse (expr1.Body, invokedExpr), expr1.Parameters);
  }

Why does it Invoke new lambda instead of just using OrElse for predicate body?

I believe it's a typing issue: Expression.OrElse returns a plain Expression, not an Expression<Func<T, bool>>.

I'm not 100% sure either, but I think the issue is ensuring that as each expression Func<T,bool> is chained, the parameter, which is an instance of T, is always the same instance for each expression.

In other words, for:

(t1 => t1>5).And(t2 => t2.Color == Blue)

we assume t1 and t2 are referentially equal, but the InvocationExpression ensures they are by essentially saying: "create a new expression that invokes expr2 with the parameter from expr1".

See also PredicateBuilder Revisited where the author, a contributor to Mono, really explicitly checks for the reference equality of the parameter.