Reputation: 71956
I'm trying to combine a list of functions like so.
I have this:
Func<int, bool>[] criteria = new Func<int, bool>[3];
criteria[0] = i => i % 2 == 0;
criteria[1] = i => i % 3 == 0;
criteria[2] = i => i % 5 == 0;
And I want this:
Func<int, bool>[] predicates = new Func<int, bool>[3];
predicates[0] = i => i % 2 == 0;
predicates[1] = i => i % 2 == 0 && i % 3 == 0;
predicates[2] = i => i % 2 == 0 && i % 3 == 0 && i % 5 == 0;
So far I've got the following code:
Expression<Func<int, bool>>[] results = new Expression<Func<int, bool>>[criteria.Length];
for (int i = 0; i < criteria.Length; i++)
{
results[i] = f => true;
for (int j = 0; j <= i; j++)
{
Expression<Func<int, bool>> expr = b => criteria[j](b);
var invokedExpr = Expression.Invoke(
expr,
results[i].Parameters.Cast<Expression>());
results[i] = Expression.Lambda<Func<int, bool>>(
Expression.And(results[i].Body, invokedExpr),
results[i].Parameters);
}
}
var predicates = results.Select(e => e.Compile()).ToArray();
Console.WriteLine(predicates[0](6)); // Returns true
Console.WriteLine(predicates[1](6)); // Returns false
Console.WriteLine(predicates[2](6)); // Throws an IndexOutOfRangeException
Does anyone know what I'm doing wrong?
Upvotes: 2
Views: 5697
Reputation: 118935
This was a guess, as I know little about this stuff, but this seems to fix it:
Func<int, bool>[] criteria = new Func<int, bool>[3];
criteria[0] = i => i % 2 == 0;
criteria[1] = i => i % 3 == 0;
criteria[2] = i => i % 5 == 0;
Expression<Func<int, bool>>[] results = new Expression<Func<int, bool>>[criteria.Length];
for (int i = 0; i < criteria.Length; i++)
{
results[i] = f => true;
for (int j = 0; j <= i; j++)
{
int ii = i;
int jj = j;
Expression<Func<int, bool>> expr = b => criteria[jj](b);
var invokedExpr = Expression.Invoke(expr, results[ii].Parameters.Cast<Expression>());
results[ii] = Expression.Lambda<Func<int, bool>>(Expression.And(results[ii].Body, invokedExpr), results[ii].Parameters);
}
}
var predicates = results.Select(e => e.Compile()).ToArray();
The key is the introduction of 'ii' and 'jj' (maybe only one matters, I didn't try). I think you are capturing a mutable variable inside a lambda, and thus when you finally reference it, you're seeing the later-mutated value rather than the original value.
Upvotes: 4
Reputation: 110221
No need to pull in Expressions...
Func<int, bool>[] criteria = new Func<int, bool>[3];
criteria[0] = i => i % 2 == 0;
criteria[1] = i => i % 3 == 0;
criteria[2] = i => i % 5 == 0;
Func<int, bool>[] predicates = new Func<int, bool>[3];
predicates[0] = criteria[0];
for (int i = 1; i < criteria.Length; i++)
{
//need j to be an unchanging int, one for each loop execution.
int j = i;
predicates[j] = x => predicates[j - 1](x) && criteria[j](x);
}
Console.WriteLine(predicates[0](6)); //True
Console.WriteLine(predicates[1](6)); //True
Console.WriteLine(predicates[2](6)); //False
Upvotes: 5