Converting a Predicate to a Func

Question

I have a class with a member Predicate which I would like to use in a Linq expression:

using System.Linq;

class MyClass
{

    public bool DoAllHaveSomeProperty()
    {
        return m_instrumentList.All(m_filterExpression);
    }

    private IEnumerable<Instrument> m_instrumentList;

    private Predicate<Instrument> m_filterExpression;
}

As I read that "Predicate<T> is [...] completely equivalent to Func<T, bool>" (see here), I would expect this to work, since All takes in as argument: Func<Instrument, bool> predicate.

However, I get the error:

Argument 2: cannot convert from 'System.Predicate<MyNamespace.Instrument>' to 'System.Type'

Is there a way to convert the predicate to an argument that this function will swallow?

score 23 · Answer 1 · answered Aug 27 '14 at 16:07

The two types represent the same logical signature, but that doesn't mean they're just interchangable. A straight assignment won't work, for example - but you can create a new Func<T, bool> from the Predicate<T, bool>. Sample code:

Predicate<string> pred = x => x.Length > 10;
// Func<string, bool> func = pred; // Error
Func<string, bool> func = new Func<string, bool>(pred); // Okay

This is a bit like having two enum types with the same values - you can convert between them, but you have to do so explicitly. They're still separate types.

In your case, this means you could write:

public bool DoAllHaveSomeProperty()
{
    return m_instrumentList.All(new Func<T, bool>(m_filterExpression));
}

The lambda expression approach suggested by other answers will work too, of course.

This `new Func(predicate)` is much much nicer than saying `x => predicate(x)`! Thank you! — florien, Aug 09 '18 at 20:10

score 16 · Accepted Answer · edited Aug 27 '14 at 16:07

16

public bool DoAllHaveSomeProperty()
{
    return m_instrumentList.All(i => m_filterExpression(i));
}

edited Aug 27 '14 at 16:07

Yuval Itzchakov

146,575
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answered Aug 27 '14 at 16:01

George Vovos

7,563
2
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Aha, that works, thanks! So Predicate and Func are clearly not **completely** the same, at least not semantically speaking! – Yellow Aug 27 '14 at 16:05
Oops, that's what I meant. :-) – Yellow Aug 28 '14 at 08:02
This has some overhead unfortunately, just fyi. I measured it, it doesn't compile or JIT away. – jackmott Apr 05 '17 at 12:56

Brian · Answer 3 · 2019-08-05T14:28:23.657

You can convert a predicate to a method by calling Invoke. All delegates have this member. Delegates don't have structural identity, but methods can be converted to matching delegates. This fix has a minor performance cost, as it adds an extra layer of indirection. However, most solutions to this problem have that problem. Eric Lippert discusses this in more detail at https://web.archive.org/web/20140625132124/http://blog.coverity.com/2014/06/18/delegates-structural-identity/ .

In your specific case, replace return m_instrumentList.All(m_filterExpression); with return m_instrumentList.All(m_filterExpression.Invoke);

Sample code demonstrating the actual problem.

void Main()
{
    Predicate<int> t1 = Foo;
    Func<int,bool> t2 = Foo;
    Predicate<int> t3 = t2.Invoke; //Legal
    Func<int,bool> t4 = t1.Invoke; //Legal
    Predicate<int> t5 = t2; //Illegal
    Func<int,bool> t6 = t1; //Illegal
}

bool Foo(int x)
{
    return x > 20;
}

score 7 · Answer 4 · edited Aug 27 '14 at 16:08

7

return m_instrumentList.All(i => m_filterExpression(i));

edited Aug 27 '14 at 16:08

Yuval Itzchakov

146,575
32
257
321

answered Aug 27 '14 at 16:01

Julien Roncaglia

17,397
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75

score 1 · Answer 5 · answered Aug 27 '14 at 16:41

Since there are a lot of answers i will add one more just for fun. If you want your code to compile you can use extention methods

//Original Code
class MyClass4
{
    public bool DoAllHaveSomeProperty()
    {
        return m_instrumentList.All(m_filterExpression);
    }

    private IEnumerable<Instrument> m_instrumentList;

    private Predicate<Instrument> m_filterExpression;
}

Add this class in the same namespace

public static class MyExtentions
{
    public static bool All(this IEnumerable<Instrument> enumer, Predicate<Instrument> pred)
    {
        return enumer.All(e => pred(e));
    }
}

score 1 · Answer 6 · answered Oct 01 '20 at 23:35

1

As Brian said, you can convert a predicate to a method via Invoke:

public bool DoAllHaveSomeProperty()
{
    return m_instrumentList.All(m_filterExpression.Invoke);
}

answered Oct 01 '20 at 23:35

Arthur Khazbs

705
8
19

1

So why repeat it? – Gert Arnold Oct 02 '20 at 07:30
@GertArnold Their answer provides a lot of details (which I respect) and the actual solution to the problem looks obstructed. I decided to put a more concise version of the answer so it would be easier to read. Maybe I should have suggested an edit to the existing answer instead. – Arthur Khazbs Oct 05 '20 at 22:57

Converting a Predicate to a Func

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