How to combine c# predicates

Question

Hi I have two predicates. And I wonder if it is possible to join them together somehow into one? First:

Predicate<object> viewFilter = f =>
{
    ExternalUser u = f as ExternalUser;
    return u.AgentName.ToLower().StartsWith(agentName.ToLower());
};

Second:

Predicate<object> viewFilter = f =>
{
    ExternalUser u = f as ExternalUser;
    return u.ExternalLogin.ToLower().StartsWith(login.ToLower());
};

And this is how joined predicate should look like.

Predicate<object> viewFilter = f =>
{
    ExternalUser u = f as ExternalUser;
    return u.ExternalLogin.ToLower().StartsWith(login.ToLower()) && u.AgentName.ToLower().StartsWith(agentName.ToLower());
};

I need this for WPF data grid filter where grid source is ICollectionView. So after I've created my predicate I set it into filter like this:

private void FilterByLogin(ICollectionView view, string login)
{
    Predicate<object> viewFilter = f =>
    {
        ExternalUser u = f as ExternalUser;
        return u.ExternalLogin.ToLower().StartsWith(login.ToLower());
    };
    view.Filter = viewFilter;
}

Aside from the question ; why do you use `as` without checking for `null` ? It implies you now for sure the type the just use a simple cast `(ExternalUser) f`. That way when for some obscure reason it'll fail you'll get an InvalidCastException rather than a NullReferenceException which is clearer about what is the problem — Sehnsucht, Sep 28 '16 at 10:25
On a sidenote: Dont use `as` if you arent checking for null. Instead cast directly. `ExternalUser u =(ExternalUser) f;` — CSharpie, Sep 28 '16 at 10:25
Possible duplicate of [Combine Multiple Predicates](http://stackoverflow.com/questions/1248232/combine-multiple-predicates) — Glen Thomas, Sep 28 '16 at 10:28

score 3 · Accepted Answer · answered Sep 28 '16 at 10:28

I would create two new methods:

public static Prediate<object> Cast<T>(Predicate<T> inner)
{
    return new Predicate<object>(o => inner((T)o));
}

public static Predicate<T> And<T>(Predicate<T> p1, Predicate<T> p2)
{
    return x => p1(x) && p2(x);
}

then you can remove the cast from your predicates and move it into the top-level:

Predicate<ExternalUser> first = u => u.AgentName.ToLower().StartsWith(agentName.ToLower());
Predicate<ExternalUser> second = u => u.ExternalLogin.ToLower().StartsWith(login.ToLower());

view.Filter = Cast(And(first, second));

score 1 · Answer 2 · answered Sep 28 '16 at 10:38

It really should be as simple as this:

Predicate<object> viewFilter1 = f =>
{
    ExternalUser u = f as ExternalUser;
    return u.AgentName.ToLower().StartsWith(agentName.ToLower());
};

Predicate<object> viewFilter2 = f =>
{
    ExternalUser u = f as ExternalUser;
    return u.ExternalLogin.ToLower().StartsWith(login.ToLower());
};

Predicate<object> viewFilter = f => viewFilter1(f) && viewFilter2(f);

How to combine c# predicates

2 Answers2

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