Check if Object is Dictionary or List

Question

Working with .NET 2 in mono, I'm using a basic JSON library that returns nested string, object Dictionary and lists.

I'm writing a mapper to map this to a jsonData class that I already have and I need to be able to determine if the underlying type of an object is a Dictionary or a List. Below is the method I'm using to perform this test, but was wondering if theres a cleaner way?

private static bool IsDictionary(object o) {
    try {
        Dictionary<string, object> dict = (Dictionary<string, object>)o;
        return true;
    } catch {
        return false;
    }
}

private static bool IsList(object o) {
    try {
        List<object> list = (List<object>)o;
        return true;
    } catch {
        return false;
    }
}

The library I'm using is litJson but the JsonMapper class essentially doesn't work on iOS, hence the reason I am writing my own mapper.

Dustin Kingen · Accepted Answer · 2015-04-22T08:56:58.950

76

Use the is keyword and reflection.

public bool IsList(object o)
{
    if(o == null) return false;
    return o is IList &&
           o.GetType().IsGenericType &&
           o.GetType().GetGenericTypeDefinition().IsAssignableFrom(typeof(List<>));
}

public bool IsDictionary(object o)
{
    if(o == null) return false;
    return o is IDictionary &&
           o.GetType().IsGenericType &&
           o.GetType().GetGenericTypeDefinition().IsAssignableFrom(typeof(Dictionary<,>));
}

edited Apr 22 '15 at 08:56

answered Jun 19 '13 at 11:39

Dustin Kingen

20,677
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system.Type doesnt seem to be available to me in mono and so cant use GetGenericTypeDefinition.. is that bit necessary? – user1711383 Jun 19 '13 at 11:53
1

It looks like the types are always `List – svick Jun 19 '13 at 11:58
o can be a string, int, long, bool, double, List or Dictionary – user1711383 Jun 19 '13 at 12:00
1

@user1711383 You can just use the `is` keyword if you are certain of the generic type parameters. e.g. `o is Dictionary` – Dustin Kingen Jun 19 '13 at 12:03
2

And you can use [`(value as IEnumerable – KyleMit Nov 30 '17 at 15:07
When I do "os is IList" it gives me an error: "CS0305: Using the generic type 'IList' requires 1 type arguments" – shieldgenerator7 Jan 01 '21 at 01:36
5

@shieldgenerator7 you need to use the non-Generic interface `IList` which is in the System.Collections namespace. – Dustin Kingen Jan 02 '21 at 02:03

score 7 · Answer 2 · answered Jun 19 '13 at 11:59

7

If you want to check that a certain object is of some type, use the is operator. For example:

private static bool IsDictionary(object o)
{
    return o is Dictionary<string, object>;
}

Though for something this simple, you probably don't need a separate method, just use the is operator directly where you need it.

answered Jun 19 '13 at 11:59

svick

236,525
50
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1

Careful - the `is` operator checks to see if the left hand object is COMPATIBLE with the right hand object, NOT if some object is necessarily the exact same type... – Ben Apr 02 '17 at 19:03
@Ben Yes, but if the type of `o` inherits from `Dictionary`, I think `true` is the right answer here. – svick Apr 02 '17 at 20:21

score 3 · Answer 3 · answered Mar 02 '15 at 13:31

Modifying the above answer. In order to use GetGenericTypeDefinition() you must preface the method with GetType(). If you look at MSDN this is how GetGenericTypeDefinition() is accessed:

public virtual Type GetGenericTypeDefinition()

Here is the link: https://msdn.microsoft.com/en-us/library/system.type.getgenerictypedefinition(v=vs.110).aspx

public bool IsList(object o)
{
    return o is IList &&
       o.GetType().IsGenericType &&
       o.GetType().GetGenericTypeDefinition().IsAssignableFrom(typeof(List<>));
}

public bool IsDictionary(object o)
{
    return o is IDictionary &&
       o.GetType().IsGenericType &&
       o.GetType().GetGenericTypeDefinition().IsAssignableFrom(typeof(Dictionary<>));
}

score 1 · Answer 4 · answered Apr 27 '17 at 05:09

If you just need to detect the object is List/Dictionary or not, you can use myObject.GetType().IsGenericType && myObject is IEnumerable

Here is some example:

var lst = new List<string>();
var dic = new Dictionary<int, int>();
string s = "Hello!";
object obj1 = new { Id = 10 };
object obj2 = null;

// True
Console.Write(lst.GetType().IsGenericType && lst is IEnumerable);
// True
Console.Write(dic.GetType().IsGenericType && dic is IEnumerable);
// False
Console.Write(s.GetType().IsGenericType && s is IEnumerable);
// False
Console.Write(obj1.GetType().IsGenericType && obj1 is IEnumerable);
// NullReferenceException: Object reference not set to an instance of 
// an object, so you need to check for null cases too
Console.Write(obj2.GetType().IsGenericType && obj2 is IEnumerable);

error CS0305: Using the generic type 'IEnumerable' requires 1 type arguments — Kristupas A., Nov 16 '22 at 14:54

score 0 · Answer 5 · answered Apr 17 '22 at 00:12

0

var t = obj.GetType();

if(t.IsGenericType && typeof(System.Collections.IList).IsAssignableFrom(t))
{
    // it is of type list

    // if its a list of integers for example then listOf will be of type int
    var listOf = t.GenericTypeArguments[0];
}

answered Apr 17 '22 at 00:12

Tono Nam

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Check if Object is Dictionary or List

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