Is there any way to determine if an object is a generic list? I'm not going to know the type of the list, I just know it's a list. How can I determine that?
This will return "True"
List<int> myList = new List<int>();
Console.Write(myList.GetType().IsGenericType && myList is IEnumerable);
Do you care to know if it's exactly a "List"... or are you ok with it being IEnumerable, and Generic?
The following method will return the item type of a generic collection type.
If the type does not implement ICollection<> then null is returned.
static Type GetGenericCollectionItemType(Type type)
{
if (type.IsGenericType)
{
var args = type.GetGenericArguments();
if (args.Length == 1 &&
typeof(ICollection<>).MakeGenericType(args).IsAssignableFrom(type))
{
return args[0];
}
}
return null;
}
Edit: The above solution assumes that the specified type has a generic parameter of its own. This will not work for types that implement ICollection<> with a hard coded generic parameter, for example:
class PersonCollection : List<Person> {}
Here is a new implementation that will handle this case.
static Type GetGenericCollectionItemType(Type type)
{
return type.GetInterfaces()
.Where(face => face.IsGenericType &&
face.GetGenericTypeDefinition() == typeof(ICollection<>))
.Select(face => face.GetGenericArguments()[0])
.FirstOrDefault();
}
The accepted answer doesn't guarantee the type of IList<>.
Check this version, it works for me:
private static bool IsList(object value)
{
var type = value.GetType();
var targetType = typeof (IList<>);
return type.GetInterfaces().Any(i => i.IsGenericType
&& i.GetGenericTypeDefinition() == targetType);
}
Try:
if(yourList.GetType().IsGenericType)
{
var genericTypeParams = yourList.GetType().GetGenericArguments;
//do something interesting with the types..
}
The question is ambiguous.
The answer depends on what you mean by a generic list.
A List<SomeType> ?
A class that derives from List<SomeType> ?
A class that implements IList<SomeType> (in which case an array can be considered to be a generic list - e.g. int[] implements IList<int>)?
A class that is generic and implements IEnumerable (this is the test proposed in the accepted answer)? But this will also consider the following rather pathological class to be a generic list:
.
public class MyClass<T> : IEnumerable
{
IEnumerator IEnumerable.GetEnumerator()
{
return null;
}
}
The best solution (e.g. whether to use GetType, IsAssignableFrom, etc) will depend on what you mean.
Theres a GetType() function in the System.Object class. Have you tried that?
Related
I'm struggling with implementing the IEquatable<> interface for a class. The class has a Parameter property that uses a generic type. Basically the class definition is like this:
public class MyClass<T> : IEquatable<MyClass<T>>
{
public T Parameter { get; }
...
}
In the Equals() method I'm using EqualityComparer<T>.Default.Equals(Parameter, other.Parameter) to compare the property. Generally, this works fine – as long as the property is not a collection, for example an IEnumerable<T>. The problem is that the default equality comparer for IEnumerable<T> is checking reference equality.
Obviously, you'd want to use SequenceEqual() to compare the IEnumerable<T>. But to get this running, you need to specify the generic type of the SequenceEqual() method. This is the closest I could get:
var parameterType = typeof(T);
var enumerableType = parameterType.GetInterfaces()
.Where(type => type.IsGenericType && type.GetGenericTypeDefinition() == typeof(IEnumerable<>))
.Select(type => type.GetGenericArguments().First()).FirstOrDefault();
if (enumerableType != null)
{
var castedThis = Convert.ChangeType(Parameter, enumerableType);
var castedOther = Convert.ChangeType(other.Parameter, enumerableType);
var isEqual = castedThis.SequenceEqual(castedOther);
}
But this does not work because Convert.ChangeType() returns an object. And of course object does not implement SequenceEqual().
How do I get this working? Thanks for any tipps!
Best regards,
Oliver
Given that you have a generic container that you want to compare various generic items, you don't want to be hard coding in various specific equality checks for certain types. There are going to be lots of situations where the default equality comparison won't work for what some particular caller is trying to do. The comments have numerous different examples of problems that can come up, but also just consider the many many classes out there who's default equality is a reference comparison by for which someone might want a value comparison. You can't have this equality comparer just hard code in a solution for all of those types.
The solution of course is easy. Let the caller provide their own equality implementation, which in C#, means an IEqualityComparer<T>. Your class can become:
public class MyClass<T> : IEquatable<MyClass<T>>
{
private IEqualityComparer<T> comparer;
public MyClass(IEqualityComparer<T> innerComparer = null)
{
comparer = innerComparer ?? EqualityComparer<T>.Default;
}
public T Parameter { get; }
...
}
And now by default the default comparer will be used for any given type, but the caller can always specify a non-default comparer for any type that needs different equality semantics.
Effectively you want a way to say
var castedThis = (IEnumerable<U>)Convert.ChangeType(Parameter, enumerableType);
where T is IEnumerable<U> and U is dynamic.
I don't think you can do that.
If you are happy with some boxing though, you can use the non-generic IEnumerable interface:
public bool Equals(MyClass<T> other)
{
var parameterType = typeof(T);
if (typeof(IEnumerable).IsAssignableFrom(parameterType))
{
var castedThis = ((IEnumerable)this.Parameter).GetEnumerator();
var castedOther = ((IEnumerable)other.Parameter).GetEnumerator();
try
{
while (castedThis.MoveNext())
{
if (!castedOther.MoveNext())
return false;
if (!Convert.Equals(castedThis.Current, castedOther.Current))
return false;
}
return !castedOther.MoveNext();
}
finally
{
(castedThis as IDisposable)?.Dispose();
(castedOther as IDisposable)?.Dispose();
}
}
else
{
return EqualityComparer<T>.Default.Equals(this.Parameter, other.Parameter);
}
}
If you are not happy with the boxing, then you can use reflection to construct and call SequenceEqual (as inspired by How do I invoke an extension method using reflection?):
public bool Equals(MyClass<T> other)
{
var parameterType = typeof(T);
if (typeof(IEnumerable).IsAssignableFrom(parameterType))
{
var enumerableType = parameterType.GetGenericArguments().First();
var sequenceEqualMethod = typeof(Enumerable)
.GetMethods(BindingFlags.Static | BindingFlags.Public)
.Where(mi => {
if (mi.Name != "SequenceEqual")
return false;
if (mi.GetGenericArguments().Length != 1)
return false;
var pars = mi.GetParameters();
if (pars.Length != 2)
return false;
return pars[0].ParameterType.IsGenericType && pars[0].ParameterType.GetGenericTypeDefinition() == typeof(IEnumerable<>) && pars[1].ParameterType.IsGenericType && pars[1].ParameterType.GetGenericTypeDefinition() == typeof(IEnumerable<>);
})
.First()
.MakeGenericMethod(enumerableType)
;
return (bool)sequenceEqualMethod.Invoke(this.Parameter, new object[] { this.Parameter, other.Parameter });
}
else
{
return EqualityComparer<T>.Default.Equals(this.Parameter, other.Parameter);
}
}
You can cache the sequenceEqualMethod for better performance.
I've got a generic method:
Func<IEnumerable<T>, bool> CreateFunction<T>()
where T can be any number of different types. This method does a bunch of stuff using reflection and if T is an IDictionary, regardless of the the dictionary's TKey and TValue I need to execute dictionary specific code.
So the method could be called:
var f = CreateFunction<string>();
var f0 = CreateFunction<SomePocoType>();
var f1 = CreateFunction<IDictionary<string,object>>();
var f2 = CreateFunction<Dictionary<string,object>>();
var f3 = CreateFunction<SomeDerivedDictionaryType<string,object>>();
etc.
Clarification per #Andy's answer
Ultimately I want to know if T inherits from/implements IDictionary even if T itself is Dictionary or some other type that derives from that interface.
if(typeof(T) == typeof(IDictionary<,>)
doesn't work because T is the generic type not the generic type definition.
And without knowing TKey and TValue (which are not known at compile time) I can't do a comparison to any concrete type that I would know about until runtime.
The only thing that I've come up with are looking at the type's name or inspecting its method with reflection, looking for methods that would lead me to believe it is a dictionary (i.e. look for ContainsKey and get_Item).
Is there any straightforward way to make this sort of determination?
You can avoid using ugly and potentially risky type name string checking using the IsGenericType and GetGenericTypeDefinition members, as follows:
var type = typeof (T);
if (typeof (IDictionary).IsAssignableFrom(type))
{
//non-generic dictionary
}
else if (type.IsGenericType &&
type.GetGenericTypeDefinition() == typeof (IDictionary<,>))
{
//generic dictionary interface
}
else if (type.GetInterfaces().Any(
i => i.IsGenericType &&
i.GetGenericTypeDefinition() == typeof (IDictionary<,>)))
{
//implements generic dictionary
}
The easy way is just this:
Type iDict = null;
if (typeof(T).GetGenericTypeDefinition() == typeof(IDictionary<,>))
iDict = typeof(T);
else
iDict = typeof(T).GetInterface(typeof(IDictionary<,>).Name);
if (iDict != null)
{
var genericParams = iDict.GetGenericArguments();
Type tKey = genericParams[0], tValue = genericParams[1];
}
Note that this will not work (throws an exception) if T implements more than one IDictionary<,> interface, but that will probably be fine for your purposes.
For the sake of completeness, here's an implementation that will work on types with multiple IDictionary<,> interfaces by using the first one:
Type iDict = t.GetType().GetInterfaces()
.Where(t => t.IsGenericType
&& t.GetGenericTypeDefinition() == typeof(IDictionary<,>))
.FirstOrDefault();
if (iDict != null)
{
var genericParams = iDict.GetGenericArguments();
Type tKey = genericParams[0], tValue = genericParams[1];
}
Note that in this second routine t is an object, whereas T is a type in the first routine.
You could do something like
class Program
{
static void Main(string[] args)
{
Example<IDictionary<int, string>>.IsDictionary();
Example<SortedDictionary<int, string>>.IsDictionary();
Example<Dictionary<int, string>>.IsDictionary();
Console.ReadKey();
}
}
public class Example<T>
{
public static void IsDictionary()
{
if (typeof(T).GetInterface(typeof(IDictionary<,>).Name) != null || typeof(T).Name.Contains("IDictionary"))
{
Console.WriteLine("Is IDictionary");
}
else
{
Console.WriteLine("Not IDictionary");
}
}
}
I think that if you call Type.GetGenericTypeDefinition() that should return the "base" generic type used to construct the concrete Type.
Note that just comparing this to IDictionary<,> is likely not enough, because if someone passes in an instance of Dictionary<,> I assume you would want to use that, as well. You could either check to see if the Type implements IDictionary<,> or you might be able to call Type.IsAssignableFrom(), although based on the doc I'm not sure how well this would work with generic Types.
I have a collection ot types:
List<Type> types;
And I want to find out which of these types inherit from a concrete generic class without caring about T:
public class Generic<T>
I've tried with:
foreach(Type type in types)
{
if (typeof(Generic<>).IsAssignableFrom(type))
{
....
}
}
But always returns false, probably due to generic element. Any ideas?
Thanks in advance.
AFAIK, no types report as inheriting from an open generic type: I suspect you'll have to loop manually:
static bool IsGeneric(Type type)
{
while (type != null)
{
if (type.IsGenericType
&& type.GetGenericTypeDefinition() == typeof(Generic<>))
{
return true;
}
type = type.BaseType;
}
return false;
}
Then the sub-list is:
var sublist = types.FindAll(IsGeneric);
or:
var sublist = types.Where(IsGeneric).ToList();
or:
foreach(var type in types) {
if(IsGeneric(type)) {
// ...
}
}
You should get first generic ancestor for the particular type in your list, and then compare generic type definition with Generic<>:
genericType.GetGenericTypeDefinition() == typeof(Generic<>)
I am trying to determine if a runtime type is some sort of collection type. What I have below works, but it seems strange that I have to name the types that I believe to be collection types in an array like I have done.
In the code below, the reason for the generic logic is because, in my app, I expect all collections to be generic.
bool IsCollectionType(Type type)
{
if (!type.GetGenericArguments().Any())
return false;
Type genericTypeDefinition = type.GetGenericTypeDefinition();
var collectionTypes = new[] { typeof(IEnumerable<>), typeof(ICollection<>), typeof(IList<>), typeof(List<>) };
return collectionTypes.Any(x => x.IsAssignableFrom(genericTypeDefinition));
}
How would I refactor this code to be smarter or simpler?
Really all of these types inherit IEnumerable. You can check only for it:
bool IsEnumerableType(Type type)
{
return (type.GetInterface(nameof(IEnumerable)) != null);
}
or if you really need to check for ICollection:
bool IsCollectionType(Type type)
{
return (type.GetInterface(nameof(ICollection)) != null);
}
Look at "Syntax" part:
List<T>
IList
ICollection
If you need to exclude strings (which are essentially an IEnumerable<char>), use the following function:
bool IsEnumerableType(Type type)
{
return (type.Name != nameof(String)
&& type.GetInterface(nameof(IEnumerable)) != null);
}
You can use this helper method to check if a type implements an open generic interface. In your case you can use DoesTypeSupportInterface(type, typeof(Collection<>))
public static bool DoesTypeSupportInterface(Type type,Type inter)
{
if(inter.IsAssignableFrom(type))
return true;
if(type.GetInterfaces().Any(i=>i. IsGenericType && i.GetGenericTypeDefinition()==inter))
return true;
return false;
}
Or you can simply check for the non generic IEnumerable. All collection interfaces inherit from it. But I wouldn't call any type that implements IEnumerable a collection.
You can use linq, search for an interface name like
yourobject.GetType().GetInterfaces().Where(s => s.Name == "IEnumerable")
If this has values is a instance of IEnumerable.
This solution will take care of ICollection and ICollection<T>.
static bool IsCollectionType(Type type)
{
return type.GetInterfaces().Any(s => s.Namespace == "System.Collections.Generic" && (s.Name == "ICollection" || s.Name.StartsWith("ICollection`")));
}
This work for me.
private static bool IsCollectionType(Type type)
{
return type.GetInterfaces().Any(s => s.Namespace == "System.Collections.Generic" && (s.Name == "IEnumerable" || s.Name.StartsWith("IEnumerable`")));
}
is there a way to retrieve type T from IEnumerable<T> through reflection?
e.g.
i have a variable IEnumerable<Child> info; i want to retrieve Child's type through reflection
IEnumerable<T> myEnumerable;
Type type = myEnumerable.GetType().GetGenericArguments()[0];
Thusly,
IEnumerable<string> strings = new List<string>();
Console.WriteLine(strings.GetType().GetGenericArguments()[0]);
prints System.String.
See MSDN for Type.GetGenericArguments.
Edit: I believe this will address the concerns in the comments:
// returns an enumeration of T where o : IEnumerable<T>
public IEnumerable<Type> GetGenericIEnumerables(object o) {
return o.GetType()
.GetInterfaces()
.Where(t => t.IsGenericType
&& t.GetGenericTypeDefinition() == typeof(IEnumerable<>))
.Select(t => t.GetGenericArguments()[0]);
}
Some objects implement more than one generic IEnumerable so it is necessary to return an enumeration of them.
Edit: Although, I have to say, it's a terrible idea for a class to implement IEnumerable<T> for more than one T.
I'd just make an extension method. This worked with everything I threw at it.
public static Type GetItemType<T>(this IEnumerable<T> enumerable)
{
return typeof(T);
}
I had a similar problem. The selected answer works for actual instances.
In my case I had only a type (from a PropertyInfo).
The selected answer fails when the type itself is typeof(IEnumerable<T>) not an implementation of IEnumerable<T>.
For this case the following works:
public static Type GetAnyElementType(Type type)
{
// Type is Array
// short-circuit if you expect lots of arrays
if (type.IsArray)
return type.GetElementType();
// type is IEnumerable<T>;
if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof (IEnumerable<>))
return type.GetGenericArguments()[0];
// type implements/extends IEnumerable<T>;
var enumType = type.GetInterfaces()
.Where(t => t.IsGenericType &&
t.GetGenericTypeDefinition() == typeof(IEnumerable<>))
.Select(t => t.GenericTypeArguments[0]).FirstOrDefault();
return enumType ?? type;
}
If you know the IEnumerable<T> (via generics), then just typeof(T) should work. Otherwise (for object, or the non-generic IEnumerable), check the interfaces implemented:
object obj = new string[] { "abc", "def" };
Type type = null;
foreach (Type iType in obj.GetType().GetInterfaces())
{
if (iType.IsGenericType && iType.GetGenericTypeDefinition()
== typeof(IEnumerable<>))
{
type = iType.GetGenericArguments()[0];
break;
}
}
if (type != null) Console.WriteLine(type);
Thank you very much for the discussion. I used it as a basis for the solution below, which works well for all cases that are of interest to me (IEnumerable, derived classes, etc). Thought I should share here in case anyone needs it also:
Type GetItemType(object someCollection)
{
var type = someCollection.GetType();
var ienum = type.GetInterface(typeof(IEnumerable<>).Name);
return ienum != null
? ienum.GetGenericArguments()[0]
: null;
}
I know this is a bit old, but I believe this method will cover all the problems and challenges stated in the comments. Credit to Eli Algranti for inspiring my work.
/// <summary>Finds the type of the element of a type. Returns null if this type does not enumerate.</summary>
/// <param name="type">The type to check.</param>
/// <returns>The element type, if found; otherwise, <see langword="null"/>.</returns>
public static Type FindElementType(this Type type)
{
if (type.IsArray)
return type.GetElementType();
// type is IEnumerable<T>;
if (ImplIEnumT(type))
return type.GetGenericArguments().First();
// type implements/extends IEnumerable<T>;
var enumType = type.GetInterfaces().Where(ImplIEnumT).Select(t => t.GetGenericArguments().First()).FirstOrDefault();
if (enumType != null)
return enumType;
// type is IEnumerable
if (IsIEnum(type) || type.GetInterfaces().Any(IsIEnum))
return typeof(object);
return null;
bool IsIEnum(Type t) => t == typeof(System.Collections.IEnumerable);
bool ImplIEnumT(Type t) => t.IsGenericType && t.GetGenericTypeDefinition() == typeof(IEnumerable<>);
}
public static Type GetInnerGenericType(this Type type)
{
// Attempt to get the inner generic type
Type innerType = type.GetGenericArguments().FirstOrDefault();
// Recursively call this function until no inner type is found
return innerType is null ? type : innerType.GetInnerGenericType();
}
This is a recursive function that will go depth first down the list of generic types until it gets a concrete type definition with no inner generic types.
I tested this method with this type:
ICollection<IEnumerable<ICollection<ICollection<IEnumerable<IList<ICollection<IEnumerable<T>>>>>>>>
which should return T
Just use typeof(T)
EDIT:
Or use .GetType().GetGenericParameter() on an instantiated object if you don't have T.
An alternative for simpler situations where it's either going to be an IEnumerable<T> or T - note use of GenericTypeArguments instead of GetGenericArguments().
Type inputType = o.GetType();
Type genericType;
if ((inputType.Name.StartsWith("IEnumerable"))
&& ((genericType = inputType.GenericTypeArguments.FirstOrDefault()) != null)) {
return genericType;
} else {
return inputType;
}
This is an improvement on Eli Algranti's solution in that it will also work where the IEnumerable<> type is at any level in the inheritance tree.
This solution will obtain the element type from any Type. If the type is not an IEnumerable<>, it will return the type passed in. For objects, use GetType. For types, use typeof, then call this extension method on the result.
public static Type GetGenericElementType(this Type type)
{
// Short-circuit for Array types
if (typeof(Array).IsAssignableFrom(type))
{
return type.GetElementType();
}
while (true)
{
// Type is IEnumerable<T>
if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(IEnumerable<>))
{
return type.GetGenericArguments().First();
}
// Type implements/extends IEnumerable<T>
Type elementType = (from subType in type.GetInterfaces()
let retType = subType.GetGenericElementType()
where retType != subType
select retType).FirstOrDefault();
if (elementType != null)
{
return elementType;
}
if (type.BaseType == null)
{
return type;
}
type = type.BaseType;
}
}
typeof(IEnumerable<Foo>).GetGenericArguments()[0] will return the first generic argument - in this case typeof(Foo).
this is how I usually do it (via extension method):
public static Type GetIEnumerableUnderlyingType<T>(this T iEnumerable)
{
return typeof(T).GetTypeInfo().GetGenericArguments()[(typeof(T)).GetTypeInfo().GetGenericArguments().Length - 1];
}
Here's my unreadable Linq query expression version ..
public static Type GetEnumerableType(this Type t) {
return !typeof(IEnumerable).IsAssignableFrom(t) ? null : (
from it in (new[] { t }).Concat(t.GetInterfaces())
where it.IsGenericType
where typeof(IEnumerable<>)==it.GetGenericTypeDefinition()
from x in it.GetGenericArguments() // x represents the unknown
let b = it.IsConstructedGenericType // b stand for boolean
select b ? x : x.BaseType).FirstOrDefault()??typeof(object);
}
Note the method also takes non-generic IEnumerable into account, it returns object in this case, because it takes a Type rather than a concrete instance as the argument. By the way, for x represents the unknown, I found this video insteresting, though it is irrelevant ..