Using IsAssignableFrom with 'open' generic types
From the answer to another question:
public static bool IsAssignableToGenericType(Type givenType, Type genericType)
{
var interfaceTypes = givenType.GetInterfaces();
foreach (var it in interfaceTypes)
{
if (it.IsGenericType && it.GetGenericTypeDefinition() == genericType)
return true;
}
if (givenType.IsGenericType && givenType.GetGenericTypeDefinition() == genericType)
return true;
Type baseType = givenType.BaseType;
if (baseType == null) return false;
return IsAssignableToGenericType(baseType, genericType);
}
The exact code you posted does not return surprising results.
This says "false":
Type g1 = typeof(generic1<>);
Type g2 = typeof(generic2<>);
Console.WriteLine("g1.IsAssignableFrom(g2): {0}", g1.IsAssignableFrom(g2));
This says "true":
Type g1 = typeof(generic1<class1>);
Type g2 = typeof(generic2<class1>);
Console.WriteLine("g1.IsAssignableFrom(g2): {0}", g1.IsAssignableFrom(g2));
The difference is that open generic types cannot have instances, so one is not "assignable" to the other.
From the docs:
Returns
true
ifc
and the currentType
represent the same type, or if the currentType
is in the inheritance hierarchy ofc
, or if the currentType
is an interface thatc
implements, or ifc
is a generic type parameter and the currentType
represents one of the constraints ofc
.false
if none of these conditions are true, or ifc
isnull
.
In this case, clearly none of these conditions are true. And there's an extra note:
A generic type definition is not assignable from a closed constructed type. That is, you cannot assign the closed constructed type
MyGenericList<int>
(MyGenericList(Of Integer)
in Visual Basic) to a variable of typeMyGenericList<T>
.
In the following case use the method Konrad Rudolph provided could be wrong, like: IsAssignableToGenericType(typeof(A), typeof(A<>));// return false
I think here's a better answer
public static bool IsAssignableFrom(Type extendType, Type baseType)
{
while (!baseType.IsAssignableFrom(extendType))
{
if (extendType.Equals(typeof(object)))
{
return false;
}
if (extendType.IsGenericType && !extendType.IsGenericTypeDefinition)
{
extendType = extendType.GetGenericTypeDefinition();
}
else
{
extendType = extendType.BaseType;
}
}
return true;
}
the test case, see Using IsAssignableFrom with C# generics for detail
using System;
/**
* Sam Sha - yCoder.com
*
* */
namespace Test2
{
class MainClass
{
public static void Main (string[] args)
{
string a = "ycoder";
Console.WriteLine(a is object);
A aa = new A();
//Console.WriteLine(aa is A<>);//con't write code like this
typeof(A<>).IsAssignableFrom(aa.GetType());//return false
Trace(typeof(object).IsAssignableFrom(typeof(string)));//true
Trace(typeof(A<>).IsAssignableFrom(typeof(A)));//false
AAA aaa = new AAA();
Trace("Use IsTypeOf:");
Trace(IsTypeOf(aaa, typeof(A<>)));
Trace(IsTypeOf(aaa, typeof(AA)));
Trace(IsTypeOf(aaa, typeof(AAA<>)));
Trace("Use IsAssignableFrom from stackoverflow - not right:");
Trace(IsAssignableFrom(typeof(A), typeof(A<>))); // error
Trace(IsAssignableFrom(typeof(AA), typeof(A<>)));
Trace(IsAssignableFrom(typeof(AAA), typeof(A<>)));
Trace("Use IsAssignableToGenericType:");
Trace(IsAssignableToGenericType(typeof(A), typeof(A<>)));
Trace(IsAssignableToGenericType(typeof(AA), typeof(A<>)));
Trace(IsAssignableToGenericType(typeof(AAA), typeof(A<>)));
}
static void Trace(object log){
Console.WriteLine(log);
}
public static bool IsTypeOf(Object o, Type baseType)
{
if (o == null || baseType == null)
{
return false;
}
bool result = baseType.IsInstanceOfType(o);
if (result)
{
return result;
}
return IsAssignableFrom(o.GetType(), baseType);
}
public static bool IsAssignableFrom(Type extendType, Type baseType)
{
while (!baseType.IsAssignableFrom(extendType))
{
if (extendType.Equals(typeof(object)))
{
return false;
}
if (extendType.IsGenericType && !extendType.IsGenericTypeDefinition)
{
extendType = extendType.GetGenericTypeDefinition();
}
else
{
extendType = extendType.BaseType;
}
}
return true;
}
//from stackoverflow - not good enough
public static bool IsAssignableToGenericType(Type givenType, Type genericType) {
var interfaceTypes = givenType.GetInterfaces();
foreach (var it in interfaceTypes)
if (it.IsGenericType)
if (it.GetGenericTypeDefinition() == genericType) return true;
Type baseType = givenType.BaseType;
if (baseType == null) return false;
return baseType.IsGenericType &&
baseType.GetGenericTypeDefinition() == genericType ||
IsAssignableToGenericType(baseType, genericType);
}
}
class A{}
class AA : A{}
class AAA : AA{}
}