How to determine if a type implements a specific generic interface type

You have to go up through the inheritance tree and find all the interfaces for each class in the tree, and compare typeof(IBar<>) with the result of calling Type.GetGenericTypeDefinition if the interface is generic. It's all a bit painful, certainly.

See this answer and these ones for more info and code.

public interface IFoo<T> : IBar<T> {}
public class Foo : IFoo<Foo> {}

var implementedInterfaces = typeof( Foo ).GetInterfaces();
foreach( var interfaceType in implementedInterfaces ) {
    if ( false == interfaceType.IsGeneric ) { continue; }
    var genericType = interfaceType.GetGenericTypeDefinition();
    if ( genericType == typeof( IFoo<> ) ) {
        // do something !
        break;
    }
}

I'm using a slightly simpler version of @GenericProgrammers extension method:

public static bool Implements<TInterface>(this Type type) where TInterface : class {
    var interfaceType = typeof(TInterface);

    if (!interfaceType.IsInterface)
        throw new InvalidOperationException("Only interfaces can be implemented.");

    return (interfaceType.IsAssignableFrom(type));
}

Usage:

    if (!featureType.Implements<IFeature>())
        throw new InvalidCastException();

As a helper method extension

public static bool Implements<I>(this Type type, I @interface) where I : class
{
    if(((@interface as Type)==null) || !(@interface as Type).IsInterface)
        throw new ArgumentException("Only interfaces can be 'implemented'.");

    return (@interface as Type).IsAssignableFrom(type);
}

Example usage:

var testObject = new Dictionary<int, object>();
result = testObject.GetType().Implements(typeof(IDictionary<int, object>)); // true!

To tackle the type system completely, I think you need to handle recursion, e.g. IList<T> : ICollection<T> : IEnumerable<T>, without which you wouldn't know that IList<int> ultimately implements IEnumerable<>.

    /// <summary>Determines whether a type, like IList&lt;int&gt;, implements an open generic interface, like
    /// IEnumerable&lt;&gt;. Note that this only checks against *interfaces*.</summary>
    /// <param name="candidateType">The type to check.</param>
    /// <param name="openGenericInterfaceType">The open generic type which it may impelement</param>
    /// <returns>Whether the candidate type implements the open interface.</returns>
    public static bool ImplementsOpenGenericInterface(this Type candidateType, Type openGenericInterfaceType)
    {
        Contract.Requires(candidateType != null);
        Contract.Requires(openGenericInterfaceType != null);

        return
            candidateType.Equals(openGenericInterfaceType) ||
            (candidateType.IsGenericType && candidateType.GetGenericTypeDefinition().Equals(openGenericInterfaceType)) ||
            candidateType.GetInterfaces().Any(i => i.IsGenericType && i.ImplementsOpenGenericInterface(openGenericInterfaceType));

    }

In case you wanted an extension method that would support generic base types as well as interfaces, I've expanded sduplooy's answer:

    public static bool InheritsFrom(this Type t1, Type t2)
    {
        if (null == t1 || null == t2)
            return false;

        if (null != t1.BaseType &&
            t1.BaseType.IsGenericType &&
            t1.BaseType.GetGenericTypeDefinition() == t2)
        {
            return true;
        }

        if (InheritsFrom(t1.BaseType, t2))
            return true;

        return
            (t2.IsAssignableFrom(t1) && t1 != t2)
            ||
            t1.GetInterfaces().Any(x =>
              x.IsGenericType &&
              x.GetGenericTypeDefinition() == t2);
    }

var genericType = typeof(ITest<>);
Console.WriteLine(typeof(Test).GetInterfaces().Any(x => x.GetGenericTypeDefinition().Equals(genericType))); // prints: "True"

interface ITest<T> { };

class Test : ITest<string> { }

This worked for me.

You have to check against a constructed type of the generic interface.

You will have to do something like this:

foo is IBar<String>

because IBar<String> represents that constructed type. The reason you have to do this is because if T is undefined in your check, the compiler doesn't know if you mean IBar<Int32> or IBar<SomethingElse>.

First of all public class Foo : IFoo<T> {} does not compile because you need to specify a class instead of T, but assuming you do something like public class Foo : IFoo<SomeClass> {}

then if you do

Foo f = new Foo();
IBar<SomeClass> b = f as IBar<SomeClass>;

if(b != null)  //derives from IBar<>
    Blabla();

Method to check if the type inherits or implements a generic type:

   public static bool IsTheGenericType(this Type candidateType, Type genericType)
    {
        return
            candidateType != null && genericType != null &&
            (candidateType.IsGenericType && candidateType.GetGenericTypeDefinition() == genericType ||
             candidateType.GetInterfaces().Any(i => i.IsGenericType && i.GetGenericTypeDefinition() == genericType) ||
             candidateType.BaseType != null && candidateType.BaseType.IsTheGenericType(genericType));
    }

Try the following extension.

public static bool Implements(this Type @this, Type @interface)
{
    if (@this == null || @interface == null) return false;
    return @interface.GenericTypeArguments.Length>0
        ? @interface.IsAssignableFrom(@this)
        : @this.GetInterfaces().Any(c => c.Name == @interface.Name);
}

To test it. create

public interface IFoo { }
public interface IFoo<T> : IFoo { }
public interface IFoo<T, M> : IFoo<T> { }
public class Foo : IFoo { }
public class Foo<T> : IFoo { }
public class Foo<T, M> : IFoo<T> { }
public class FooInt : IFoo<int> { }
public class FooStringInt : IFoo<string, int> { }
public class Foo2 : Foo { }

and the test method

public void Test()
{
    Console.WriteLine(typeof(Foo).Implements(typeof(IFoo)));
    Console.WriteLine(typeof(FooInt).Implements(typeof(IFoo)));
    Console.WriteLine(typeof(FooInt).Implements(typeof(IFoo<>)));
    Console.WriteLine(typeof(FooInt).Implements(typeof(IFoo<int>)));
    Console.WriteLine(typeof(FooInt).Implements(typeof(IFoo<string>)));
    Console.WriteLine(typeof(FooInt).Implements(typeof(IFoo<,>)));
    Console.WriteLine(typeof(FooStringInt).Implements(typeof(IFoo<,>)));
    Console.WriteLine(typeof(FooStringInt).Implements(typeof(IFoo<string,int>)));
    Console.WriteLine(typeof(Foo<int,string>).Implements(typeof(IFoo<string>)));
 }

You can add the below extension method:

public static TypeExtension
{
    public static bool IsImplement<T>(this Type type)
    {
         return type.IsImplement(typeof(T));
    }

    public static bool IsImplement(this Type type, Type interfaceType)
    {
        if (!interfaceType.IsInterface)
              throw new InvalidOperationException("Only interfaces can be implemented.");

       return type.IsAssignableTo(interfaceType) ||
           interfaceType.IsGenericType && type.GetInterfaces()
            .Any(i => i.IsGenericType && i.GetGenericTypeDefinition() == interfaceType.GetGenericTypeDefinition());
   }
}