GetInterfaces() returns generic interface type with FullName = null

Question

Can anyone explain to me why GetInterfaces() in the below code returns an interface type that has FullName = null?

public class Program
{
    static void Main(string[] args)
    {
        Type[] interfaces = typeof (Data<>).GetInterfaces();
        foreach (Type @interface in interfaces)
        {
            Console.WriteLine("Name='{0}' FullName='{1}'", @interface.Name, @interface.FullName ?? "null");
        }
    }
}

public class Data<T> : IData<T>
{
    public T Content { get; set; }
}

public interface IData<T>
{
    T Content { get; set; }
}

The output of the program is:

Name=IData`1' FullName='null'

I kind of expected:

Name=IData`1'
FullName='ConsoleApplication2.IData`1'

Please enlighten me :)

Answer 1

https://docs.microsoft.com/archive/blogs/haibo_luo/type-fullname-returns-null-when

Update: Microsoft documentation is improved:

https://msdn.microsoft.com/en-us/library/system.type.fullname.aspx

Type.FullName is null if the current instance represents a generic type parameter, an array type, pointer type, or byref type based on a type parameter, or a generic type that is not a generic type definition but contains unresolved type parameters.

Here is an example of a situation where Type.FullName is null , boiled down from the documentation:

    [Fact]
    public void FullNameOfUnresolvedGenericArgumentIsNull()
    {
        Type openGenericType = typeof(Nullable<>);
        Type typeOfUnresolvedGenericArgument = openGenericType.GetGenericArguments()[0];

        Assert.Null(typeOfUnresolvedGenericArgument.FullName);
    }

Answer 2

You can create an extension method to fix the type reference:

public static Type FixTypeReference(this Type type)
{
    if (type.FullName != null)
        return type;

    string typeQualifiedName = type.DeclaringType != null
        ? type.DeclaringType.FullName + "+" + type.Name + ", " + type.Assembly.FullName
        : type.Namespace + "." + type.Name + ", " + type.Assembly.FullName;

    return Type.GetType(typeQualifiedName, true);
}