为什么IntelliSense在foreach循环中无法按预期工作？

Question

While typing the following code snippet, I noticed that Intellisense didn't work as expected:

StringBuilder sb = new StringBuilder();

foreach (PropertyDescriptor prop in TypeDescriptor.GetProperties(sb))
{
    var name = prop.DisplayName;
    Console.WriteLine("{0}", name);
}

In the foreach statement, if I start typing prop.Di, Intellisense completes as expexcted with prop.DisplayName. However, I use the var keyword instead of PropertyDescriptor , then I see only the method inherited from object.

As TypeDescriptor.GetProperties() returns a collection of TypeDescriptor, I thought that Visual Studio would be able to infer the correct type for prop .

Why is it not working?

Answer 1

GetProperties returns a PropertyDescriptorCollection which only implements IEnumerable , not IEnumerable<PropertyDescriptor> . If you use var , the type of prop is inferred to be object not PropertyDescriptor .

Answer 2

It returns PropertyDescriptorCollection , and the return type of GetEnumerator method of PropertyDescriptorCollection is IEnumerator (non-generic). So even you can write

foreach (int prop in TypeDescriptor.GetProperties(sb))

And you won't get any exception in compile time,but at runtime you will get an InvalidCastException .

Here is an example that demonstrates the difference:

Normally, this is invalid:

foreach(string x in new List<int>())

Because List<T> implements IEnumerable<T> .But if you wrap it with a class and implement non-generic IEnumerable interface:

class NonGenericCollection : IEnumerable
{
    public List<int> List { get; set; }


    public NonGenericCollection()
    {
        List = new List<int>();
    }

    public IEnumerator GetEnumerator()
    {
        return List.GetEnumerator();
    }
}

You can write any type you want and don't get any exceptions in compile time:

foreach(string x in new NonGenericCollection())

Because the return type is inferred as object and the actual type is not known until the run-time.