为什么我需要指定多种类型的绑定标志来获取单个字段的信息？

Question

When trying to get field information using reflection, I need to use the code

SomeObject.GetType().GetField(
    "FieldName",
    System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance)

In particular, I must specify both BindingFlags.NonPublic and BindingFlags.Instance . If I only specify one, I get a null return.

If I am only looking at a single field, why do I need to specify multiple binding flag types?

Answer 1

These flags play a role of some sort of a filter.

If you omit BindingFlags.NonPublic , GetField() function does not look for private , internal and protected fields.

And you have to specify either BindingFlags.Instance or BindingFlags.Static to define what you are looking for.

See http://msdn.microsoft.com/en-us/library/6ztex2dc.aspx :

You must specify either BindingFlags.Instance or BindingFlags.Static in order to get a return.

...

Specify BindingFlags.NonPublic to include non-public fields (that is, private, internal, and protected fields) in the search. Only protected and internal fields on base classes are returned; private fields on base classes are not returned.

Answer 2

It's how the search is implemented. From MSDN (in the Note section):

You must specify Instance or Static along with Public or NonPublic or no members will be returned.

Answer 3

every BindingFlag has a meaning

flags do not specify how many fields you'll receive but what kind of field GetField method should look for

for example if you do not specify NonPublic then you may not be able to retrieve any private, protected or internal fields

in your case

• NonPublic - your desired property is not public
• Instance - your desired field is an instance field not a static field

other common flags

• DeclaredOnly - which are declared in the class, not inherited one
• FlattenHierarchy - it will look into every base class for the given name
• IgnoreCase - as implies ignored the casing of the name
• Public - looks for public members
• Static - looks for static members

more on BindingFlags

Answer 4

The flags form a filter. They define the types of fields returned. If your field is not public and is an instance field (ie is not a static field), you need to include these flags.

You could as well specify additional flags like BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static in order to include other filed types.

Answer 5

if it is going about: System.Reflection.BindingFlags.Instance - from msdn:

You must specify either BindingFlags.Instance or BindingFlags.Static in order to get a return.

(source)

That flag is required to determine wheter you want to get Static or Non-Static members (or both).

Next System.Reflection.BindingFlags.NonPublic tells, that you want to get non-public member (that by default are not visible outside class). Fields are usually private that is why probably you get null when trying to retreive field without BindingFlags.Public .