"Types may declare locations that are associated with the type rather than any particular value of the type. Such locations are static fields of the type. As such, static fields declare a location that is shared by all values of the type. Just like non-static (instance) fields, a static field is typed and that type never changes. Static fields are always restricted to a single application domain basis, but they may also be allocated on a per-thread basis."
The paragraph above is the definition of static field extracted from the CLI specification. The text means that when we create a static field "f" inside a class "C", the value will not belong to any of the instances we create of C, instead its value is shared across all the instances and therefore "belongs" to the type C itself.
I've created a very simple class with one static and two instance fields, which you can see below.
1: public class BusinessLogic
2: {
3: public int instanceField1;
4: public int instanceField2;
5:
6: public static int staticField3 = 3;
7:
8: public BusinessLogic()
9: {
10: instanceField1 = 1;
11: instanceField2 = 2;
12: }
13: }
I've run the code and I've created 5 instances of the class above, which we will check with WinDBG. To do it we can execute the next command:
!DumpHeap -type BusinessLogic
That will show something similar to the next:
Address MT Size
01dfa98c 006668c4 16
01dfe898 006668c4 16
01e027a4 006668c4 16
01e0668c 006668c4 16
01e13104 006668c4 16
total 5 objects
Statistics:
MT Count TotalSize Class Name
006668c4 5 80 BusinessLogic
Total 5 objects
Once we have the address of the 5 instances we created we can dump the objects one by one by using the command !DumpObj [Address]. If we take the first one it will display something similar to:
Name: BusinessLogic
MethodTable: 001c68bc
EEClass: 00281abc
Size: 16(0x10) bytes
Fields:
MT Field Offset Type VT Attr Value Name
6f642b38 4000001 4 System.Int32 1 instance 1 instanceField1
6f642b38 4000002 8 System.Int32 1 instance 2 instanceField2
6f642b38 4000003 1c System.Int32 1 static 3 staticField3
What we have done with this command is to examine the fields of one of the instances in memory of BusinessLogic. We can see that the object has the fields instanceField1, instanceField2 and staticField3 and its values are 1,2 and 3 respectively. So, all is as expected.
Lets focus now on the cool thing of static fields. We can see how "staticField3" is shared across al the instances we create of BusinessLogic by examing the EEClass of the objects. The 5 instances we created before have the same EEClass: "00281abc". If we examine it with the command !DumpClass 00281abc we will see that "staticField3" is present at EEClass level and its value is already initialized.
Class Name: BusinessLogic
mdToken: 02000002
Parent Class: 6f3d3ef0
Module: 001c64f0
Method Table: 001c68bc
Vtable Slots: 4
Total Method Slots: 6
Class Attributes: 100001
NumInstanceFields: 2
NumStaticFields: 1
MT Field Offset Type VT Attr Value Name
6f642b38 4000001 4 System.Int32 1 instance instanceField1
6f642b38 4000002 8 System.Int32 1 instance instanceField2
6f642b38 4000003 1c System.Int32 1 static 3 staticField3
At this point we have demonstrated how the static fields and their values are shared across all the instances we create of a class.
In this sample we have used value types for the static field, but this also applies for reference types. This is very cool, because it allow us creating class designs where objects with a heavy load construction can be instantiated just once i.e.
static object staticField;
....
if (staticField == null)
staticField = new object();
This simple code will allow sharing the same instance of staticField across the entire application domain helping with the performance if staticField is hard to construct.
This does not mean that from now you must create all your "hard to construct" fields as static, because like always the gold hammer does not exist and static fields have a downside.
As we have seen the static field values are related to the EEClass, which are allocated on the loader heaps that are AppDomain specific, so this means they will be in memory until the AppDomain is unloaded.