Whenever protected resources are accessed by an assembly, it's permissions are determined by the code access security system of CLR. Each permission set granted to an assembly is based on the assembly's evidence (such as its URL or publisher certificate, strong name), which in turn is based on configurable security policy.

Code groups are the building blocks of security policies. A Code Group is made of an association between an evidence value and a permission set.

• A hierarchical structure of Code Groups defines a security policy. The .NET framework comes with three different security policies: Enterprise, Machine, User. Additionally an host can define application domain-level policy by calling the AppDomain.SetAppDomainPolicy method on the System.AppDomain class. The first three policies are typically set by administrator while the latter is eventually defined by developers. 
  
  
  

There are number of built-in permission sets as shown below.

• FullTrust
• Everything
• Internet
• LocalIntranet
• Execution
• SkipVerification
• Nothing

Let's see how to create/delete a code group at runtime.

Register CodeGroup.

We need to decide on the following while creating a code-group.

1. At what level we need to set the code group?
2. What evidence value is to be set?
3. What permission set we need to provide for this code-group?

To access a security level.

Imports System.Security
Imports System.Reflection
Imports System.Security.Permissions
Imports System.Security.Policy

We can use SecurityManager.PolicyHierarchy()

Dim secLevels As IEnumerator = SecurityManager.PolicyHierarchy()
Dim policyMachineLevel As PolicyLevel = Nothing
Dim machineCodeGroupRoot As CodeGroup = Nothing
While secLevels.MoveNext()
Dim level As PolicyLevel = secLevels.Current '
If Not (level Is Nothing) And level.Label = "Machine" Then 'used to check whether the level is Machine Level
policyMachineLevel = level
machineCodeGroupRoot = level.RootCodeGroup
Exit While
End If
End While
Return policyMachineLevel

To provide evidence value.

We can use Assembly.GetExecutingAssembly() to get the assembly object and then we can use assembly.Evidence to get the evidence information.