CLR Integration Programming Model Restrictions
When you are building a managed stored procedure or other managed database object, there are certain code checks performed by SQL Server that need to be considered. SQL Server performs checks on the managed code assembly when it is first registered in the database, using the CREATE ASSEMBLY statement, and also at runtime. The managed code is also checked at runtime because in an assembly there may be code paths that may never actually be reached at runtime. This provides flexibility for registering third party assemblies, especially, so that an assembly wouldn't be blocked where there is 'unsafe' code designed to run in a client environment but would never be executed in the hosted CLR. The requirements that the managed code must meet depend on whether the assembly is registered as SAFE, EXTERNAL_ACCESS, or UNSAFE, SAFE being the strictest, and are listed below.
In addition to restrictions being placed on the managed code assemblies, there are also code security permissions that are granted. The common language runtime (CLR) supports a security model called code access security (CAS) for managed code. In this model, permissions are granted to assemblies based on the identity of the code. SAFE, EXTERNAL_ACCESS, and UNSAFE assemblies have different CAS permissions. For more information, see CLR Integration Code Access Security.
When the CREATE ASSEMBLY statement is run, the following checks are performed for each security level. If any check fails, CREATE ASSEMBLY will fail with an error message.
All referenced assemblies must meet one or more of the following criteria:
The assembly is already registered in the database.
The assembly is one of the supported assemblies. For more information, see Supported .NET Framework Libraries.
You are using CREATE ASSEMBLY FROM<location>, and all the referenced assemblies and their dependencies are available in <location>.
You are using CREATE ASSEMBLY FROM<bytes …>, and all the references are specified via space separated bytes.
All EXTERNAL_ACCESS assemblies must meet the following criteria:
Static fields are not used to store information. Read-only static fields are allowed.
PEVerify test is passed. The PEVerify tool (peverify.exe), which checks that the MSIL code and associated metadata meet type safety requirements, is provided with the .NET Framework SDK.
Synchronization, for example with the SynchronizationAttribute class, is not used.
Finalizer methods are not used.
The following custom attributes are disallowed in EXTERNAL_ACCESS assemblies:
All EXTERNAL_ACCESS assembly conditions are checked.
At runtime, the code assembly is checked for the following conditions. If any of these conditions are found, the managed code will not be allowed to run and an exception will be thrown.
Loading an assembly—either explicitly by calling the System.Reflection.Assembly.Load() method from a byte array, or implicitly through the use of Reflection.Emit namespace—is not permitted.
All UNSAFE conditions are checked.
All types and methods annotated with the following host protection attribute (HPA) values in the supported list of assemblies are disallowed.
For more information about HPAs and a list of disallowed types and members in the supported assemblies, see Host Protection Attributes and CLR Integration Programming.
All EXTERNAL_ACCESS conditions are checked.