CLR integration overview
Applies to: 
SQL Server
Azure SQL Managed Instance
The common language runtime (CLR) is the heart of the .NET Framework and provides the execution environment for all .NET Framework code. Code that runs within the CLR is referred to as managed code. The CLR provides various functions and services required for program execution, including just-in-time (JIT) compilation, allocating and managing memory, enforcing type safety, exception handling, thread management, and security. For more information, see .NET Framework development guide.
Note
For more information about using the new .NET with SQL Server Language Extensions, see How to call the .NET runtime in SQL Server Language Extensions.
With the CLR hosted in SQL Server (called CLR integration), you can author stored procedures, triggers, user-defined functions, user-defined types, and user-defined aggregates in managed code. Because managed code compiles to native code before execution, you can achieve significant performance increases in some scenarios.
Code access security
In SQL Server 2016 (13.x) and earlier versions, Code Access Security (CAS) prevented assemblies from performing certain operations.
CLR uses Code Access Security (CAS) in the .NET Framework, which is no longer supported as a security boundary. A CLR assembly created with PERMISSION_SET = SAFE might be able to access external system resources, call unmanaged code, and acquire sysadmin privileges. In SQL Server 2017 (14.x) and later versions, the sp_configure option, clr strict security, enhances the security of CLR assemblies. clr strict security is enabled by default, and treats SAFE and EXTERNAL_ACCESS assemblies as if they were marked UNSAFE. The clr strict security option can be disabled for backward compatibility, but isn't recommended.
We recommend that you sign all assemblies by a certificate or asymmetric key, with a corresponding login that has been granted UNSAFE ASSEMBLY permission in the master database. SQL Server administrators can also add assemblies to a list of assemblies, which the Database Engine should trust. For more information, see sys.sp_add_trusted_assembly.
Advantages of CLR integration
Transact-SQL is designed for direct data access and manipulation in the database. While Transact-SQL excels at data access and management, it's not a full-fledged programming language. For example, Transact-SQL doesn't support arrays, collections, for-each loops, bit shifting, or classes. While some of these constructs can be simulated in Transact-SQL, managed code has integrated support for these constructs. Depending on the scenario, these features can provide a compelling reason to implement certain database functionality in managed code.
Visual Basic and C# offer object-oriented capabilities such as encapsulation, inheritance, and polymorphism. Related code can now be easily organized into classes and namespaces. When you're working with large amounts of server code, these capabilities allow you to more easily organize and maintain your code.
Managed code is better suited than Transact-SQL for calculations and complicated execution logic, and features extensive support for many complex tasks, including string handling and regular expressions. With the functionality found in the .NET Framework, you have access to thousands of prebuilt classes and routines. These classes can be easily accessed from any stored procedure, trigger, or user defined function. The base class library (BCL) includes classes that provide functionality for string manipulation, advanced math operations, file access, cryptography, and more.
Note
While many of these classes are available for use from within CLR code in SQL Server, those that aren't appropriate for server-side use (for example, windowing classes), aren't available. For more information, see Supported .NET Framework libraries.
One of the benefits of managed code is type safety, or the assurance that code accesses types only in well-defined, permissible ways. Before managed code is executed, the CLR verifies that the code is safe. For example, the code is checked to ensure that no memory is read that wasn't previously written. The CLR can also help ensure that code doesn't manipulate unmanaged memory.
CLR integration offers the potential for improved performance. For information, see Performance of CLR integration architecture.
Choose between Transact-SQL and managed code
When you write stored procedures, triggers, and user-defined functions, you must decide whether to use traditional Transact-SQL, or a .NET Framework language such as Visual Basic or C#. Use Transact-SQL when the code mostly performs data access with little or no procedural logic. Use managed code for CPU-intensive functions and procedures that feature complex logic, or when you want to make use of the BCL of the .NET Framework.
Choose between execution in the server and execution in the client
Another factor in your decision about whether to use Transact-SQL or managed code is where you would like your code to reside, the server computer or the client computer. Both Transact-SQL and managed code can be run on the server. This places code and data close together, and allows you to take advantage of the processing power of the server. On the other hand, you might wish to avoid placing processor intensive tasks on your database server. Most client computers today are powerful, and you might wish to take advantage of this processing power by placing as much code as possible on the client. Managed code can run on a client computer, while Transact-SQL can't.
Choose between extended stored procedures and managed code
Extended stored procedures can be built to perform functionality not possible with Transact-SQL stored procedures. Extended stored procedures can, however, compromise the integrity of the SQL Server process, while managed code that is verified to be type-safe can't. Further, memory management, scheduling of threads and fibers, and synchronization services are more deeply integrated between the managed code of the CLR and SQL Server. With CLR integration, you have a more secure way than extended stored procedures to write the stored procedures you need to perform tasks not possible in Transact-SQL. For more information about CLR integration and extended stored procedures, see Performance of CLR integration architecture.