If you are using an external library, whose sources are not available to you, it does not seem feasible to use the attributes there for specifying code annotations.
In this case, you can use External Annotations, which allow you to complement the already compiled entities with attributes recognized by ReSharper's code analysis engine. External annotations let you 'cheat' the engine, by making it see the attributes (for methods, parameters and other declarations), which were not declared at the time the library was compiled.
External annotations are specified in XML files. When loading solution, ReSharper looks for specifically named XML files in specific locations and reads annotations from there. These XML files have a structure similar to XmlDoc. For example, to annotate the method XmlReader.Create(Stream input) from the assembly System.Xml of .NET Framework 4.0, with the NotNull contracts, the XML file would look as follows:
<assemblyname="System.Xml, Version=4.0.0.0"><!--The attribute name contains the assembly name. If you don't specify the version, this file's attributes will be applied to all versions of the assemblies of that name --><membername="M:System.Xml.XmlReader.Create(System.IO.Stream)"><!--This shows the name of the member whose attributes are complemented; the notation is the same as XmlDoc --><attributector="M:JetBrains.Annotations.NotNullAttribute.#ctor"/><!--attribute constructor names are also specified using XmlDoc notation --><parametername="input"><attributector="M:JetBrains.Annotations.NotNullAttribute.#ctor"/></parameter></member></assembly>
JetBrains external annotations
The approach described above was used to annotate a huge amount of symbols in the .NET Framework Class Library, as well as in other frequently used libraries. External annotations for these libraries are installed with ReSharper to ensure better code analysis. Some features (e.g. ASP.NET MVC 5.2 support) are also rely on external annotations.
Creating external annotations
You can create external annotations to specify contracts for any compiled assembly. To make sure ReSharper recognizes the XML file with external annotations, this file should be named [AssemblyName].ExternalAnnotations.xml and placed in the same folder with the assembly *.dll file.
To illustrate creating external annotations, imagine that you use a TestLib assembly that has MyTestClass class with static string ReverseString(String inputString) method. Suppose, that from the assembly documentation, you know that the method is pure and never returns null, and its parameter should never be null.
Initially, ReSharper is not aware of how ReverseString works, and therefore it finds no problems in the following code:
To create external annotations for a compiled assembly
Locate the TestLib.dll on your file system and create a file named TestLib.ExternalAnnotations.xml in the same folder.
Open TestLib.ExternalAnnotations.xml for editing and type in the following:
Reload your solution. Now ReSharper detects incorrect usage of ReverseString and highlights the problem with the null parameter: ... and the unnecessary check of the return value:
To see, which external annotations are applied to a library symbol, you can use the Quick Documentation feature. Just press Ctrl+Q over the symbol to study its attributes. In our case, we can see that ReverseString is annotated with [NotNull] and [Pure] attributes and the parameter is annotated with [NotNull]: The grey color of the attributes means that they come not from assembly code, but from external annotations.
Distributing external annotations
Another case of using external annotations is publishing external annotations for a library that you distribute, or for any library you like. In this case, the users of your library who also use ReSharper or Rider, will get more suggestions and fixes.
