Handling enumerables in Roslyn Analyzers and Code Generators

Roslyn Analyzers and Code Generators are essential tools in the world of .NET development. Analyzers offer real-time code analysis at compile time, catching issues and promoting best practices by interpreting the code before compilation. This ensures that potential problems are identified early in the development process. Code Generators, also operating at compile time, automate the creation of repetitive code snippets. By doing so, they enhance code quality, boost efficiency, and streamline the development process.

The requirement for an enumerable to be traversed using a foreach is that the type must provide a public parameterless method named GetEnumerator() that returns an instance of an enumerator. The returned enumerator type must provide a public parameterless method named MoveNext() that returns bool, and also a public readable property named Current. Alternatively, the GetEnumerator() method can be defined as an extension method.

It’s not correct to simply check if the type derives from IEnumerable<T> or IAsyncEnumerable<T>.

This makes the correct handling of enumerables in Roslyn Analyzers and Code Generators very complicated.

NetFabric.CodeAnalysis

NetFabric.CodeAnalysis provides extension methods for the interface ITypeSymbol that can correctly validate if the type it represents can be used as the source in foreach or await foreach statements.

IsEnumerable

public static bool IsEnumerable(this ITypeSymbol typeSymbol, Compilation compilation,
[NotNullWhen(true)] out EnumerableSymbols? enumerableSymbols,
    out IsEnumerableError error)

The methods return true if the type represented by ITypeSymbol can be used in a foreach statement; otherwise false. It supports all the cases including when GetEnumerator() is defined as an extension method.

The method IsEnumerable() only returns true if both the enumerable and the enumerator are valid.

If the method IsEnumerable() returns true, the enumerableSymbols output parameter contains all the IMethodSymbol> and IPropertySymbol for the methods and properties that are going to be actually used by the foreach statement. The GetEnumerator() of the enumerable, the property Current and the method MoveNext() of the enumerator. It may also contain info for methods Reset() and Dispose() of the enumerator, if defined.

Is the methods return false, the errors output parameter indicates why the type is not considered an enumerable. It can be Error.MissingGetEnumerator, Error.MissingCurrent or Error.MissingMoveNext.

The output parameter also includes a ForEachUsesIndexer boolean property that indicates that, although the collection provides an enumerator, foreach will use the indexer instead. That’s the case for arrays and spans.

You can use these info values to further validate the enumerable and its respective enumerator. For example, use the following to find if the Current property of the enumerator returns by reference:

enumerableSymbols.EnumeratorSymbols.Current.ReturnsByRef;

IsAsyncEnumerable

public static bool IsAsyncEnumerable(this ITypeSymbol typeSymbol, Compilation compilation,
    [NotNullWhen(true)] out AsyncEnumerableSymbols? enumerableSymbols,
    out IsAsyncEnumerableError error)

The methods return true if the type represented by ITypeSymbol can be used in an await foreach statement; otherwise false. It supports all the cases including when GetAsyncEnumerator() is defined as an extension method.

The method IsAsyncEnumerable() only returns true if both the enumerable and the enumerator are valid.

If the method IsAsyncEnumerable() returns true, the enumerableSymbols output parameter contains all the IMethodSymbol and IPropertySymbol for the methods and properties that are going to be actually used by the await foreach statement. The GetAsyncEnumerator() of the enumerable, the property Current and the method MoveNextAsync() of the enumerator. It may also contain info for method DisposeAsync() of the enumerator, if defined.

Is the methods return false, the errors output parameter indicates why the type is not considered an enumerable. It can be Error.MissingGetAsyncEnumerator, Error.MissingCurrent or Error.MissingMoveNextAsync.

You can use these info values to further validate the async enumerable or its respective enumerator.

NetFabric.Hyperlinq.Analyzer

NetFabric.Hyperlinq.Analyzer is an Roslyn Analyzer that provides diagnostics and code fixes related to enumerables in C# projects. It makes use of the NetFabric.CodeAnalysis package.

If you’re developing a Roslyn Analyzer or a Code Generator, and would like to use NetFabric.CodeAnalysis, you can use its source code as reference.

If you’re developing any other kind of C# project, you can add this analyzer to your projects so that you can easily find where to use the tips I wrote about in the articles I’ve been publishing. You can find the current list of rules in its repository.