graphql-dotnet / parser

A lexer and parser for GraphQL in .NET

GraphQL.NET Parser

This library contains a lexer and parser as well as the complete GraphQL AST model that allows you to work with GraphQL documents compatible with the October 2021 spec.

The parser from this library is used by the GraphQL.NET project and was verified by many test data sets.

Preview versions of this package are available on GitHub Packages.

1. Lexer

Generates token based on input text. Lexer takes advantage of ReadOnlyMemory<char> and in most cases does not allocate memory on the managed heap at all.

Usage:

var token = Lexer.Lex("\"str\"");

Lex method always returns the first token it finds. In this case case the result would look like following.

2. Parser

Parses provided GraphQL expression into AST (abstract syntax tree). Parser also takes advantage of ReadOnlyMemory<char> but still allocates memory for AST.

Usage:

var ast1 = Parser.Parse(@"
{
  field
}");

var ast2 = Parser.Parse(@"
{
  field
}", new ParserOptions { Ignore = IgnoreOptions.Comments });

By default ParserOptions.Ignore is IgnoreOptions.None. If you want to ignore all comments use IgnoreOptions.Comments. If you don't need information about tokens locations in the source document, then use flag IgnoreOptions.Locations. Or just use IgnoreOptions.All and this will maximize the saving of memory allocated in the managed heap for AST.

3. ASTVisitor

ASTVisitor provides API to traverse AST of the parsed GraphQL document. Default implementation traverses all AST nodes of the provided one. You can inherit from it and override desired methods to implement your own AST processing algorithm.

SDLPrinter

For printing SDL from AST, you can use SDLPrinter. This is a highly optimized visitor for asynchronous non-blocking SDL output into provided TextWriter. In the majority of cases it does not allocate memory in the managed heap at all. Extension methods are also provided for printing directly to a string, which utilize the StringBuilder and StringWriter classes.

var document = Parser.Parse("query { hero { name age } }");

// print to a string with default options
var sdl = new SDLPrinter().Print(document);

// print to a string builder
var sb = new StringBuilder();
new SDLPrinter().Print(document, sb);

// print to a string with some options
var sdlPrinter = new SDLPrinter(
    new SDLPrinterOptions
    {
        PrintComments = true,
        EachDirectiveLocationOnNewLine = true,
        EachUnionMemberOnNewLine = true,
    });
var sdl = sdlPrinter.Print(document);

// print to a stream asynchronously
using var writer = new StreamWriter(stream);
await sdlPrinter.PrintAsync(document, writer, default);
await writer.FlushAsync();

Output:

query {
  hero {
    name
    age
  }
}

SDLSorter

An AST document can be sorted with the SDLSorter using a predefined sort order. You can specify the string comparison; by default it uses a culture-invariant case-insensitive comparison. Any futher customization is possible by deriving from SDLSorterOptions and overriding the Compare methods.

var document = Parser.Parse("query { hero { name age } }");
SDLSorter.Sort(document);
var sdl = new SDLPrinter().Print(document);

Output:

query {
  hero {
    age
    name
  }
}

StructurePrinter

You can also find a StructurePrinter visitor that prints AST into the provided TextWriter as a hierarchy of node types. It can be useful when debugging for better understanding the AST structure. Consider the following GraphQL document:

query a { name age }

After StructurePrinter processing the output text will be

Document
  OperationDefinition
    Name [a]
    SelectionSet
      Field
        Name [name]
      Field
        Name [age]

Usage:

public static async Task PrintStructure(string sdl)
{
    var document = Parser.Parse(sdl);
    using var writer = new StringWriter(); 
    var printer = new StructurePrinter()
    await printer.PrintAsync(document, writer);
    var rendered = writer.ToString();
    Console.WriteLine(rendered);
}

Contributors

This project exists thanks to all the people who contribute.

PRs are welcome! Looking for something to work on? The list of open issues is a great place to start. You can help the project by simply responding to some of the asked questions.