Prerequisites

Introduction

Today, the Visual Basic and C# compilers are black boxes - text goes in and bytes come out - with no transparency into the intermediate phases of the compilation pipeline. With the .NET Compiler Platform (formerly known as "Roslyn"), tools and developers can leverage the exact same data structures and algorithms the compiler uses to analyze and understand code with confidence that information is accurate and complete.

In this walkthrough we'll explore the Symbol and Binding APIs. The Syntax API exposes the parsers, the syntax trees themselves, and utilities for reasoning about and constructing them.

Understanding Compilations and Symbols

The Syntax API allows you to look at the structure of a program. However, often you'll want richer information about the semantics or meaning of a program. And while a loose code file or snippet of VB or C# code can be syntactically analyzed in isolation it's not very meaningful to ask questions such as "what's the type of this variable" in a vacuum. The meaning of a type name may be dependent on assembly references, namespace imports, or other code files. That's where the Compilation class comes in.

A Compilation is analogous to a single project as seen by the compiler and represents everything needed to compile a Visual Basic or C# program such as assembly references, compiler options, and the set of source files to be compiled. With this context you can reason about the meaning of code. Compilations allow you to find Symbols - entities such as types, namespaces, members, and variables which names and other expressions refer to. The process of associating names and expressions with Symbols is called Binding.

Like SyntaxTree, Compilation is an abstract class with language-specific derivatives. When creating an instance of Compilation you must invoke a factory method on the CSharpCompilation (or VisualBasicCompilation) class.

Example - Creating a compilation

This example shows how to create a Compilation by adding assembly references and source files. Like the syntax trees, everything in the Symbols API and the Binding API is immutable.

Create a new C# Stand-Alone Code Analysis Tool project.

In Visual Studio, choose File -> New -> Project... to display the New Project dialog.
Under Visual C# -> Extensibility, choose Stand-Alone Code Analysis Tool.
Name your project "SemanticsCS" and click OK.

Replace the contents of your Program.cs with the following:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;
 
namespace SemanticsCS
{
    class Program
    {
        static void Main(string[] args)
        {
            SyntaxTree tree = CSharpSyntaxTree.ParseText(
@"using System;
using System.Collections.Generic;
using System.Text;
 
namespace HelloWorld
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(""Hello, World!"");
        }
    }
}");
 
            var root = (CompilationUnitSyntax)tree.GetRoot();
         }
    }
}

Next, add this code to the end of your Main method to construct a CSharpCompilation object:

            var compilation = CSharpCompilation.Create("HelloWorld")
                                               .AddReferences(
                                                    MetadataReference.CreateFromFile(
                                                        typeof(object).Assembly.Location))
                                               .AddSyntaxTrees(tree);

Move your cursor to the line containing the closing brace of your Main method and set a breakpoint there.

In Visual Studio, choose Debug -> Toggle Breakpoint.

Run the program.

In Visual Studio, choose Debug -> Start Debugging.

Inspect the root variable in the debugger by hovering over it and expanding the datatip.

The SemanticModel

Once you have a Compilation you can ask it for a SemanticModel for any SyntaxTree contained in that Compilation. SemanticModels can be queried to answer questions like "What names are in scope at this location?" "What members are accessible from this method?" "What variables are used in this block of text?" and "What does this name/expression refer to?"

Example - Binding a name

This example shows how to obtain a SemanticModel object for our HelloWorld SyntaxTree. Once the model is obtained, the name in the first using directive is bound to retrieve a Symbol for the System namespace.

Add this code to the end of your Main method. The code gets a SemanticModel for the HelloWorld SyntaxTree and stores it in a new variable:

            var model = compilation.GetSemanticModel(tree);

Set this statement as the next statement to be executed and execute it.

Right-click this line and choose Set Next Statement.
In Visual Studio, choose Debug -> Step Over, to execute this statement and initialize the new variable.
You will need to repeat this process for each of the following steps as we introduce new variables and inspect them with the debugger.

Now add this code to bind the Name of the "using System;" directive using the SemanticModel.GetSymbolInfo method:

            var nameInfo = model.GetSymbolInfo(root.Usings[0].Name);

Execute this statement and hover over the nameInfo variable and expand the datatip to inspect the SymbolInfo object returned.

Note the Symbol property. This property returns the Symbol this expression refers to. For expressions which don't refer to anything (such as numeric literals) this property will be null.
- Note that the Symbol.Kind property returns the value SymbolKind.Namespace.

Cast the symbol to a NamespaceSymbol instance and store it in a new variable:

            var systemSymbol = (INamespaceSymbol)nameInfo.Symbol;

Execute this statement and examine the systemSymbol variable using the debugger datatips.
Stop the program.

In Visual Studio, choose Debug -> Stop debugging.

Add the following code to enumerate the sub-namespaces of the System namespace and print their names to the Console:

            foreach (var ns in systemSymbol.GetNamespaceMembers())
            {
                Console.WriteLine(ns.Name);
            }

Press Ctrl+F5 to run the program. You should see the following output:

Collections
Configuration
Deployment
Diagnostics
Globalization
IO
Reflection
Resources
Runtime
Security
StubHelpers
Text
Threading
Press any key to continue . . .

Example - Binding an expression

The previous example showed how to bind name to find a Symbol. However, there are other expressions in a C# program that can be bound that aren't names. This example shows how binding works with other expression types - in this case a simple string literal.

Add the following code to locate the "Hello, World!" string literal in the SyntaxTree and store it in a variable (it should be the only LiteralExpressionSyntax in this example):

            var helloWorldString = root.DescendantNodes()
                                       .OfType<LiteralExpressionSyntax>()
                                       .First();

Start debugging the program.
Add the following code to get the TypeInfo for this expression:

            var literalInfo = model.GetTypeInfo(helloWorldString);

Execute this statement and examine the literalInfo.

Note that its Type property is not null and returns the INamedTypeSymbol for the System.String type because the string literal expression has a compile-time type of System.String

Stop the program.
Add the following code to enumerate the public methods of the System.String class which return strings and print their names to the Console:

            var stringTypeSymbol = (INamedTypeSymbol)literalInfo.Type;
 
            Console.Clear();
            foreach (var name in (from method in stringTypeSymbol.GetMembers()
                                                              .OfType<IMethodSymbol>()
                               where method.ReturnType.Equals(stringTypeSymbol) &&
                                     method.DeclaredAccessibility == 
                                                Accessibility.Public
                               select method.Name).Distinct())
            {
                Console.WriteLine(name);
            }

Press Ctrl+F5 to run to run the program without debugging it. You should see the following output:

Join
Substring
Trim
TrimStart
TrimEnd
Normalize
PadLeft
PadRight
ToLower
ToLowerInvariant
ToUpper
ToUpperInvariant
ToString
Insert
Replace
Remove
Format
Copy
Concat
Intern
IsInterned
Press any key to continue . . .

Your Program.cs file should now look like this:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;

namespace SemanticsCS
{
    class Program
    {
        static void Main(string[] args)
        {
            SyntaxTree tree = CSharpSyntaxTree.ParseText(
@" using System;
using System.Collections.Generic;
using System.Text;
 
namespace HelloWorld
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(""Hello, World!"");
        }
    }
}");

            var root = (CompilationUnitSyntax)tree.GetRoot();

            var compilation = CSharpCompilation.Create("HelloWorld")
                                               .AddReferences(
                                                    MetadataReference.CreateFromFile(
                                                        typeof(object).Assembly.Location))
                                               .AddSyntaxTrees(tree);

            var model = compilation.GetSemanticModel(tree);

            var nameInfo = model.GetSymbolInfo(root.Usings[0].Name);

            var systemSymbol = (INamespaceSymbol)nameInfo.Symbol;

            foreach (var ns in systemSymbol.GetNamespaceMembers())
            {
                Console.WriteLine(ns.Name);
            }

            var helloWorldString = root.DescendantNodes()
                                       .OfType<LiteralExpressionSyntax>()
                                       .First();

            var literalInfo = model.GetTypeInfo(helloWorldString);

            var stringTypeSymbol = (INamedTypeSymbol)literalInfo.Type;

            Console.Clear();
            foreach (var name in (from method in stringTypeSymbol.GetMembers()
                                                              .OfType<IMethodSymbol>()
                                  where method.ReturnType.Equals(stringTypeSymbol) &&
                                        method.DeclaredAccessibility ==
                                                   Accessibility.Public
                                  select method.Name).Distinct())
            {
                Console.WriteLine(name);
            }
        }
    }
}