Dynamic C# Compilation with Roslyn

This snippet demonstrates how to dynamically compile C# code at runtime using the Roslyn compiler. It compiles a simple C# class and executes a method within it.

Prerequisites

Before you begin, make sure you have the Roslyn NuGet packages installed in your project. The essential package is Microsoft.CodeAnalysis.CSharp. You might also need Microsoft.CodeAnalysis.CSharp.Scripting for simpler scripting scenarios.

Complete Code Example

This code defines a RoslynCompiler class with a Main method and a CompileCSharpCode method. The CompileCSharpCode method takes C# code as a string, compiles it into an assembly using Roslyn, and returns the compiled assembly. If compilation fails, it prints the diagnostic messages. The Main method then instantiates and executes the dynamically compiled code.

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.Emit;
using System;
using System.IO;
using System.Reflection;

public class RoslynCompiler
{
    public static void Main(string[] args)
    {
        string codeToCompile = @"
        using System;

        namespace DynamicCode
        {
            public class MyClass
            {
                public string Greet(string name)
                {
                    return $\"Hello, {name}! This code was dynamically compiled.\";
                }
            }
        }";

        // Compile the code
        Assembly compiledAssembly = CompileCSharpCode(codeToCompile);

        if (compiledAssembly != null)
        {
            // Create an instance of the dynamically compiled class
            Type myClassType = compiledAssembly.GetType("DynamicCode.MyClass");
            object myClassInstance = Activator.CreateInstance(myClassType);

            // Invoke the 'Greet' method
            MethodInfo greetMethod = myClassType.GetMethod("Greet");
            string result = (string)greetMethod.Invoke(myClassInstance, new object[] { "Roslyn" });

            Console.WriteLine(result);
        }
        else
        {
            Console.WriteLine("Compilation failed.");
        }
    }

    public static Assembly CompileCSharpCode(string code)
    {
        string assemblyName = Path.GetRandomFileName();
        SyntaxTree syntaxTree = CSharpSyntaxTree.ParseText(code);
        string currentPath = Directory.GetCurrentDirectory();
        string refPath = Path.Combine(currentPath, "System.Private.CoreLib.dll");
        
        CSharpCompilation compilation = CSharpCompilation.Create(
            assemblyName,
            syntaxTrees: new[] { syntaxTree },
            references: new[] { 
                MetadataReference.CreateFromFile(typeof(object).Assembly.Location),
                MetadataReference.CreateFromFile(typeof(Console).Assembly.Location),
                MetadataReference.CreateFromFile(refPath)  // Add System.Private.CoreLib
                },
            options: new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary));

        using (var ms = new MemoryStream())
        {
            EmitResult result = compilation.Emit(ms);

            if (!result.Success)
            {
                Console.WriteLine("Compilation failed!");
                foreach (Diagnostic diagnostic in result.Diagnostics)
                {
                    Console.WriteLine(diagnostic.ToString());
                }
                return null;
            }

            ms.Seek(0, SeekOrigin.Begin);
            Assembly assembly = Assembly.Load(ms.ToArray());
            return assembly;
        }
    }
}

Step-by-Step Explanation

The code first defines a string containing the C# code to be compiled. This code defines a simple class MyClass with a Greet method. Then, the CompileCSharpCode method is called to compile the code. Inside this method, CSharpSyntaxTree.ParseText parses the C# code into a syntax tree. CSharpCompilation.Create creates a compilation object with the syntax tree and references to necessary assemblies like System.Console and System.Runtime. The compilation is then emitted to a memory stream, and if successful, the assembly is loaded from the stream. Finally, the Main method retrieves the type MyClass from the loaded assembly, creates an instance of it, and invokes the Greet method using reflection.

Concepts Behind the Snippet

This snippet utilizes the following key concepts:

• Roslyn Compiler: The .NET Compiler Platform, providing C# and Visual Basic compilers with rich code analysis APIs.
• Syntax Trees: A representation of the structure of the C# code.
• Compilation: Represents a single invocation of the C# compiler.
• Metadata References: References to other assemblies that the compiled code depends on.
• Emit: Generates the compiled assembly from the compilation object.
• Reflection: Allows you to inspect and manipulate types and members at runtime.

Real-Life Use Case

Dynamic compilation is useful in scenarios where you need to generate code at runtime, such as:

• Scripting Engines: Allowing users to write and execute C# scripts within your application.
• Code Generation: Generating code based on user input or data.
• Plugins and Extensions: Loading and executing custom code provided by third-party developers.
• Testing Frameworks: Dynamically generating test cases.

Best Practices

When using dynamic compilation, consider these best practices:

• Error Handling: Thoroughly handle compilation errors and provide informative messages to the user.
• Security: Be cautious about executing code from untrusted sources. Consider using sandboxing techniques to restrict the permissions of dynamically compiled code.
• Performance: Dynamic compilation can be expensive. Cache compiled assemblies whenever possible to avoid recompilation.
• Dependency Management: Ensure that all necessary assemblies are available at runtime.

Interview Tip

Be prepared to discuss the advantages and disadvantages of dynamic compilation, its use cases, and security considerations. Also, understand the role of Roslyn in the compilation process.

When to Use Them

Use dynamic compilation when:

• You need to generate code at runtime based on changing conditions.
• You need to support user-defined scripts or plugins.
• You can't know the exact structure of the code at compile time.

Memory Footprint

Dynamic compilation can increase the memory footprint of your application, especially if you are constantly compiling and loading new assemblies. Consider unloading unused assemblies to release memory. Also, reuse the same compilation objects and syntax trees when possible to reduce memory allocation.

Alternatives

Alternatives to dynamic compilation include:

• Expression Trees: A more lightweight way to generate code at runtime, but with limited functionality.
• Pre-compilation: Generate the code at design time or build time.
• Configuration: Use configuration files to control the behavior of your application.

Pros

Advantages of dynamic compilation:

• Flexibility: Allows you to generate code at runtime based on user input or changing conditions.
• Extensibility: Enables users to extend your application with custom code.
• Adaptability: Allows your application to adapt to new requirements without recompilation.

Cons

Disadvantages of dynamic compilation:

• Performance: Can be slower than pre-compiled code.
• Security: Raises security concerns if you are executing code from untrusted sources.
• Complexity: Adds complexity to your application.
• Debugging: Debugging dynamically generated code can be more difficult.