.net - How to use Mono.Cecil to create HelloWorld.exe

Question

How do you use Mono.Cecil to create a simple program from scratch? All the examples and tutorials I've been able to find so far, assume you are working with an existing assembly, reading or making small changes.

Answer 1

See the following code to get you started:

var myHelloWorldApp = AssemblyDefinition.CreateAssembly(
    new AssemblyNameDefinition("HelloWorld", new Version(1, 0, 0, 0)), "HelloWorld", ModuleKind.Console);

var module = myHelloWorldApp.MainModule;

// create the program type and add it to the module
var programType = new TypeDefinition("HelloWorld", "Program",
    Mono.Cecil.TypeAttributes.Class | Mono.Cecil.TypeAttributes.Public, module.TypeSystem.Object);

module.Types.Add(programType);

// add an empty constructor
var ctor = new MethodDefinition(".ctor", Mono.Cecil.MethodAttributes.Public | Mono.Cecil.MethodAttributes.HideBySig
    | Mono.Cecil.MethodAttributes.SpecialName | Mono.Cecil.MethodAttributes.RTSpecialName, module.TypeSystem.Void);

// create the constructor's method body
var il = ctor.Body.GetILProcessor();

il.Append(il.Create(OpCodes.Ldarg_0));

// call the base constructor
il.Append(il.Create(OpCodes.Call, module.Import(typeof(object).GetConstructor(Array.Empty<Type>()))));

il.Append(il.Create(OpCodes.Nop));
il.Append(il.Create(OpCodes.Ret));

programType.Methods.Add(ctor);

// define the 'Main' method and add it to 'Program'
var mainMethod = new MethodDefinition("Main",
    Mono.Cecil.MethodAttributes.Public | Mono.Cecil.MethodAttributes.Static, module.TypeSystem.Void);

programType.Methods.Add(mainMethod);

// add the 'args' parameter
var argsParameter = new ParameterDefinition("args",
    Mono.Cecil.ParameterAttributes.None, module.Import(typeof(string[])));

mainMethod.Parameters.Add(argsParameter);

// create the method body
il = mainMethod.Body.GetILProcessor();

il.Append(il.Create(OpCodes.Nop));
il.Append(il.Create(OpCodes.Ldstr, "Hello World"));

var writeLineMethod = il.Create(OpCodes.Call,
    module.Import(typeof(Console).GetMethod("WriteLine", new[] { typeof(string) })));

// call the method
il.Append(writeLineMethod);

il.Append(il.Create(OpCodes.Nop));
il.Append(il.Create(OpCodes.Ret));

// set the entry point and save the module
myHelloWorldApp.EntryPoint = mainMethod;
myHelloWorldApp.Write("HelloWorld.exe");