Pinned variables in Mono.Cecil

Question

How can i make a local variable pinned using Mono.Cecil ?

Answer 1

Full example:

This:

public unsafe static void Assign2(byte[] arr) 
{
    fixed (byte* ptr = arr)
        *ptr = 255;

can be translated to:

var assembly = AssemblyDefinition.CreateAssembly(new AssemblyNameDefinition("Test", new Version()), "Test", ModuleKind.Dll);

var module = assembly.MainModule;

var testClassType = new TypeDefinition(
    "TestNamespace",
    "TestClass",
    TypeAttributes.Class | TypeAttributes.Public | TypeAttributes.Abstract | TypeAttributes.Sealed,
    module.ImportReference(typeof(object)));

module.Types.Add(testClassType);

var assignMethod = new MethodDefinition(
    "Assign",
    MethodAttributes.Public | MethodAttributes.Static,
    module.ImportReference(typeof(void)));

assignMethod.Parameters.Add(new ParameterDefinition("arr", ParameterAttributes.None, module.ImportReference(typeof(byte[]))));

testClassType.Methods.Add(assignMethod);

// Get ILProcessor for the method body
var ilProcessor = assignMethod.Body.GetILProcessor();

ilProcessor.Body.Variables.Add(new VariableDefinition(module.ImportReference(typeof(byte).MakePointerType())));

// THIS IS WHAT YOU WANT!
// using Mono.Cecil.Rocks or new PinnedType(module.ImportReference(...))
ilProcessor.Body.Variables.Add(new VariableDefinition(module.ImportReference(typeof(byte[])).MakePinnedType()));

var instrLabelA = ilProcessor.Create(OpCodes.Ldc_I4_0);
var instrLabelB = ilProcessor.Create(OpCodes.Ldloc_1);
var instrLabelC = ilProcessor.Create(OpCodes.Ldloc_0);

ilProcessor.Emit(OpCodes.Ldarg_0);
ilProcessor.Emit(OpCodes.Dup);
ilProcessor.Emit(OpCodes.Stloc_1);
ilProcessor.Emit(OpCodes.Brfalse_S, instrLabelA);

ilProcessor.Emit(OpCodes.Ldloc_1);
ilProcessor.Emit(OpCodes.Ldlen);
ilProcessor.Emit(OpCodes.Conv_I4);
ilProcessor.Emit(OpCodes.Brtrue_S, instrLabelB);

ilProcessor.Append(instrLabelA);
ilProcessor.Emit(OpCodes.Conv_U);
ilProcessor.Emit(OpCodes.Stloc_0);
ilProcessor.Emit(OpCodes.Br_S, instrLabelC);

ilProcessor.Append(instrLabelB);
ilProcessor.Emit(OpCodes.Ldc_I4_0);
ilProcessor.Emit(OpCodes.Ldelema, module.ImportReference(typeof(byte)));
ilProcessor.Emit(OpCodes.Conv_U);
ilProcessor.Emit(OpCodes.Stloc_0);

ilProcessor.Append(instrLabelC);
ilProcessor.Emit(OpCodes.Ldc_I4, 255);
ilProcessor.Emit(OpCodes.Stind_I1);
ilProcessor.Emit(OpCodes.Ldnull);
ilProcessor.Emit(OpCodes.Stloc_1);
ilProcessor.Emit(OpCodes.Ret);

// Save the assembly to disk
assembly.Write(@"test.dll");

The IL code was taken from SharpLab.

What you want is:

ilProcessor.Body.Variables.Add(new VariableDefinition(module.ImportReference(typeof(byte[])).MakePinnedType()));

remember to use

using Mono.Cecil.Rocks;

Without using Rocks you can directly:

ilProcessor.Body.Variables.Add(new VariableDefinition(new PinnedType(module.ImportReference(typeof(byte[])))));

Note that this:

public unsafe static void Assign(byte[] arr) 
{
    fixed (byte* ptr = &arr[0])
        *ptr = 255;

should generate the same code, but it doesn't.

var ilProcessor = assignMethod.Body.GetILProcessor();

// THIS IS WHAT YOU WANT!
// using Mono.Cecil.Rocks or new PinnedType(module.ImportReference(...))
ilProcessor.Body.Variables.Add(new VariableDefinition(module.ImportReference(typeof(byte)).MakePinnedType()));

ilProcessor.Emit(OpCodes.Ldarg_0);
ilProcessor.Emit(OpCodes.Ldc_I4_0);
ilProcessor.Emit(OpCodes.Ldelema, module.ImportReference(typeof(byte)));
ilProcessor.Emit(OpCodes.Stloc_0);
ilProcessor.Emit(OpCodes.Ldloc_0);
ilProcessor.Emit(OpCodes.Conv_U);
ilProcessor.Emit(OpCodes.Ldc_I4, 255);
ilProcessor.Emit(OpCodes.Stind_I1);

ilProcessor.Emit(OpCodes.Ldc_I4_0);
ilProcessor.Emit(OpCodes.Conv_U);
ilProcessor.Emit(OpCodes.Stloc_0);
ilProcessor.Emit(OpCodes.Ret);

I'll say that this happens because the case arr == null || arr.Length == 0 must be explicitly handled by code in the first case, while it is handled by the .NET (that will throw an exception), in the case &arr[0].