Member is declared in another module and needs to be imported

Question

I use Mono.Cecil to create a new custom attribute type and then add it to an existing type.

To demonstrate it, I have a pre-existing DLL called "Sample" with type that is called "SampleType".

I want to use Mono.Cecil to weave in a new type in "Sample" called "NewAttribute" and then add this attribute to "SampleType".

The code looks like this: (not exactly but its good enough for example)

static void AddCustomeAttribute()
{
    var module = ModuleDefinition.ReadModule(AssemblyName);
    var attrType = NewAttributeProvider.Add(module);
    var ctor = attrType.GetConstructors().First();
    //module.Import(ctor);
    CustomAttribute attribute = new CustomAttribute(ctor);
    attribute.ConstructorArguments.Add(new CustomAttributeArgument(module.TypeSystem.String, "InternalClass"));
    module.CustomAttributes.Add(attribute);
    module.Write(AssemblyName); //error
}

-

public static TypeDefinition Add(ModuleDefinition targetModule)
{
    var type = targetModule.AddType("Namespace", "NewAttribute", TypeAttributes.Public | TypeAttributes.Class, targetModule.Import(typeof(Attribute)));
    var stringType = targetModule.TypeSystem.String;
    var nameField = type.AddField(stringType, "_name");
    var nameProp = type.AddSimpleProperty(stringType, "Name", nameField);

    // generate a constructor body
    var constructor = type.AddConstructor(MethodAttributes.Public | MethodAttributes.SpecialName | MethodAttributes.RTSpecialName, targetModule.TypeSystem.Void, new[] { stringType });
    constructor.Body.Instructions.Add(Instruction.Create(OpCodes.Ldarg_0));
    constructor.Body.Instructions.Add(Instruction.Create(OpCodes.Ldarg_1));
    constructor.Body.Instructions.Add(Instruction.Create(OpCodes.Stfld, nameField));
    constructor.Body.Instructions.Add(Instruction.Create(OpCodes.Ret));

    var attrUsageType = targetModule.Import(typeof(AttributeUsageAttribute)).Resolve();
    //var att = targetModule.Import(typeof(AttributeUsageAttribute));
    //targetModule.Import(attrUsageType);
    var attributeTargetsType = targetModule.Import(typeof(AttributeTargets));
    //targetModule.Import(attributeTargetsType);
    var propertiesToSet = new Dictionary<string, Tuple<TypeReference, object>>
    {
        {"AllowMultiple", Tuple.Create(targetModule.TypeSystem.Boolean, (object)true)}
    };
    var usageAttr = type.AddCustomAttribute(attrUsageType, new[] { attributeTargetsType }, propertiesToSet);
    //targetModule.Import(usageAttr.AttributeType);
    targetModule.Types.Add(type);
    return type;
}

-

public static CustomAttribute AddCustomAttribute(this TypeDefinition type, TypeDefinition attrType, TypeReference[] ctorParameters, Dictionary<string, Tuple<TypeReference, object>> propetiesToSet)
{
    var attrUsageCtor = attrType.GetConstructors().Single(ctor => ctor.Parameters.Count == ctorParameters.Length && ValidateParameters(ctor.Parameters, ctorParameters));
    type.Module.Import(attrUsageCtor);
    Collection<CustomAttributeNamedArgument> properties = new Collection<CustomAttributeNamedArgument>();
    foreach (KeyValuePair<string, Tuple<TypeReference, object>> typeReference in propetiesToSet)
    {
        properties.Add(new CustomAttributeNamedArgument(typeReference.Key, new CustomAttributeArgument(typeReference.Value.Item1, typeReference.Value.Item2)));
    }
    var customeAttr = new CustomAttribute(attrUsageCtor);
    foreach (var property in properties)
    {
        customeAttr.Properties.Add(property);
    }
    type.CustomAttributes.Add(customeAttr);
    return customeAttr;
}

As you see, the comments in the code are attempts that I did to fix the problem but without success. I'm sure I'm missing something but I don't know what..

Answer 1

The Import methods in Cecil have the following signature:

TypeReference Import(TypeReference type)
MethodReference Import(MethodReference method)

Import takes a type or a method, no matter where they are defined, and create a reference for them for the current module. If you don't use what they return, your code is not correct.

For instance, you write:

var attrUsageCtor = attrType.GetConstructors().Single(ctor => ...);
type.Module.Import(attrUsageCtor);

In that case, you're creating a CustomAttribute for your module, but using the constructor defined in mscorlib. You instead need to create a reference for the constructor in your module and use that reference: the result of Import is what you must use when creating the custom attribute.