Emit call to System.Lazy constructor with Mono.Cecil

Question

I\'m trying to emit a method that instantiates a System.Lazy and failing with a PEVerify error of \"Invalid token\", at the line newobj instance void class [mscorl

Answer 1

I've discovered the answer buried in a years-old mailing list archive (thanks to Gábor Kozár!). I was not properly creating/importing generic types and their methods. The code that properly loads the Lazy<T> and Func<T> types follows:

var genericArgument = lazyElementType;
var funcType = ModuleDefinition.ImportReference(typeof(Func<>)).MakeGenericInstanceType(genericArgument);
var funcCtor =
    ModuleDefinition.ImportReference(funcType.Resolve()
                                    .Methods.First(m => m.IsConstructor && m.Parameters.Count == 2))
                    .MakeHostInstanceGeneric(genericArgument);

var lazyType = ModuleDefinition.ImportReference(typeof(Lazy<>)).MakeGenericInstanceType(genericArgument);
var lazyCtor =
    ModuleDefinition.ImportReference(lazyType.Resolve()
                                    .GetConstructors()
                                    .First(m => m.Parameters.Count == 1
                                             && m.Parameters[0].ParameterType.Name.StartsWith("Func")))
                    .MakeHostInstanceGeneric(genericArgument);

// Method body as above

Key to the above is the extension method MakeHostInstanceGeneric, which is defined as

public static MethodReference MakeHostInstanceGeneric(
                                  this MethodReference self,
                                  params TypeReference[] args)
{
    var reference = new MethodReference(
        self.Name,
        self.ReturnType,
        self.DeclaringType.MakeGenericInstanceType(args))
    {
        HasThis = self.HasThis,
        ExplicitThis = self.ExplicitThis,
        CallingConvention = self.CallingConvention
    };

    foreach (var parameter in self.Parameters) {
        reference.Parameters.Add(new ParameterDefinition(parameter.ParameterType));
    }

    foreach (var genericParam in self.GenericParameters) {
        reference.GenericParameters.Add(new GenericParameter(genericParam.Name, reference));
    }

    return reference;
}