Reference Importing

.NET modules use entries in the TypeRef or MemberRef tables to reference types or members from external assemblies. Importing references into the current module, therefore, form a key role when creating new- or modifying existing .NET modules. When a member is not imported into the current module, a MemberNotImportedException will be thrown when you are trying to create a PE image or write the module to the disk.

AsmResolver provides the ReferenceImporter class that does most of the heavy lifting. Obtaining an instance of ReferenceImporter can be done in two ways.

Either instantiate one yourself:

ModuleDefinition module = ...
var importer = new ReferenceImporter(module);

Or obtain the default instance that comes with every ModuleDefinition object. This avoids allocating new reference importers every time.

ModuleDefinition module = ...
var importer = module.DefaultImporter;

The example snippets that will follow in this article assume that there is such a ReferenceImporter object instantiated using either of these two methods, and is stored in an importer variable.

Importing existing members

Metadata members from external modules can be imported using the ReferenceImporter class using one of the following members:

Member type to import	Method to use	Result type
`IResolutionScope`	`ImportScope`	`IResolutionScope`
`AssemblyReference`	`ImportScope`	`IResolutionScope`
`AssemblyDefinition`	`ImportScope`	`IResolutionScope`
`ModuleReference`	`ImportScope`	`IResolutionScope`
`ITypeDefOrRef`	`ImportType`	`ITypeDefOrRef`
`TypeDefinition`	`ImportType`	`ITypeDefOrRef`
`TypeReference`	`ImportType`	`ITypeDefOrRef`
`TypeSpecification`	`ImportType`	`ITypeDefOrRef`
`IMethodDefOrRef`	`ImportMethod`	`IMethodDefOrRef`
`MethodDefinition`	`ImportMethod`	`IMethodDefOrRef`
`MethodSpecification`	`ImportMethod`	`IMethodDefOrRef`
`IFieldDescriptor`	`ImportField`	`IFieldDescriptor`
`FieldDefinition`	`ImportField`	`IFieldDescriptor`

Below an example of how to import a type definition called SomeType:

ModuleDefinition externalModule = ModuleDefinition.FromFile(...);
TypeDefinition typeToImport = externalModule.TopLevelTypes.First(t => t.Name == "SomeType");

ITypeDefOrRef importedType = importer.ImportType(typeToImport);

These types also implement the IImportable interface. This means you can also use the member.ImportWith method instead:

ModuleDefinition externalModule = ModuleDefinition.FromFile(...);
TypeDefinition typeToImport = externalModule.TopLevelTypes.First(t => t.Name == "SomeType");

ITypeDefOrRef importedType = typeToImport.ImportWith(importer);

Importing existing type signatures

Type signatures can also be imported using the ReferenceImporter class, but these should be imported using the ImportTypeSignature method instead.

Note

If a corlib type signature is imported, the appropriate type from the CorLibTypeFactory of the target module will be selected, regardless of whether CorLib versions are compatible with each other.

Importing using System.Reflection

Types and members can also be imported by passing on an instance of various System.Reflection classes.

Member type to import	Method to use	Result type
`Type`	`ImportType`	`ITypeDefOrRef`
`Type`	`ImportTypeSignature`	`TypeSignature`
`MethodBase`	`ImportMethod`	`IMethodDefOrRef`
`MethodInfo`	`ImportMethod`	`IMethodDefOrRef`
`ConstructorInfo`	`ImportMethod`	`IMethodDefOrRef`
`FieldInfo`	`ImportScope`	`MemberReference`

There is limited support for importing complex types. Types that can be imported through reflection include:

Pointer types.
By-reference types.
Array types (If an array contains only one dimension, a SzArrayTypeSignature is returned. Otherwise a ArrayTypeSignature is created).
Generic parameters.
Generic type instantiations.
Function pointer types (.NET 8.0+ only. TFM doesn't matter for this, the runtime used at runtime is all that matters.) (!!WARNING!! )

Warning

Function pointer Type instances lose their calling conventions unless attained with GetModified(Field/Property/Parameter)Type. This includes typeof! typeof(delegate*unmanaged[Cdecl]<void>) is the same as typeof(delegate*managed<void>). Import(Field/Method) will also strip the calling conventions from function pointers that are the types of fields or in method signatures. If you need to handle this, manually set the types in the resulting IMethodDefOrRef or MemberReference to the appropriate type from ImportType.

Instantiations of generic methods are also supported.

Creating new references

Member references can also be created and imported without having direct access to its member definition or System.Reflection instance. It is possible to create new instances of TypeReference and MemberReference using the constructors, but the preferred way is to use the factory methods that allow for a more fluent syntax. Below is an example of how to create a fully imported reference to void System.Console.WriteLine(string):

var factory = module.CorLibTypeFactory;
var importedMethod = factory.CorLibScope
    .CreateTypeReference("System", "Console")
    .CreateMemberReference("WriteLine", MethodSignature.CreateStatic(
        factory.Void, factory.String))
    .ImportWith(importer);

// importedMethod now references "void System.Console.WriteLine(string)"

Generic type instantiations can also be created using MakeGenericInstanceType:

ModuleDefinition module = ...

var factory = module.CorLibTypeFactory;
var importedMethod = factory.CorLibScope
    .CreateTypeReference("System.Collections.Generic", "List`1")
    .MakeGenericInstanceType(factory.Int32)
    .ToTypeDefOrRef()
    .CreateMemberReference("Add", MethodSignature.CreateInstance(
        factory.Void,
        new GenericParameterSignature(GenericParameterType.Type, 0)))
    .ImportWith(importer);

// importedMethod now references "System.Collections.Generic.List`1<System.Int32>.Add(!0)"

Similarly, generic method instantiations can be constructed using MakeGenericInstanceMethod:

ModuleDefinition module = ...

var factory = module.CorLibTypeFactory;
var importedMethod = factory.CorLibScope
    .CreateTypeReference("System", "Array")
    .CreateMemberReference("Empty", MethodSignature.CreateStatic(
        new GenericParameterSignature(GenericParameterType.Method, 0).MakeSzArrayType(), 1))
    .MakeGenericInstanceMethod(factory.String)
    .ImportWith(importer);

// importedMethod now references "!0[] System.Array.Empty<System.String>()"

Common Caveats using the Importer

Caching and reuse of instances

The default implementation of ReferenceImporter does not maintain a cache. Each call to any of the import methods will result in a new instance of the imported member. The exception to this rule is when the member passed onto the importer is defined in the module the importer is targeting itself, or was already a reference imported by an importer into the target module. In both of these cases, the same instance of this member definition or reference will be returned instead.

Importing cross-framework versions

The ReferenceImporter does not support importing across different versions of the target framework. Members are being imported as-is, and are not automatically adjusted to conform with other versions of a library.

As a result, trying to import from for example a library part of the .NET Framework into a module targeting .NET Core or vice versa has a high chance of producing an invalid .NET binary that cannot be executed by the runtime. For example, attempting to import a reference to [System.Runtime] System.DateTime into a module targeting .NET Framework will result in a new reference targeting a .NET Core library (System.Runtime) as opposed to the appropriate .NET Framework library (mscorlib).

This is a common mistake when trying to import using metadata provided by System.Reflection. For example, if the host application that uses AsmResolver targets .NET Core but the input file is targeting .NET Framework, then you will run in the exact issue described in the above.

var reference = importer.ImportType(typeof(DateTime));

// `reference` will target `[mscorlib] System.DateTime` when running on .NET Framework, and `[System.Runtime] System.DateTime` when running on .NET Core.

Therefore, always make sure you are importing from a .NET module that is compatible with the target .NET module.

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