C# Compiler Options that specify inputs

The following options control compiler inputs. The new MSBuild syntax is shown in Bold. The older csc.exe syntax is shown in code style.

  • References / -reference or -references: Reference metadata from the specified assembly file or files.
  • AddModules / -addmodule: Add a module (created with target:module to this assembly.)
  • EmbedInteropTypes / -link: Embed metadata from the specified interop assembly files.


The References option causes the compiler to import public type information in the specified file into the current project, enabling you to reference metadata from the specified assembly files.

<Reference Include="filename" />

filename is the name of a file that contains an assembly manifest. To import more than one file, include a separate Reference element for each file. You can define an alias as a child element of the Reference element:

<Reference Include="filename.dll">

In the previous example, LS is the valid C# identifier that represents a root namespace that will contain all namespaces in the assembly filename.dll. The files you import must contain a manifest. Use AdditionalLibPaths to specify the directory in which one or more of your assembly references is located. The AdditionalLibPaths topic also discusses the directories in which the compiler searches for assemblies. In order for the compiler to recognize a type in an assembly, and not in a module, it needs to be forced to resolve the type, which you can do by defining an instance of the type. There are other ways to resolve type names in an assembly for the compiler: for example, if you inherit from a type in an assembly, the type name will then be recognized by the compiler. Sometimes it is necessary to reference two different versions of the same component from within one assembly. To do this, use the Aliases element on the References element for each file to distinguish between the two files. This alias will be used as a qualifier for the component name, and will resolve to the component in one of the files.


In Visual Studio, use the Add Reference command. For more information, see How to: Add or Remove References By Using the Reference Manager.


This option adds a module that was created with the <TargetType>module</TargetType> switch to the current compilation:

<AddModule Include=file1 />
<AddModule Include=file2 />

Where file, file2 are output files that contain metadata. The file can't contain an assembly manifest. To import more than one file, separate file names with either a comma or a semicolon. All modules added with AddModules must be in the same directory as the output file at run time. That is, you can specify a module in any directory at compile time but the module must be in the application directory at run time. If the module isn't in the application directory at run time, you'll get a TypeLoadException. file can't contain an assembly. For example, if the output file was created with TargetType option of module, its metadata can be imported with AddModules.

If the output file was created with a TargetType option other than module, its metadata cannot be imported with AddModules but can be imported with the References option.


Causes the compiler to make COM type information in the specified assemblies available to the project that you are currently compiling.


Where file1;file2;file3 is a semicolon-delimited list of assembly file names. If the file name contains a space, enclose the name in quotation marks. The EmbedInteropTypes option enables you to deploy an application that has embedded type information. The application can then use types in a runtime assembly that implement the embedded type information without requiring a reference to the runtime assembly. If various versions of the runtime assembly are published, the application that contains the embedded type information can work with the various versions without having to be recompiled. For an example, see Walkthrough: Embedding Types from Managed Assemblies.

Using the EmbedInteropTypes option is especially useful when you're working with COM interop. You can embed COM types so that your application no longer requires a primary interop assembly (PIA) on the target computer. The EmbedInteropTypes option instructs the compiler to embed the COM type information from the referenced interop assembly into the resulting compiled code. The COM type is identified by the CLSID (GUID) value. As a result, your application can run on a target computer that has installed the same COM types with the same CLSID values. Applications that automate Microsoft Office are a good example. Because applications like Office usually keep the same CLSID value across different versions, your application can use the referenced COM types as long as .NET Framework 4 or later is installed on the target computer and your application uses methods, properties, or events that are included in the referenced COM types. The EmbedInteropTypes option embeds only interfaces, structures, and delegates. Embedding COM classes isn't supported.


When you create an instance of an embedded COM type in your code, you must create the instance by using the appropriate interface. Attempting to create an instance of an embedded COM type by using the CoClass causes an error.

Like the References compiler option, the EmbedInteropTypes compiler option uses the Csc.rsp response file, which references frequently used .NET assemblies. Use the NoConfig compiler option if you don't want the compiler to use the Csc.rsp file.

// The following code causes an error if ISampleInterface is an embedded interop type.
ISampleInterface<SampleType> sample;

Types that have a generic parameter whose type is embedded from an interop assembly cannot be used if that type is from an external assembly. This restriction doesn't apply to interfaces. For example, consider the Range interface that is defined in the Microsoft.Office.Interop.Excel assembly. If a library embeds interop types from the Microsoft.Office.Interop.Excel assembly and exposes a method that returns a generic type that has a parameter whose type is the Range interface, that method must return a generic interface, as shown in the following code example.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Microsoft.Office.Interop.Excel;

public class Utility
    // The following code causes an error when called by a client assembly.
    public List<Range> GetRange1()
        return null;

    // The following code is valid for calls from a client assembly.
    public IList<Range> GetRange2()
        return null;

In the following example, client code can call the method that returns the IList generic interface without error.

public class Client
    public void Main()
        Utility util = new Utility();

        // The following code causes an error.
        List<Range> rangeList1 = util.GetRange1();

        // The following code is valid.
        List<Range> rangeList2 = (List<Range>)util.GetRange2();