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Walkthrough: Embedding Types from Managed Assemblies (C# and Visual Basic)

If you embed type information from a strong-named managed assembly, you can loosely couple types in an application to achieve version independence. That is, your program can be written to use types from multiple versions of a managed library without having to be recompiled for each version.

Type embedding is frequently used with COM interop, such as an application that uses automation objects from Microsoft Office. Embedding type information enables the same build of a program to work with different versions of Microsoft Office on different computers. However, you can also use type embedding with a fully managed solution.

Type information can be embedded from an assembly that has the following characteristics:

  • The assembly exposes at least one public interface.

  • The embedded interfaces are annotated with a ComImport attribute and a Guid attribute (and a unique GUID).

  • The assembly is annotated with the ImportedFromTypeLib attribute or the PrimaryInteropAssembly attribute, and an assembly-level Guid attribute. (By default, Visual Basic and Visual C# project templates include an assembly-level Guid attribute.)

After you have specified the public interfaces that can be embedded, you can create runtime classes that implement those interfaces. A client program can then embed the type information for those interfaces at design time by referencing the assembly that contains the public interfaces and setting the Embed Interop Types property of the reference to True. This is equivalent to using the command line compiler and referencing the assembly by using the /link compiler option. The client program can then load instances of your runtime objects typed as those interfaces. If you create a new version of your strong-named runtime assembly, the client program does not have to be recompiled with the updated runtime assembly. Instead, the client program continues to use whichever version of the runtime assembly is available to it, using the embedded type information for the public interfaces.

Because the primary function of type embedding is to support embedding of type information from COM interop assemblies, the following limitations apply when you embed type information in a fully managed solution:

  • Only attributes specific to COM interop are embedded; other attributes are ignored.

  • If a type uses generic parameters and the type of the generic parameter is an embedded type, that type cannot be used across an assembly boundary. Examples of crossing an assembly boundary include calling a method from another assembly or a deriving a type from a type defined in another assembly.

  • Constants are not embedded.

  • The System.Collections.Generic.Dictionary<TKey, TValue> class does not support an embedded type as a key. You can implement your own dictionary type to support an embedded type as a key.

In this walkthrough, you will do the following:

  • Create a strong-named assembly that has a public interface that contains type information that can be embedded.

  • Create a strong-named runtime assembly that implements that public interface.

  • Create a client program that embeds the type information from the public interface and creates an instance of the class from the runtime assembly.

  • Modify and rebuild the runtime assembly.

  • Run the client program to see that the new version of the runtime assembly is being used without having to recompile the client program.

NoteNote

Your computer might show different names or locations for some of the Visual Studio user interface elements in the following instructions. The Visual Studio edition that you have and the settings that you use determine these elements. For more information, see Visual Studio Settings.

To create the type equivalence interface project

  1. In Visual Studio, on the File menu, point to New and then click Project.

  2. In the New Project dialog box, in the Project Types pane, make sure that Windows is selected. Select Class Library in the Templates pane. In the Name box, type TypeEquivalenceInterface, and then click OK. The new project is created.

  3. In Solution Explorer, right-click the Class1.vb or Class1.cs file and click Rename. Rename the file to ISampleInterface.vb or ISampleInterface.cs and press ENTER. Renaming the file will also rename the class to ISampleInterface. This class will represent the public interface for the class.

  4. Right-click the TypeEquivalenceInterface project and click Properties. Click the Compile tab in Visual Basic or the Build tab in Visual C#. Set the output path to a valid location on your development computer, such as C:\TypeEquivalenceSample. This location will also be used in a later step in this walkthrough. 

  5. While still editing the project properties, click the Signing tab. Select the Sign the assembly option. In the Choose a strong name key file list, click <New...>. In the Key file name box, type key.snk. Clear the Protect my key file with a password check box. Click OK.

  6. Open the ISampleInterface.vb or ISampleInterface.cs file. Add the following code to the ISampleInterface class file to create the ISampleInterface interface.

    
    using System;
    using System.Runtime.InteropServices;
    
    namespace TypeEquivalenceInterface
    {
        [ComImport]
        [Guid("8DA56996-A151-4136-B474-32784559F6DF")]
        public interface ISampleInterface
        {
            void GetUserInput();
            string UserInput { get; }
    
    
    ...
    
    
        }
    }
    
    
    
  7. On the Tools menu, click Create Guid. In the Create GUID dialog box, click Registry Format and then click Copy. Click Exit.

  8. In the Guid attribute, delete the sample GUID and paste in the GUID that you copied from the Create GUID dialog box. Remove the braces ({}) from the copied GUID.

  9. In Visual Basic, on the Project menu, click Show All Files. Skip this step if you are using Visual C#.

  10. In Solution Explorer, expand the My Project folder if you are using Visual Basic. Expand the Properties folder if you are using Visual C#. Double-click the AssemblyInfo.vb or AssemblyInfo.cs file. Add the following attribute to the file.

    
    [assembly: ImportedFromTypeLib("")]
    
    
    

    Save the file.

  11. Save the project.

  12. Right-click the TypeEquivalenceInterface project and click Build. The class library .dll file is compiled and saved to the specified build output path (for example, C:\TypeEquivalenceSample).

To create the type equivalence runtime project

  1. In Visual Studio, on the File menu, point to New and then click Project.

  2. In the New Project dialog box, in the Project Types pane, make sure that Windows is selected. Select Class Library in the Templates pane. In the Name box, type TypeEquivalenceRuntime, and then click OK. The new project is created.

  3. In Solution Explorer, right-click the Class1.vb or Class1.cs file and click Rename. Rename the file to SampleClass.vb or SampleClass.cs and press ENTER. Renaming the file also renames the class to SampleClass. This class will implement the ISampleInterface interface.

  4. Right-click the TypeEquivalenceRuntime project and click Properties. Click the Compile tab in Visual Basic or the Build tab in Visual C#. Set the output path to the same location you used in the TypeEquivalenceInterface project, for example, C:\TypeEquivalenceSample.

  5. While still editing the project properties, click the Signing tab. Select the Sign the assembly option. In the Choose a strong name key file list, click <New...>. In the Key file name box, type key.snk. Clear the Protect my key file with a password check box. Click OK.

  6. Right-click the TypeEquivalenceRuntime project and click Add Reference. Click the Browse tab and browse to the output path folder. Select the TypeEquivalenceInterface.dll file and click OK.

  7. In Visual Basic, on the Project menu, click Show All Files. Skip this step if you are using Visual C#.

  8. In Solution Explorer, expand the References folder. Select the TypeEquivalenceInterface reference. In the Properties window for the TypeEquivalenceInterface reference, set the Specific Version property to False.

  9. Add the following code to the SampleClass class file to create the SampleClass class.

    
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using TypeEquivalenceInterface;
    
    namespace TypeEquivalenceRuntime
    {
        public class SampleClass : ISampleInterface
        {
            private string p_UserInput;
            public string UserInput { get { return p_UserInput; } }
    
            public void GetUserInput()
            {
                Console.WriteLine("Please enter a value:");
                p_UserInput = Console.ReadLine();
            }
    
    
    ...
    
    
        }
    }
    
    
    
  10. Save the project.

  11. Right-click the TypeEquivalenceRuntime project and click Build. The class library .dll file is compiled and saved to the specified build output path (for example, C:\TypeEquivalenceSample).

To create the type equivalence client project

  1. In Visual Studio, on the File menu, point to New and then click Project.

  2. In the New Project dialog box, in the Project Types pane, make sure that Windows is selected. Select Console Application in the Templates pane. In the Name box, type TypeEquivalenceClient, and then click OK. The new project is created.

  3. Right-click the TypeEquivalenceClient project and click Properties. Click the Compile tab in Visual Basic or the Build tab in Visual C#. Set the output path to the same location you used in the TypeEquivalenceInterface project, for example, C:\TypeEquivalenceSample.

  4. Right-click the TypeEquivalenceClient project and click Add Reference. Click the Browse tab and browse to the output path folder. Select the TypeEquivalenceInterface.dll file (not the TypeEquivalenceRuntime.dll) and click OK.

  5. In Visual Basic, on the Project menu, click Show All Files. Skip this step if you are using Visual C#.

  6. In Solution Explorer, expand the References folder. Select the TypeEquivalenceInterface reference. In the Properties window for the TypeEquivalenceInterface reference, set the Embed Interop Types property to True.

  7. Add the following code to the Module1.vb or Program.cs file to create the client program.

    
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using TypeEquivalenceInterface;
    using System.Reflection;
    
    namespace TypeEquivalenceClient
    {
        class Program
        {
            static void Main(string[] args)
            {
                Assembly sampleAssembly = Assembly.Load("TypeEquivalenceRuntime");
                ISampleInterface sampleClass = 
                    (ISampleInterface)sampleAssembly.CreateInstance("TypeEquivalenceRuntime.SampleClass");
                sampleClass.GetUserInput();
                Console.WriteLine(sampleClass.UserInput);
                Console.WriteLine(sampleAssembly.GetName().Version.ToString());
                Console.ReadLine();
            }
        }
    }
    
    
    
  8. Press CTRL+F5 to build and run the program.

To modify the interface

  1. In Visual Studio, on the File menu, point to Open, and then click Project/Solution.

  2. In the Open Project dialog box, right-click the TypeEquivalenceInterface project, and then click Properties. Click the Application tab. Click the Assembly Information button. Change the Assembly Version and File Version values to 2.0.0.0.

  3. Open the ISampleInterface.vb or ISampleInterface.cs file. Add the following line of code to the ISampleInterface interface.

    
    DateTime GetDate();
    
    
    

    Save the file.

  4. Save the project.

  5. Right-click the TypeEquivalenceInterface project and click Build. A new version of the class library .dll file is compiled and saved in the specified build output path (for example, C:\TypeEquivalenceSample).

To modify the runtime class

  1. In Visual Studio, on the File menu, point to Open, and then click Project/Solution.

  2. In the Open Project dialog box, right-click the TypeEquivalenceRuntime project and click Properties. Click the Application tab. Click the Assembly Information button. Change the Assembly Version and File Version values to 2.0.0.0.

  3. Open the SampleClass.vb or SampleClass.cs file. Add the following lines of code to the SampleClass class.

    
    public DateTime GetDate()
    {
        return DateTime.Now;
    }
    
    
    

    Save the file.

  4. Save the project.

  5. Right-click the TypeEquivalenceRuntime project and click Build. An updated version of the class library .dll file is compiled and saved in the previously specified build output path (for example, C:\TypeEquivalenceSample).

  6. In Windows Explorer, open the output path folder (for example, C:\TypeEquivalenceSample). Double-click the TypeEquivalenceClient.exe to run the program. The program will reflect the new version of the TypeEquivalenceRuntime assembly without having been recompiled.

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