COM and .NET Interoperability - Working with Legacy Components

Microsoft .NET Framework is used to develop web applications and Windows applications by applying objects and components. When you're building small applications, you might not need to borrow readymade components from other applications. But if your application becomes complex, you'll want to consume those components to save time and effort.

Apart from saving time, COM and .NET interoperability also allows you to continue with your existing code while using other components, thereby protecting your existing investment. This flexibility means you don't have to throw away years of work just to adopt new technology.

The Flexibility of Interoperability

Here's the best part: you don't need to use other components entirely. .NET has the capability to use even a small part of other components with your existing code. The flexibility or interoperability between COM and .NET allows you to either use COM components from .NET or use .NET components from COM.

This bidirectional compatibility means you can modernize your applications gradually, without having to rewrite everything at once. You can keep what works and enhance it with new features.

Using COM Components in .NET

To use COM components from .NET, there's a tool that takes a COM component or type library and creates a managed assembly, also called a Runtime Callable Wrapper (RCW). This wrapper assembly is the DLL file created by tlbimp.exe or the Visual Studio .NET IDE.

The RCW acts as a bridge, translating calls between your managed .NET code and the unmanaged COM component. It handles all the messy details of marshalling data types and managing memory across the boundary.

# Generate a Runtime Callable Wrapper from a COM type library
tlbimp MyComComponent.tlb /out:MyComComponent.dll

# Use the generated assembly in your .NET project
# Add reference to MyComComponent.dll in Visual Studio

// Using a COM component in .NET (Early-bound)
using MyComComponent;

// Create an instance of the COM object
var comObject = new MyComClass();

// Call methods on the COM object
string result = comObject.DoSomething("parameter");
Console.WriteLine(result);

Using .NET Components in COM

Similarly, to use .NET components from COM, you can use a wrapper class that organizes the COM calls through to the managed code. In addition, you should create interfaces for .NET components to be available in COM. These interfaces can be created either manually or automatically.

This approach allows your legacy COM applications to take advantage of new .NET functionality without requiring a complete rewrite of the COM application.

using System;
using System.Runtime.InteropServices;

// Make the class COM-visible
[ComVisible(true)]
[Guid("12345678-1234-1234-1234-123456789012")]
[ClassInterface(ClassInterfaceType.None)]
public class MyDotNetComponent : IMyInterface
{
    public string ProcessData(string input)
    {
        return $"Processed: {input}";
    }
}

// Define a COM-visible interface
[ComVisible(true)]
[Guid("87654321-4321-4321-4321-210987654321")]
[InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
public interface IMyInterface
{
    string ProcessData(string input);
}

Late-Bound vs. Early-Bound References

You can make use of existing COM components by using late-bound and early-bound references. Both are objects that are created and set to object types, but there's an important difference between them.

Late-bound references are set to object type at runtime. The late-bound COM reference is used by writing the Server.CreateObject() method. This gives you flexibility but sacrifices performance and compile-time type checking.

Early-bound references are set to object type at design time. The early-bound COM reference uses a built-in utility that creates a proxy class. This approach offers better performance and IntelliSense support in Visual Studio.

' Late-bound reference (runtime binding)
Dim lateBound As Object
lateBound = CreateObject("Excel.Application")
lateBound.Visible = True

' Early-bound reference (design-time binding)
' Add reference to Microsoft Excel Object Library first
Dim earlyBound As New Excel.Application
earlyBound.Visible = True
' IntelliSense available, better performance

Best Practices for COM Interop

When working with COM and .NET interoperability, consider these best practices. Always prefer early binding when possible for better performance and type safety. Release COM objects properly to avoid memory leaks, especially in long-running applications.

Handle exceptions carefully since COM errors translate differently in .NET. Test thoroughly, as interop can introduce subtle bugs related to threading, memory management, and type conversions. Document your interop layer well, as it can be confusing for developers unfamiliar with both technologies.