How to Use C# .NET Reflection to Dynamically Invoke Internal Classes from Third-Party DLL Assemblies

When developing enterprise C# .NET applications, engineers frequently rely on third-party Class Library assemblies (DLLs) to interface with proprietary hardware, parse obscure file formats, or connect to legacy APIs. Occasionally, developers discover that the third-party vendor has restricted access to crucial helper methods or underlying data structures by marking those classes or methods with the internal or private access modifiers. While the compiler strictly prevents direct access to these restricted members, the Common Language Runtime (CLR) retains the metadata required to execute them. By utilizing .NET Reflection, advanced developers can bypass compiler access modifiers, dynamically inspect the assembly at runtime, and forcefully invoke these restricted internal methods.

The Mechanics of .NET Reflection

Reflection is the ability of managed code to read its own metadata. Housed within the System.Reflection namespace, these APIs allow an application to interrogate loaded assemblies, enumerate the modules within them, and discover all defined types (classes, interfaces, structs). Once a type is discovered, Reflection can map out every constructor, property, field, and method—regardless of their access modifiers.

Because Reflection circumvents the static type checking performed by the C# compiler, it inherently sacrifices compile-time safety. If the third-party vendor updates their DLL and renames the internal method, your application will compile successfully but crash with a MissingMethodException at runtime.

Loading the Third-Party Assembly

To dynamically invoke a method, you must first load the target assembly into the current application domain. Assuming you have a proprietary DLL named LegacyDriver.dll, you load it using the Assembly class.

using System;
using System.Reflection;

class ReflectionDemo
{
    static void Main()
    {
        // Load the external DLL from the absolute path
        Assembly externalAssembly = Assembly.LoadFrom(@"C:\Libs\LegacyDriver.dll");
        
        // Output confirmation
        Console.WriteLine($"Successfully loaded: {externalAssembly.FullName}");
    }
}

Discovering and Instantiating the Internal Class

Next, you must locate the specific internal class within the assembly. You must know the fully qualified namespace and class name. Once the type is located, you use the Activator class to dynamically spawn an instance of the object in memory.

// 1. Get the internal type representation
Type targetType = externalAssembly.GetType("LegacyDriver.Core.HiddenHardwareController");

if (targetType == null)
{
    throw new Exception("The internal type could not be found.");
}

// 2. Dynamically instantiate the object using the default constructor
object instance = Activator.CreateInstance(targetType);

Bypassing Access Modifiers to Invoke Methods

With the object instantiated in memory, the final step is to locate and execute the restricted method. The crucial parameter here is the BindingFlags enumeration.

By default, Reflection only searches for public methods. To instruct the CLR to search for internal or private methods, you must explicitly combine the NonPublic flag with the Instance (for standard object methods) or Static flags.

// 3. Define the search criteria using BindingFlags
BindingFlags flags = BindingFlags.NonPublic | BindingFlags.Instance;

// 4. Locate the specific internal method named 'ResetHardwareBuffer'
MethodInfo hiddenMethod = targetType.GetMethod("ResetHardwareBuffer", flags);

if (hiddenMethod == null)
{
    throw new Exception("The internal method could not be found.");
}

// 5. Invoke the method on the dynamic instance
// If the method requires parameters, pass them as an object array: new object[] { param1, param2 }
hiddenMethod.Invoke(instance, null);

Console.WriteLine("Internal method successfully executed.");

Performance and Security Considerations

While exceptionally powerful for overcoming vendor limitations or facilitating deep unit testing, dynamic invocation via Reflection carries significant penalties. Utilizing MethodInfo.Invoke() is orders of magnitude slower than a standard, strongly-typed method call because the CLR must perform late-bound security checks, parameter type validation, and boxing/unboxing operations on every single invocation.

If the internal method must be called within a high-frequency loop, developers should utilize Reflection solely during the initialization phase to create a strongly-typed Delegate pointing to the internal method, which can then be invoked repeatedly with near-native performance.