Introduction

The .NET Framework provides a powerful capability for exploring metadata and examining code at runtime using the System.Reflection namespace. System.Reflection enables you to dynamically load and inspect assemblies, types, methods, and various kinds of metadata about them. This makes it a very useful tool for performing various kinds of dynamic code analysis, including debugging, profiling, and code generation.

Exploring Assemblies

An assembly is a collection of types and resources that are built and deployed as a single unit. System.Reflection provides methods for loading assemblies dynamically at runtime, for example, using Assembly.Load or Assembly.LoadFrom. Once an assembly is loaded, you can use the reflection API to examine the types and members contained in the assembly.

For example, you can extract all the types contained in an assembly using the GetTypes method of the Assembly object. You can then examine the properties, fields, methods, and other members of each type using the reflection API. This is very useful for generating documentation, creating object models, or just for exploring the structure of an unfamiliar assembly.

Examining Types

System.Reflection provides a lot of methods and properties for examining the properties, methods, events, fields, and other members of a type. You can use these methods to analyze how the type was implemented, what its dependencies are, and how it interacts with other types in the system. For example, you can use the following code to get a list of all the public and private fields declared in a class:

```

Type t = typeof(MyClass);

FieldInfo[] fields = t.GetFields(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.Instance);

```

This will return an array of FieldInfo objects, which you can then use to inspect the attributes and other properties of each field. Similarly, you can use the GetProperties and GetMethods methods to get a list of all the public properties and methods declared in a class, respectively.

Method Invocation

One of the most powerful features of System.Reflection is the ability to dynamically invoke methods at runtime. You can use the Invoke method of a MethodInfo object to call the method with the specified arguments.

For example, consider the following class:

```

class MyClass {

public int Add(int a, int b) {

return a + b;

}

}

```

You can use the reflection API to invoke the Add method as follows:

```

Type t = typeof(MyClass);

object obj = Activator.CreateInstance(t);

MethodInfo mi = t.GetMethod("Add");

int result = (int)mi.Invoke(obj, new object[] { 2, 3 });

```

This will create a new instance of the MyClass class, get a reference to the Add method using the GetMethod method, and then invoke the method with the arguments (2, 3). The result of the method call is then returned as an integer value.

Code Generation

Another powerful use case for System.Reflection is generating code dynamically at runtime. You can use the reflection API to create new classes, methods, fields, and other members at runtime, and then execute the generated code using reflection.

For example, you can use the following code to dynamically generate a new class with a single method that computes the sum of two integers:

```

AssemblyName assemblyName = new AssemblyName("MyAssembly");

AssemblyBuilder assemblyBuilder = AppDomain.CurrentDomain.DefineDynamicAssembly(assemblyName, AssemblyBuilderAccess.RunAndSave);

ModuleBuilder moduleBuilder = assemblyBuilder.DefineDynamicModule("MyModule");

TypeBuilder typeBuilder = moduleBuilder.DefineType("MyClass", TypeAttributes.Public);

MethodBuilder methodBuilder = typeBuilder.DefineMethod("Add", MethodAttributes.Public, typeof(int), new Type[] { typeof(int), typeof(int) });

ILGenerator ilGenerator = methodBuilder.GetILGenerator();

ilGenerator.Emit(OpCodes.Ldarg_1);

ilGenerator.Emit(OpCodes.Ldarg_2);

ilGenerator.Emit(OpCodes.Add);

ilGenerator.Emit(OpCodes.Ret);

Type newType = typeBuilder.CreateType();

object obj = Activator.CreateInstance(newType);

MethodInfo mi = newType.GetMethod("Add");

int result = (int)mi.Invoke(obj, new object[] { 2, 3 });

```

This code will generate a new assembly with a single module, a single class named MyClass, and a single method named Add. The IL code for computing the sum of the two arguments is generated using the Emit method of the ILGenerator class. Finally, the code creates a new instance of the generated class, invokes the Add method, and prints the result to the console.

Conclusion

System.Reflection is a powerful and versatile API for performing dynamic code analysis in the .NET Framework. It provides a wealth of information and capabilities for exploring assembly metadata, analyzing types and members, invoking methods dynamically, and generating code at runtime. This makes it a very useful tool for debugging, profiling, testing, and code generation, among other use cases. With a good understanding of the System.Reflection API, you can explore the internals of the .NET Framework and create powerful and flexible applications that can adapt to changing requirements and behaviors at runtime.