Using C# Interceptors to Reduce Repeated Code in .NET Applications

.NET applications often contain repeated code for logging, validation, caching, performance monitoring, and other cross-cutting concerns.

For example, multiple service methods may need execution-time logging. Developers might add similar logging statements to every method, creating duplication and making future changes difficult.

Traditional approaches such as middleware, decorators, filters, and dependency injection can reduce this repetition. However, they do not cover every scenario, particularly when developers want to intercept specific method calls during compilation.

C# interceptors provide a compiler-level mechanism for redirecting certain method calls to alternative implementations.

Instead of manually changing every call site, an interceptor can replace a selected invocation during compilation.

This article explains how C# interceptors work, where they can reduce repeated code, how to configure them, and what limitations developers should consider before using them in production applications.

What Are C# Interceptors?

C# interceptors are a compiler feature that allows a method invocation to be redirected to another method during compilation.

The original source code continues to call the original method, but the compiler can bind that invocation to an interceptor method.

Consider a simple method:

public static class MessageService
{
    public static void Send(string message)
    {
        Console.WriteLine(message);
    }
}

An application calls it normally:

MessageService.Send("Order created");

With an applicable interceptor, the compiler can redirect this invocation to another method.

The interceptor might perform additional work before or after calling the original implementation.

Conceptually:

Original Method Call
        |
        v
Compiler Interception
        |
        v
Interceptor Method
        |
        v
Original Implementation

The important distinction is that interception happens during compilation rather than through a runtime proxy or dynamic method replacement.

This makes interceptors particularly relevant to compiler tooling and source-generation scenarios.

How C# Interceptors Work

Interceptors are associated with specific source locations.

An interceptor identifies the original invocation using compiler-recognized location information. When the compiler finds a matching invocation, it redirects that call to the interceptor.

This means an interceptor is not automatically applied to every call of a method.

For example, if a method is called from five different source locations, an interceptor configured for one location does not necessarily intercept the other four.

This behavior makes interceptors different from traditional aspect-oriented programming systems that may apply behavior to every invocation matching a method or type.

The compiler handles the redirection as part of the compilation process.

Understand the Role of Source Generators

C# interceptors are closely related to source generators and compiler tooling.

A source generator can analyze application code and produce additional source files during compilation.

An interceptor generator can use this information to produce methods that redirect selected invocations.

A simplified workflow looks like this:

Application Source Code
          |
          v
Compiler Analysis
          |
          v
Source Generator
          |
          v
Generated Interceptor
          |
          v
Compilation
          |
          v
Redirected Method Invocation

This approach can reduce repetitive code without requiring developers to manually rewrite every method call.

However, interceptors are a specialized compiler feature. They should not be treated as a general replacement for dependency injection, middleware, or decorators.

Configure an Interceptor

Interceptor support has evolved across C# and .NET releases. Availability, restrictions, and required compiler configuration depend on the SDK and language version.

Before implementing an interceptor, verify that the selected SDK supports the required feature and that the project uses the appropriate compiler configuration.

The following example illustrates the general source-location-based interception model.

First, consider the original method:

public static class OrderService
{
    public static void CreateOrder(int orderId)
    {
        Console.WriteLine(
            $"Creating order: {orderId}");
    }
}

An application invokes the method:

OrderService.CreateOrder(101);

An interceptor can be defined using the compiler's interceptor-related attribute mechanism.

The exact attribute and location metadata are compiler-version-specific. Conceptually, an interceptor has a compatible method signature:

public static class OrderInterceptor
{
    public static void CreateOrder(
        int orderId)
    {
        Console.WriteLine(
            "Order interception started.");

        OrderService.CreateOrder(orderId);

        Console.WriteLine(
            "Order interception completed.");
    }
}

This example demonstrates the intended structure, not a complete, independently compilable interceptor.

A real interceptor must be associated with the original invocation through the supported compiler mechanism. Its signature must also satisfy the compiler's interception requirements.

Use Interceptors for Repeated Cross-Cutting Behavior

Interceptors can be useful when a source generator or compiler extension needs to introduce behavior at selected call sites.

Potential scenarios include:

Diagnostic instrumentation

A compiler extension can generate interception code around selected operations to collect diagnostic information.

For example, it may record the start and completion of a specific service invocation.

Specialized logging

A generator can add logging around known method calls without requiring developers to manually insert logging statements at every selected location.

Generated validation

A code-generation system may intercept selected calls and apply validation logic before forwarding them to the original method.

However, ordinary business validation should usually remain explicit in the application or be implemented through established validation libraries.

Testing and instrumentation

Specialized testing tools can use interception mechanisms to redirect selected calls to alternative implementations during compilation.

This can be useful for generated test infrastructure, although dependency injection and interfaces are generally easier to maintain for ordinary application testing.

Compare Interceptors With Other .NET Techniques

Interceptors solve a different problem from several commonly used .NET features.

Technique	How it works	Suitable scenarios
Middleware	Processes HTTP requests through a pipeline	Authentication, request logging
Decorator	Wraps an implementation with additional behavior	Caching, logging, validation
Dependency injection	Supplies registered implementations	Service composition and testability
Source generator	Generates code during compilation	Repetitive code generation
C# interceptor	Redirects selected method invocations during compilation	Specialized call-site interception

For example, ASP.NET Core middleware is a better choice for logging every HTTP request.

A decorator is generally a better choice when every invocation of a service interface should receive additional behavior.

Interceptors are more appropriate when a compiler or source-generation workflow needs to redirect specific calls.

Choosing the correct mechanism helps avoid unnecessary compiler complexity.

Understand Interceptor Limitations

C# interceptors have important restrictions that developers should understand before adopting them.

They are tied to specific source locations

An interceptor generally targets a particular invocation location rather than every call to a method.

Moving code between files or changing source structure may affect the generated interception mapping.

They depend on compiler support

Interceptors require compatible compiler and SDK behavior.

A project using an older compiler may not support the required functionality.

They can make code harder to understand

A developer reading a normal method call may not immediately realize that the compiler redirects it elsewhere.

This can make debugging and code reviews more complicated.

They are not a universal runtime interception mechanism

Interceptors do not automatically provide dynamic interception for arbitrary method calls.

If the application needs runtime behavior based on registered services or configuration, decorators and dependency injection are usually more suitable.

They require careful tooling and diagnostics

Generated interceptor code should be inspected and tested.

Developers need to understand how the generator identifies source locations, produces interceptor declarations, and handles changes to the original source code.

Debugging and Troubleshooting Interceptors

Because interception occurs during compilation, troubleshooting requires examining both the original source and the generated code.

Start with these checks:

Confirm that the project uses a compatible .NET SDK and compiler.
Verify the configured C# language version.
Confirm that the generator or compiler feature is enabled.
Inspect generated source files.
Check whether the interceptor targets the expected invocation.
Verify that the interceptor method signature matches the original invocation requirements.
Review compiler diagnostics for unsupported or invalid interception declarations.

For generated source inspection, the following MSBuild command can help:

dotnet build \
  -p:EmitCompilerGeneratedFiles=true

The generated files can then be inspected to understand how the generator produces its output.

The exact generated-file location depends on the project configuration.

Also verify that the interceptor does not accidentally call itself through an incorrectly redirected invocation. Recursive interception can create difficult runtime failures.

Common Mistakes to Avoid

Using interceptors for every service call

Not every repeated operation requires compiler-level interception. Middleware, decorators, and dependency injection are often simpler.

Assuming all calls are intercepted

An interceptor associated with one source location does not automatically affect every invocation of the same method.

Ignoring generated source code

Generated files are part of the compilation result. Inspect them when diagnosing unexpected behavior.

Depending on unsupported compiler behavior

Interception support and restrictions can vary between SDK and compiler versions. Use documented capabilities rather than relying on internal compiler assumptions.

Hiding important business logic

Interceptors should not make essential business behavior difficult to discover. Keep important rules explicit and maintainable.

Ignoring build and debugging complexity

Additional generators and compiler features increase the importance of consistent build environments and clear diagnostics.

Best Practices for Using C# Interceptors

Use interceptors only when source-location-based redirection provides a clear benefit.
Prefer middleware for HTTP request-level behavior.
Prefer decorators for reusable service-level behavior.
Keep business rules visible in normal application code.
Use compatible SDK and compiler versions.
Inspect generated source during development.
Add tests for both intercepted and non-intercepted call sites.
Review generated code when refactoring source locations.
Document why a particular invocation is intercepted.
Avoid relying on experimental or unsupported compiler behavior in production builds.

The most important consideration is maintainability. A small reduction in repeated code is not worth introducing a compiler mechanism that makes the application difficult to understand or troubleshoot.

Summary

C# interceptors provide a compiler-level mechanism for redirecting selected method invocations to alternative implementations.

They can support specialized source-generation, diagnostic instrumentation, and call-site transformation scenarios. However, they are not a general-purpose replacement for middleware, decorators, or dependency injection.

Developers should understand that interception is associated with specific source locations and depends on compiler support and restrictions.

For ordinary cross-cutting concerns such as HTTP logging, service validation, and caching, established .NET patterns are often easier to maintain.

Use C# interceptors when compiler-level redirection solves a specific problem that existing application patterns cannot address cleanly. Always validate generated code, compiler compatibility, and long-term maintainability before adopting them in production applications.