Interceptors allow a method to declare it should be used in place of another call elsewhere in the project's source code by specifying a filepath, line and column using the InterceptsLocationAttribute.

This is useful for code generation scenarios where a code-generated methods could add new functionality but could not replace or remove existing code unless that code was specifically written in a way to permit replacement or removal.

Code

Now

// User-written "Program.cs"
partial class Program
{
    public void PerformOperation()
    {
        Initialize();
    }

    public void Initialize()
    {
        // ...
    }
}

// Code-generated "Program.generated.cs"
static class ProgramInterceptors
{
    [InterceptsLocation("Program.cs", 6, 8)]
    public static void SpecialInitialize(this Program program)
    {
        // ...
    }
}

Before

// User-written "Program.cs"
partial class Program
{
    private Action initializer = () => Initialize();
    partial void PerformOperationStartHook();

    public void PerformOperation()
    {
        initializer = Initialize;
        PerformOperationStartHook();
        initializer();
    }

    public void Initialize()
    {
        // ...
    }
}

// Code-generated "Program.generated.cs"
partial class Program
{
    partial void PerformOperationStartHook()
    {
        initializer = SpecialInitialize;
    }

    public void SpecialInitialize()
    {
        // ...
    }
}

Notes

• Interceptors are opt-in and require explicit project configuration
• The interceptor must be a static method whose signature matches the intercepted call. For instance methods, the first parameter is the receiver and is annotated with this just like an extension method ^{C# 3.0}
• The containing namespace must be listed in the <InterceptorsPreviewNamespaces> MSBuild property

More information

• What's new in C# 12.0
• Proposal
• Interceptors @ JetBrains