How can I dynamically inject code before and after C# method execution at runtime?

Runtime Method Behavior Modification in C#

This article explores techniques for dynamically altering the execution flow of C# methods at runtime, enabling the injection of code before and after method execution. The solution leverages the Harmony library.

Harmony, a powerful open-source library, simplifies runtime method manipulation in C#. It replaces original methods with proxy methods, strategically inserting custom code sequences. This allows developers to add new features or modify existing ones without recompiling.

Implementation Details:

Harmony employs a combination of dynamic method generation and assembly modifications. It creates proxy methods for each target method, generating Intermediate Language (IL) code that calls user-defined methods at precise points during execution.

Code Example:

Let's examine a sample method:

<code class="language-csharp">public class SomeGameClass
{
    public int DoSomething()
    {
        // Original method logic
        return 0;
    }
}</code>

Using Harmony's attribute-based patching:

<code class="language-csharp">using HarmonyLib;

[HarmonyPatch(typeof(SomeGameClass))]
[HarmonyPatch("DoSomething")]
class Patch
{
    static void Prefix()
    {
        // Code executed before the original method
    }

    static void Postfix(ref int __result)
    {
        // Code executed after the original method, __result is the return value
    }
}</code>

Alternatively, manual patching using reflection:

<code class="language-csharp">using HarmonyLib;
using System.Reflection;

Harmony harmony = new Harmony("mymod");
harmony.Patch(
    typeof(SomeGameClass).GetMethod("DoSomething"),
    new HarmonyMethod(typeof(Patch).GetMethod("Prefix")),
    new HarmonyMethod(typeof(Patch).GetMethod("Postfix"))
);</code>

Summary:

Harmony offers a robust solution for dynamic method enhancement in C#, particularly valuable for game development and plugin creation. Its clear documentation and intuitive API facilitate seamless method customization, enabling developers to adapt and extend applications efficiently.

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