Mono.Cecil: Log method's entry and exit point

Question

I am writing a program that can change the IL of target program to log method's entry and exit point. I am using Mono.Cecil I want this program to insert log statements in the beginning and end of the target method.

I tried a basic program as sample.

public class Target
{
    // My target method. 
    public void Run()
    {
        Console.WriteLine("Run method body");
    }

    // This is my log method, which i want to call in begining of Run() method. 
    public void LogEntry()
    {
        System.Console.WriteLine("******Entered in RUN method.***********");
    }
}

Source program.

public class Sample 
{
    private readonly string _targetFileName;
    private readonly ModuleDefinition _module;

    public ModuleDefinition TargetModule { get { return _module; } }

    public Sample(string targetFileName)
    {
        _targetFileName = targetFileName;

        // Read the module with default parameters
        _module = ModuleDefinition.ReadModule(_targetFileName);
    }

    public void Run()
    {

        // Retrive the target class. 
        var targetType = _module.Types.Single(t => t.Name == "Target");

        // Retrieve the target method.
        var runMethod = targetType.Methods.Single(m => m.Name == "Run");

        // Get a ILProcessor for the Run method
        var processor = runMethod.Body.GetILProcessor();

        // get log entry method ref to create instruction
        var logEntryMethodReference = targetType.Methods.Single(m => m.Name == "LogEntry");

        var newInstruction = processor.Create(OpCodes.Call, logEntryMethodReference);

        var firstInstruction = runMethod.Body.Instructions[0];

        processor.InsertBefore(firstInstruction, newInstruction);

        // Write the module with default parameters
        _module.Write(_targetFileName);
    }
}

When I run my source program to change the IL of target program, I am getting following error message.

System.InvalidProgramException: Common Language Runtime detected an invalid program. at CecilDemoTarget.Target.Run() at CecilDemoTarget.Program.Main(String[] args).

Answer 1

I believe the issue is that you're calling instance method without specifying this .

To fix this, you have two choices:

• Make LogEntry static .
• Load this on the evaluation stack by adding the ldarg.0 instruction before the call .

To verify that what I'm saying is correct and to diagnose similar issues in the future, you can run Peverify on the modified assembly.