如何使用mono.cecil按类型在方法体中找到变量？

Question

I've had a look around the cecil questions here and I haven't seen anything regarding this particular question.

What I am trying to achieve is find a variable in method.Body.Variables that is of a certain type ( System.Exception in my case)

I wrote the following code thinking it would do the trick:

var exceptionTypeReference = module.Import(typeof(Exception));
var exceptionVariable = method.Body.Variables.First(x => x.VariableType == exceptionTypeReference);

What seems strange to me even though I am sure the reason is my noobness with cecil is that I get a "Sequence contains no matching elements" error at runtime.

I've stepped through the code and I know there is a variable there and that it's type is System.Exception , but it does not want to match up with exceptionTypeReference .

I'm certain that this is simple and that my brain is fried from learning cecil. Even so, any pointers, smacks across the face with a wet fish, etc., would be much appreciated.

Answer 1

Each time you import a type it is a different instance of TypeReference

So this

var typeReference1 = moduleDefinition.Import(typeof (Exception));
var typeReference2 = moduleDefinition.Import(typeof (Exception));
Debug.WriteLine(typeReference1 == typeReference2);

Will output false .

So when you are doing the query

• VariableType may be an instance of TypeReference representing Exception
• exceptionTypeReference will be an instance of TypeReference representing Exception

But they are not the same reference and there is no built in equality checking on TypeReference .

What you need to do is

var exceptionType = module.Import(typeof(Exception));
var exceptionVariable = method
              .Body
              .Variables
              .First(x => x.VariableType.FullName == exceptionType.FullName);

Also remember you have to handle inherited exception types.

As a side not be careful using .Import(typeof (Exception)) . The reason is that it give you the Exception type of the current code, not the Exception type of the target assembly. For example you could be processing a WinRT assembly using a .net4 assembly. Importing the .net4 Exception type will probably give you some strange behavior.

So you are just as safe doing this

var exceptionVariable = method
                   .Body
                   .Variables
                   .First(x => x.VariableType.FullName == "System.Exception");