How can I throw an Exception with a certain HResult?

Question

I want to test the following code:

private bool TestException(Exception ex)
{
    if ((Marshal.GetHRForException(ex) & 0xFFFF) == 0x4005)
    {
        return true;
    }
    return false;
}

I'd like to set up the Exception object somehow to return the correct HResult, but I can't see a field in the Exception class which allows this.

How would I do this?

Answer 1

I found three ways to do this:

1. Use the System.Runtime.InteropServices.ExternalException class, passing in the error code as a parameter:

   var ex = new ExternalException("-", 0x4005);
   

   Thanks to @HansPassant for his comment explaining this.

2. Pass a mock exception using inheritance to access a protected field:

   private class MockException : Exception
   {
       public MockException() { HResult = 0x4005; }
   }
   
   var ex = new MockException();
   

3. Use .NET Reflection to set the underlying field:

   BindingFlags flags = BindingFlags.Instance | BindingFlags.NonPublic;
   FieldInfo hresultFieldInfo = typeof(Exception).GetField("_HResult", flags);
   
   var ex = new Exception();
   hresultFieldInfo.SetValue(ex, 0x4005);
   

Passing any one of these exceptions to the method in the question, will result in that method returning true. I suspect the first method is most useful.

Answer 2

I find it useful to create an extension to do this.

using System.Reflection;

namespace Helper
{
    public static class ExceptionHelper 
    {
       public static Exception SetCode(this Exception e, int value)
       {
           BindingFlags flags = BindingFlags.Instance | BindingFlags.NonPublic;
           FieldInfo fieldInfo = typeof(Exception).GetField("_HResult", flags);

           fieldInfo.SetValue(e, value);

           return e;
        }
}

Then throw exception:

using Helper;

public void ExceptionTest()
{
    try
    {
        throw new Exception("my message").SetCode(999);
    }
    catch (Exception e)
    {
        string message = e.Message;
        int code = e.HResult;
    }
}