Yesterday Miguel blogged about a nice new feature in Mono. I added the IKVM_VERBOSE_CAST environment variable to IKVM to do something similar a while ago.

public class test {
  public static void main(String[] args) {
    System.out.println((String)(Object)args);
  }
}

C:\j>\ikvm-0.36.0.11\bin\ikvm test
Exception in thread "main" java.lang.ClassCastException
        at test.main(test.java)

C:\j>set IKVM_VERBOSE_CAST=1

C:\j>\ikvm-0.36.0.11\bin\ikvm test
Exception in thread "main" java.lang.ClassCastException: Object of type "System.String[], mscorlib, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" cannot be cast to "System.String, mscorlib, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
        at test.main(test.java)

Note that the assembly qualified type names are displayed, as I believe this feature is particularly useful when trying to debug issues that arise from having loaded multiple assemblies that contain the "same" types.

While writing this I discovered that both JDK 1.6 and .NET 2.0 always generate descriptive exception messages for invalid casts:

C:\j>\jdk1.6\bin\java test
Exception in thread "main" java.lang.ClassCastException: [Ljava.lang.String; can not be cast to java.lang.String
        at test.main(test.java:5)

C:\j>\ikvm\bin\ikvm test
Exception in thread "main" java.lang.ClassCastException: Unable to cast object of type 'System.String[]' to type 'System.String'.
        at test.main(test.java:5)

This last result is my local ikvm development version running on .NET 2.0 with a patch to enable taking the exception message from the .NET InvalidCastException, which I didn't previously do because on .NET 1.1 this message didn't contain any useful information.