Invoking the Default Host: Running Managed Executables

The default host for running managed executables is implemented in the function _CorExeMain in the CLR startup shim (mscoree.dll). Control is passed to _CorExeMain differently depending on the operating system on which you're running. On computers running Microsoft Windows XP and later, the operating system loader has explicit knowledge of managed code executables and calls _CorExeMain directly when a managed executable is launched.

For earlier versions of the Windows operating system, the process for getting to _CorExeMain isn't as direct. When a compiler emits a managed executable, it sets the main routine (in the Portable Executable [PE] header) to a small stub that calls _CorExeMain instead of the executable's "real" entry point. Launching the executable thereby passes control to the CLR startup shim, where the process of running the program begins.

How Does Windows XP Determine Whether a File Contains Managed Code?

Managed executable files are stored in the standard PE format. An additional directory entry has been added to the standard file header that identifies whether the file contains managed code. You can see this directory entry by looking for the COM Descriptor Directory in the output from running dumpbin /headers on a file.

If a nonzero address is specified for the COM Descriptor Directory, the file contains managed code as shown in the following output from dumpbin:

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OPTIONAL HEADER VALUES              10B magic # (PE32)             6.00 linker version             1000 size of code             2000 size of initialized data                0 size of uninitialized data             265E entry point (0040265E)             2000 base of code             4000 base of data           400000 image base (00400000 to 00407FFF)             2000 section alignment             1000 file alignment             4.00 operating system version             0.00 image version             4.00 subsystem version                0 Win32 version             8000 size of image             1000 size of headers            13331 checksum                3 subsystem (Windows CUI)              400 DLL characteristics                    No safe exception handler           100000 size of stack reserve             1000 size of stack commit           100000 size of heap reserve             1000 size of heap commit                0 loader flags               10 number of directories                0 [       0] RVA [size] of Export Directory             2604 [      57] RVA [size] of Import Directory             4000 [     860] RVA [size] of Resource Directory                0 [       0] RVA [size] of Exception Directory                0 [       0] RVA [size] of Certificates Directory             6000 [       C] RVA [size] of Base Relocation Directory             20A0 [      1C] RVA [size] of Debug Directory                0 [       0] RVA [size] of Architecture Directory                0 [       0] RVA [size] of Global Pointer Directory                0 [       0] RVA [size] of Thread Storage Directory                0 [       0] RVA [size] of Load Configuration  Directory                0 [       0] RVA [size] of Bound Import Directory             2000 [       8] RVA [size] of Import Address Table  Directory                0 [       0] RVA [size] of Delay Import Directory             2008 [      48] RVA [size] of COM Descriptor  Directory[1]                 0 [      0] RVA [size] of Reserved Directory

^[1] A nonzero value identifies a managed code file.

^[1] A nonzero value identifies a managed code file.