Ilasm.exe (IL Assembler)

Article
07/23/2022

The IL Assembler generates a portable executable (PE) file from intermediate language (IL) assembly. (For more information on IL, see Managed Execution Process.) You can run the resulting executable, which contains IL and the required metadata, to determine whether the IL performs as expected.

This tool is automatically installed with Visual Studio. To run the tool, use Visual Studio Developer Command Prompt or Visual Studio Developer PowerShell.

At the command prompt, type the following:

Syntax

ilasm [options] filename [[options]filename...]

Parameters

Argument	Description
`filename`	The name of the .il source file. This file consists of metadata declaration directives and symbolic IL instructions. Multiple source file arguments can be supplied to produce a single PE file with Ilasm.exe. Note: Ensure that the last line of code in the .il source file has either trailing white space or an end-of-line character.

Option	Description
/32bitpreferred	Creates a 32-bit-preferred image (PE32).
/alignment: `integer`	Sets FileAlignment to the value specified by `integer` in the NT Optional header. If the .alignment IL directive is specified in the file, this option overrides it.
/appcontainer	Produces a .dll or .exe file that runs in the Windows app container, as output.
/arm	Specifies the Advanced RISC Machine (ARM) as the target processor. If no image bitness is specified, the default is /32bitpreferred.
/base: `integer`	Sets ImageBase to the value specified by `integer` in the NT Optional header. If the .imagebase IL directive is specified in the file, this option overrides it.
/clock	Measures and reports the following compilation times in milliseconds for the specified .il source file: Total Run: The total time spent performing all the specific operations that follow. Startup: Loading and opening the file. Emitting MD: Emitting metadata. Ref to Def Resolution: Resolving references to definitions in the file. CEE File Generation: Generating the file image in memory. PE File Writing: Writing the image to a PE file.
/debug[:IMPL\|OPT]	Includes debug information (local variable and argument names, and line numbers). Creates a PDB file. /debug with no additional value disables JIT optimization and uses sequence points from the PDB file. IMPL disables JIT optimization and uses implicit sequence points. OPT enables JIT optimization and uses implicit sequence points.
/dll	Produces a .dll file as output.
/enc: `file`	Creates Edit-and-Continue deltas from the specified source file. This argument is for academic use only and is not supported for commercial use.
/exe	Produces an executable file as output. This is the default.
/flags: `integer`	Sets ImageFlags to the value specified by `integer` in the common language runtime header. If the .corflags IL directive is specified in the file, this option overrides it. See CorHdr.h, COMIMAGE_FLAGS for a list of valid values for integer.
/fold	Folds identical method bodies into one.
/highentropyva	Produces an output executable that supports high-entropy address space layout randomization (ASLR). (Default for /appcontainer.)
/include: `includePath`	Sets a path to search for files included with `#include`.
/itanium	Specifies Intel Itanium as the target processor. If no image bitness is specified, the default is /pe64.
/key: `keyFile`	Compiles `filename` with a strong signature using the private key contained in `keyFile`.
/key: @`keySource`	Compiles `filename` with a strong signature using the private key produced at `keySource`.
/listing	Produces a listing file on the standard output. If you omit this option, no listing file is produced. This parameter is not supported in the .NET Framework 2.0 or later.
/mdv: `versionString`	Sets the metadata version string.
/msv: `major`.`minor`	Sets the metadata stream version, where `major` and `minor` are integers.
/noautoinherit	Disables default inheritance from Object when no base class is specified.
/nocorstub	Suppresses generation of the CORExeMain stub.
/nologo	Suppresses the Microsoft startup banner display.
/output: `file.ext`	Specifies the output file name and extension. By default, the output file name is the same as the name of the first source file. The default extension is .exe. If you specify the /dll option, the default extension is .dll. Note: Specifying /output:myfile.dll does not set the /dll option. If you do not specify /dll, the result will be an executable file named myfile.dll.
/optimize	Optimizes long instructions to short. For example, `br` to `br.s`.
/pe64	Creates a 64-bit image (PE32+). If no target processor is specified, the default is `/itanium`.
/pdb	Creates a PDB file without enabling debug information tracking.
/quiet	Specifies quiet mode; does not report assembly progress.
/resource: `file.res`	Includes the specified resource file in .res format in the resulting .exe* or .dll file. Only one .res file can be specified with the /resource option.
/ssver: `int`.`int`	Sets the subsystem version number in the NT optional header. For /appcontainer and /arm the minimum version number is 6.02.
/stack: `stackSize`	Sets the SizeOfStackReserve value in the NT Optional header to `stackSize`.
/stripreloc	Specifies that no base relocations are needed.
/subsystem: `integer`	Sets subsystem to the value specified by `integer` in the NT Optional header. If the .subsystem IL directive is specified in the file, this command overrides it. See winnt.h, IMAGE_SUBSYSTEM for a list of valid values for `integer`.
/x64	Specifies a 64-bit AMD processor as the target processor. If no image bitness is specified, the default is /pe64.
/?	Displays command syntax and options for the tool.

Note

All options for Ilasm.exe are case-insensitive and recognized by the first three letters. For example, /lis is equivalent to /listing and /res:myresfile.res is equivalent to /resource:myresfile.res. Options that specify arguments accept either a colon (:) or an equal sign (=) as the separator between the option and the argument. For example, /output:file.ext is equivalent to /output=file.ext.

Remarks

The IL Assembler helps tool vendors design and implement IL generators. Using Ilasm.exe, tool and compiler developers can concentrate on IL and metadata generation without being concerned with emitting IL in the PE file format.

Similar to other compilers that target the runtime, such as C# and Visual Basic, Ilasm.exe does not produce intermediate object files and does not require a linking stage to form a PE file.

The IL Assembler can express all the existing metadata and IL features of the programming languages that target the runtime. This allows managed code written in any of these programming languages to be adequately expressed in IL Assembler and compiled with Ilasm.exe.

Note

Compilation might fail if the last line of code in the .il source file does not have either trailing white space or an end-of-line character.

You can use Ilasm.exe in conjunction with its companion tool, Ildasm.exe. Ildasm.exe takes a PE file that contains IL code and creates a text file suitable as input to Ilasm.exe. This is useful, for example, when compiling code in a programming language that does not support all the runtime metadata attributes. After compiling the code and running the output through Ildasm.exe, the resulting IL text file can be hand-edited to add the missing attributes. You can then run this text file through the Ilasm.exe to produce a final executable file.

You can also use this technique to produce a single PE file from several PE files originally generated by different compilers.

Note

Currently, you cannot use this technique with PE files that contain embedded native code (for example, PE files produced by Visual C++).

To make this combined use of Ildasm.exe and Ilasm.exe as accurate as possible, by default the assembler does not substitute short encodings for long ones you might have written in your IL sources (or that might be emitted by another compiler). Use the /optimize option to substitute short encodings wherever possible.

Note

Ildasm.exe only operates on files on disk. It does not operate on files installed in the global assembly cache.

For more information about the grammar of IL, see the asmparse.grammar file in the Windows SDK.

Version Information

Starting with .NET Framework 4.5, you can attach a custom attribute to an interface implementation by using code similar to the following:

.class interface public abstract auto ansi IMyInterface
{
  .method public hidebysig newslot abstract virtual
    instance int32 method1() cil managed
  {
  } // end of method IMyInterface::method1
} // end of class IMyInterface
.class public auto ansi beforefieldinit MyClass
  extends [mscorlib]System.Object
  implements IMyInterface
  {
    .interfaceimpl type IMyInterface
    .custom instance void
      [mscorlib]System.Diagnostics.DebuggerNonUserCodeAttribute::.ctor() = ( 01 00 00 00 )
      …

Starting with .NET Framework 4.5, you can specify an arbitrary marshal BLOB (binary large object) by using its raw binary representation, as shown in the following code:

.method public hidebysig abstract virtual
        instance void
        marshal({ 38 01 02 FF })
        Test(object A_1) cil managed

For more information about the grammar of IL, see the asmparse.grammar file in the Windows SDK.

Examples

The following command assembles the IL file myTestFile.il and produces the executable myTestFile.exe.

ilasm myTestFile

The following command assembles the IL file myTestFile.il and produces the .dll file myTestFile.dll.

ilasm myTestFile /dll

The following command assembles the IL file myTestFile.il and produces the .dll file myNewTestFile.dll.

ilasm myTestFile /dll /output:myNewTestFile.dll

The following code example shows an extremely simple application that displays "Hello World!" to the console. You can compile this code and then use the Ildasm.exe tool to generate an IL file.

using System;

public class Hello
{
    public static void Main(String[] args)
    {
        Console.WriteLine("Hello World!");
    }
}

The following IL code example corresponds to the previous C# code example. You can compile this code into an assembly using the IL Assembler tool. Both IL and C# code examples display "Hello World!" to the console.

// Metadata version: v2.0.50215
.assembly extern mscorlib
{
  .publickeytoken = (B7 7A 5C 56 19 34 E0 89 )                         // .z\V.4..
  .ver 2:0:0:0
}
.assembly sample
{
  .custom instance void [mscorlib]System.Runtime.CompilerServices.CompilationRelaxationsAttribute::.ctor(int32) = ( 01 00 08 00 00 00 00 00 )
  .hash algorithm 0x00008004
  .ver 0:0:0:0
}
.module sample.exe
// MVID: {A224F460-A049-4A03-9E71-80A36DBBBCD3}
.imagebase 0x00400000
.file alignment 0x00000200
.stackreserve 0x00100000
.subsystem 0x0003       // WINDOWS_CUI
.corflags 0x00000001    //  ILONLY
// Image base: 0x02F20000

// =============== CLASS MEMBERS DECLARATION ===================

.class public auto ansi beforefieldinit Hello
       extends [mscorlib]System.Object
{
  .method public hidebysig static void  Main(string[] args) cil managed
  {
    .entrypoint
    // Code size       13 (0xd)
    .maxstack  8
    IL_0000:  nop
    IL_0001:  ldstr      "Hello World!"
    IL_0006:  call       void [mscorlib]System.Console::WriteLine(string)
    IL_000b:  nop
    IL_000c:  ret
  } // end of method Hello::Main

  .method public hidebysig specialname rtspecialname
          instance void  .ctor() cil managed
  {
    // Code size       7 (0x7)
    .maxstack  8
    IL_0000:  ldarg.0
    IL_0001:  call       instance void [mscorlib]System.Object::.ctor()
    IL_0006:  ret
  } // end of method Hello::.ctor

} // end of class Hello

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Ilasm.exe (IL Assembler)

Syntax

Parameters

Remarks

Version Information

Examples

See also

Additional resources