This is an add-in for Fody which lets you inject arbitrary IL into your assembly at compile time.

using static InlineIL.IL.Emit;

public static class Example
{
    public static void ZeroInit<T>(ref T value)
        where T : struct
    {
        Ldarg(nameof(value));
        Ldc_I4_0();
        Sizeof(typeof(T));
        Unaligned(1);
        Initblk();
    }
}

.method public hidebysig static void ZeroInit
    <valuetype .ctor
      ([System.Runtime]System.ValueType) T> (
    !!T& 'value'
  ) cil managed 
{
  .maxstack 8

  IL_0000: ldarg.0
  IL_0001: ldc.i4.0
  IL_0002: sizeof !!T
  IL_0008: unaligned. 1
  IL_000b: initblk
  IL_000d: ret
}

• Include the Fody and InlineIL.Fody NuGet packages with a PrivateAssets="all" attribute on their <PackageReference /> items. Installing Fody explicitly is needed to enable weaving.

  <ItemGroup>
    <PackageReference Include="Fody" Version="..." PrivateAssets="all" />
    <PackageReference Include="InlineIL.Fody" Version="..." PrivateAssets="all" />
  </ItemGroup>

• If you have a FodyWeavers.xml file in the root directory of your project, add the <InlineIL /> tag there. Otherwise, the following file will be created on the first build if it doesn't exist:

  <?xml version="1.0" encoding="utf-8" ?>
  <Weavers>
    <InlineIL />
  </Weavers>

See Fody usage for general guidelines, and Fody configuration for additional options.

Call static methods of the InlineIL.IL.Emit class to emit IL instructions. That's it. 😉

A few more things which are good to know:

• The InlineIL.IL class exposes methods for handling labels and local variables, and some utilities (see below).

• System.Type is implicitly convertible to InlineIL.TypeRef: when you see a TypeRef parameter, you can use the typeof keyword in most cases.

• You can use the IL.Push method to push anything onto the evaluation stack. In particular, this lets you access C# variables, as you cannot emit an ldloc instruction for those.

• You can add the using static InlineIL.IL.Emit; directive to get rid of the IL.Emit prefix.

• You don't have to emit instructions in their short form, they will be shortened automatically (e.g. ldarg.0 will be emitted instead of ldarg 0).

• Most types used as operands declare instance methods which change their meaning and can be chained, for instance: new TypeRef(...).MakeArrayType().MakeByRefType().

• You can combine InlineIL instructions with existing C# code: a given method doesn't have to be entirely written in IL. After weaving, the reference to the InlineIL assembly is removed from your project.

• IL.Emit.*
  Every method call on the IL.Emit class will be replaced by the IL instruction it represents.
  Note that all arguments to these methods need to be constructed in place (i.e. the instruction needs to be representable in IL).

• IL.DeclareLocals
  Declares local variables. Supports changing the init flag and pinned variables. Local variables can be referenced by name or by index.

• IL.MarkLabel
  Marks a label at a given position in the code.

• IL.Push, IL.PushInRef, IL.PushOutRef
  Pushes the provided argument onto the evaluation stack. Does not require a constant argument, any expression should work. If an error is raised in optimized builds, try to use IL.EnsureLocal to change the layout of the IL emitted by the compiler, so it becomes compatible.

• IL.Pop
  Pops a value from the evaluation stack into a local variable or static field.

• IL.Unreachable
  Marks an unreachable code region, for instance just after a ret instruction. Necessary when the compiler's control flow analysis requires a code statement.
  Usage: throw IL.Unreachable();

• IL.Return, IL.ReturnRef, IL.ReturnPointer
  Helper methods to return the value from the top of the evaluation stack.
  Example usage: return IL.Return<int>();

• IL.EnsureLocal
  Helper method that forces the compiler to emit an IL local for the supplied local variable, instead of keeping its value on the evaluation stack only. This can let a local variable be used with IL.Push in optimized builds, where the IL layout wouldn't allow it otherwise.

• TypeRef
  A class which represents a type. Note that System.Type is implicitly convertible to TypeRef, so you can directly write typeof(int) for instance where a TypeRef parameter is expected.

• MethodRef
  A method reference. Exposes a simple constructor for methods without overloads, and some more detailed ones for overload disambiguation. Additional static factory methods for referencing underlying methods of properties and events are provided for convenience.
  Use TypeRef.TypeGenericParameters[N] and TypeRef.MethodGenericParameters[N] to represent the generic parameter of index N in MethodRef calls which involve overload resolution.

• FieldRef
  A field reference.

• StandAloneMethodSig
  A method signature for use as an operand to the calli instruction.

• LocalVar
  Declares a local variable (with an optional name), for use with IL.DeclareLocals. Implicitly convertible from System.Type if you don't want to use named locals or pinning.

The <InlineIL /> element in FodyWeavers.xml accepts the following attributes:

• SequencePoints="True|False|Debug|Release", default value: Debug
  Defines if sequence points should be generated for each emitted IL instruction. The default Debug value improves the debugging experience in Debug builds without impacting the JIT codegen in Release builds.

• Warnings="Warnings|Ignore|Errors", default value: Warnings
  Defines how warnings should be handled. Ignore hides them, while Errors turns them into errors.

• A reimplementation of the System.Runtime.CompilerServices.Unsafe class using InlineIL is provided as an example (compare to the original IL code).

• Unit tests can also serve as examples of API usage. See verifiable and unverifiable test cases.

• See also the simple example from above.