TranspilerBasics.cs

static IEnumerable<CodeInstruction> Transpiler(IEnumerable<CodeInstruction> instructions, ILGenerator il, MethodBase original)
{
    // create new local variables
    var local = il.DeclareLocal(typeof(TheType));
    
    // create new labels for jumping
    var label = il.DefineLabel();
    
    // inspect original methods properties
    var args = original.GetParameters();
    
    // alternative 1 - looping and emitting
    //
    yield return ...; // add new codes before
    foreach (CodeInstruction instruction in instructions)
    {
         // skip instructions
         if (...)
             continue;
             
         // add new instructions
         if (...)
             yield return new CodeInstruction(...);
         
         // edit instructions
         if (...)
         {
             instruction.opcode = OpCodes.Brtrue;
             instruction.operand = label;
             // or
             instruction.labels.Add(label); // add label
             // or
             instruction.blocks.Add(new ExceptionBlock(ExceptionBlockType.BeginCatchBlock)); // add try/catch boundaries
         }
         yield return instruction;
    }
    yield return ...; // add new codes after
    
    // alternative 2 - convert to list
    //
    var list = instructions.ToList();
    
    // search in list (to find existing local variable for example)
    var index = list.FindIndex(instr => instr.opcode == OpCodes.Ldloc);
    var someLocal = list[index].operand; // LocalBuilder object representing variable
    // reuse someLocal here
    
    // move existing labels
    int idx = ...;
    var someLabels = list[idx].labels;
    list[idx].labels.Clear();
    list.Insert(idx, new CodeInstruction(OpCodes.Nop) { labels = someLabels });
    list[idx].labels.AddRange(someLabels);
    
    return list.AsEnumerable();
    
    // alternative 3 - abort and dont modify
    //
    if (...)
         return null;
}